src/tools/miri/src/shims/foreign_items.rs

   1 use std::{collections::hash_map::Entry, io::Write, iter};
   2
   3 use log::trace;
   4
   5 use rustc_apfloat::Float;
   6 use rustc_ast::expand::allocator::AllocatorKind;
   7 use rustc_hir::{
   8     def::DefKind,
   9     def_id::{CrateNum, DefId, LOCAL_CRATE},
  10 };
  11 use rustc_middle::middle::{
  12     codegen_fn_attrs::CodegenFnAttrFlags, dependency_format::Linkage,
  13     exported_symbols::ExportedSymbol,
  14 };
  15 use rustc_middle::mir;
  16 use rustc_middle::ty;
  17 use rustc_session::config::CrateType;
  18 use rustc_span::Symbol;
  19 use rustc_target::{
  20     abi::{Align, Size},
  21     spec::abi::Abi,
  22 };
  23
  24 use super::backtrace::EvalContextExt as _;
  25 use crate::helpers::{convert::Truncate, target_os_is_unix};
  26 use crate::*;
  27
  28 /// Returned by `emulate_foreign_item_by_name`.
  29 pub enum EmulateByNameResult<'mir, 'tcx> {
  30     /// The caller is expected to jump to the return block.
  31     NeedsJumping,
  32     /// Jumping has already been taken care of.
  33     AlreadyJumped,
  34     /// A MIR body has been found for the function.
  35     MirBody(&'mir mir::Body<'tcx>, ty::Instance<'tcx>),
  36     /// The item is not supported.
  37     NotSupported,
  38 }
  39
  40 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
  41 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
  42     /// Returns the minimum alignment for the target architecture for allocations of the given size.
  43     fn min_align(&self, size: u64, kind: MiriMemoryKind) -> Align {
  44         let this = self.eval_context_ref();
  45         // List taken from `library/std/src/sys/common/alloc.rs`.
  46         // This list should be kept in sync with the one from libstd.
  47         let min_align = match this.tcx.sess.target.arch.as_ref() {
  48             "x86" | "arm" | "mips" | "powerpc" | "powerpc64" | "asmjs" | "wasm32" => 8,
  49             "x86_64" | "aarch64" | "mips64" | "s390x" | "sparc64" => 16,
  50             arch => bug!("Unsupported target architecture: {}", arch),
  51         };
  52         // Windows always aligns, even small allocations.
  53         // Source: <https://support.microsoft.com/en-us/help/286470/how-to-use-pageheap-exe-in-windows-xp-windows-2000-and-windows-server>
  54         // But jemalloc does not, so for the C heap we only align if the allocation is sufficiently big.
  55         if kind == MiriMemoryKind::WinHeap || size >= min_align {
  56             return Align::from_bytes(min_align).unwrap();
  57         }
  58         // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
  59         fn prev_power_of_two(x: u64) -> u64 {
  60             let next_pow2 = x.next_power_of_two();
  61             if next_pow2 == x {
  62                 // x *is* a power of two, just use that.
  63                 x
  64             } else {
  65                 // x is between two powers, so next = 2*prev.
  66                 next_pow2 / 2
  67             }
  68         }
  69         Align::from_bytes(prev_power_of_two(size)).unwrap()
  70     }
  71
  72     fn malloc(
  73         &mut self,
  74         size: u64,
  75         zero_init: bool,
  76         kind: MiriMemoryKind,
  77     ) -> InterpResult<'tcx, Pointer<Option<Provenance>>> {
  78         let this = self.eval_context_mut();
  79         if size == 0 {
  80             Ok(Pointer::null())
  81         } else {
  82             let align = this.min_align(size, kind);
  83             let ptr = this.allocate_ptr(Size::from_bytes(size), align, kind.into())?;
  84             if zero_init {
  85                 // We just allocated this, the access is definitely in-bounds and fits into our address space.
  86                 this.write_bytes_ptr(
  87                     ptr.into(),
  88                     iter::repeat(0u8).take(usize::try_from(size).unwrap()),
  89                 )
  90                 .unwrap();
  91             }
  92             Ok(ptr.into())
  93         }
  94     }
  95
  96     fn free(
  97         &mut self,
  98         ptr: Pointer<Option<Provenance>>,
  99         kind: MiriMemoryKind,
 100     ) -> InterpResult<'tcx> {
 101         let this = self.eval_context_mut();
 102         if !this.ptr_is_null(ptr)? {
 103             this.deallocate_ptr(ptr, None, kind.into())?;
 104         }
 105         Ok(())
 106     }
 107
 108     fn realloc(
 109         &mut self,
 110         old_ptr: Pointer<Option<Provenance>>,
 111         new_size: u64,
 112         kind: MiriMemoryKind,
 113     ) -> InterpResult<'tcx, Pointer<Option<Provenance>>> {
 114         let this = self.eval_context_mut();
 115         let new_align = this.min_align(new_size, kind);
 116         if this.ptr_is_null(old_ptr)? {
 117             if new_size == 0 {
 118                 Ok(Pointer::null())
 119             } else {
 120                 let new_ptr =
 121                     this.allocate_ptr(Size::from_bytes(new_size), new_align, kind.into())?;
 122                 Ok(new_ptr.into())
 123             }
 124         } else {
 125             if new_size == 0 {
 126                 this.deallocate_ptr(old_ptr, None, kind.into())?;
 127                 Ok(Pointer::null())
 128             } else {
 129                 let new_ptr = this.reallocate_ptr(
 130                     old_ptr,
 131                     None,
 132                     Size::from_bytes(new_size),
 133                     new_align,
 134                     kind.into(),
 135                 )?;
 136                 Ok(new_ptr.into())
 137             }
 138         }
 139     }
 140
 141     /// Lookup the body of a function that has `link_name` as the symbol name.
 142     fn lookup_exported_symbol(
 143         &mut self,
 144         link_name: Symbol,
 145     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 146         let this = self.eval_context_mut();
 147         let tcx = this.tcx.tcx;
 148
 149         // If the result was cached, just return it.
 150         // (Cannot use `or_insert` since the code below might have to throw an error.)
 151         let entry = this.machine.exported_symbols_cache.entry(link_name);
 152         let instance = *match entry {
 153             Entry::Occupied(e) => e.into_mut(),
 154             Entry::Vacant(e) => {
 155                 // Find it if it was not cached.
 156                 let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum)> = None;
 157                 // `dependency_formats` includes all the transitive informations needed to link a crate,
 158                 // which is what we need here since we need to dig out `exported_symbols` from all transitive
 159                 // dependencies.
 160                 let dependency_formats = tcx.dependency_formats(());
 161                 let dependency_format = dependency_formats
 162                     .iter()
 163                     .find(|(crate_type, _)| *crate_type == CrateType::Executable)
 164                     .expect("interpreting a non-executable crate");
 165                 for cnum in iter::once(LOCAL_CRATE).chain(
 166                     dependency_format.1.iter().enumerate().filter_map(|(num, &linkage)| {
 167                         // We add 1 to the number because that's what rustc also does everywhere it
 168                         // calls `CrateNum::new`...
 169                         #[allow(clippy::integer_arithmetic)]
 170                         (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1))
 171                     }),
 172                 ) {
 173                     // We can ignore `_export_info` here: we are a Rust crate, and everything is exported
 174                     // from a Rust crate.
 175                     for &(symbol, _export_info) in tcx.exported_symbols(cnum) {
 176                         if let ExportedSymbol::NonGeneric(def_id) = symbol {
 177                             let attrs = tcx.codegen_fn_attrs(def_id);
 178                             let symbol_name = if let Some(export_name) = attrs.export_name {
 179                                 export_name
 180                             } else if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
 181                                 tcx.item_name(def_id)
 182                             } else {
 183                                 // Skip over items without an explicitly defined symbol name.
 184                                 continue;
 185                             };
 186                             if symbol_name == link_name {
 187                                 if let Some((original_instance, original_cnum)) = instance_and_crate
 188                                 {
 189                                     // Make sure we are consistent wrt what is 'first' and 'second'.
 190                                     let original_span =
 191                                         tcx.def_span(original_instance.def_id()).data();
 192                                     let span = tcx.def_span(def_id).data();
 193                                     if original_span < span {
 194                                         throw_machine_stop!(
 195                                             TerminationInfo::MultipleSymbolDefinitions {
 196                                                 link_name,
 197                                                 first: original_span,
 198                                                 first_crate: tcx.crate_name(original_cnum),
 199                                                 second: span,
 200                                                 second_crate: tcx.crate_name(cnum),
 201                                             }
 202                                         );
 203                                     } else {
 204                                         throw_machine_stop!(
 205                                             TerminationInfo::MultipleSymbolDefinitions {
 206                                                 link_name,
 207                                                 first: span,
 208                                                 first_crate: tcx.crate_name(cnum),
 209                                                 second: original_span,
 210                                                 second_crate: tcx.crate_name(original_cnum),
 211                                             }
 212                                         );
 213                                     }
 214                                 }
 215                                 if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 216                                     throw_ub_format!(
 217                                         "attempt to call an exported symbol that is not defined as a function"
 218                                     );
 219                                 }
 220                                 instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 221                             }
 222                         }
 223                     }
 224                 }
 225
 226                 e.insert(instance_and_crate.map(|ic| ic.0))
 227             }
 228         };
 229         match instance {
 230             None => Ok(None), // no symbol with this name
 231             Some(instance) => Ok(Some((this.load_mir(instance.def, None)?, instance))),
 232         }
 233     }
 234
 235     /// Emulates calling a foreign item, failing if the item is not supported.
 236     /// This function will handle `goto_block` if needed.
 237     /// Returns Ok(None) if the foreign item was completely handled
 238     /// by this function.
 239     /// Returns Ok(Some(body)) if processing the foreign item
 240     /// is delegated to another function.
 241     fn emulate_foreign_item(
 242         &mut self,
 243         def_id: DefId,
 244         abi: Abi,
 245         args: &[OpTy<'tcx, Provenance>],
 246         dest: &PlaceTy<'tcx, Provenance>,
 247         ret: Option<mir::BasicBlock>,
 248         unwind: StackPopUnwind,
 249     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 250         let this = self.eval_context_mut();
 251         let link_name = this.item_link_name(def_id);
 252         let tcx = this.tcx.tcx;
 253
 254         // First: functions that diverge.
 255         let ret = match ret {
 256             None =>
 257                 match link_name.as_str() {
 258                     "miri_start_panic" => {
 259                         // `check_shim` happens inside `handle_miri_start_panic`.
 260                         this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 261                         return Ok(None);
 262                     }
 263                     // This matches calls to the foreign item `panic_impl`.
 264                     // The implementation is provided by the function with the `#[panic_handler]` attribute.
 265                     "panic_impl" => {
 266                         // We don't use `check_shim` here because we are just forwarding to the lang
 267                         // item. Argument count checking will be performed when the returned `Body` is
 268                         // called.
 269                         this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 270                         let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 271                         let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 272                         return Ok(Some((
 273                             this.load_mir(panic_impl_instance.def, None)?,
 274                             panic_impl_instance,
 275                         )));
 276                     }
 277                     #[rustfmt::skip]
 278                     | "exit"
 279                     | "ExitProcess"
 280                     => {
 281                         let exp_abi = if link_name.as_str() == "exit" {
 282                             Abi::C { unwind: false }
 283                         } else {
 284                             Abi::System { unwind: false }
 285                         };
 286                         let [code] = this.check_shim(abi, exp_abi, link_name, args)?;
 287                         // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 288                         let code = this.read_scalar(code)?.to_i32()?;
 289                         throw_machine_stop!(TerminationInfo::Exit { code: code.into(), leak_check: false });
 290                     }
 291                     "abort" => {
 292                         let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 293                         throw_machine_stop!(TerminationInfo::Abort(
 294                             "the program aborted execution".to_owned()
 295                         ))
 296                     }
 297                     _ => {
 298                         if let Some(body) = this.lookup_exported_symbol(link_name)? {
 299                             return Ok(Some(body));
 300                         }
 301                         this.handle_unsupported(format!(
 302                             "can't call (diverging) foreign function: {link_name}"
 303                         ))?;
 304                         return Ok(None);
 305                     }
 306                 },
 307             Some(p) => p,
 308         };
 309
 310         // Second: functions that return immediately.
 311         match this.emulate_foreign_item_by_name(link_name, abi, args, dest)? {
 312             EmulateByNameResult::NeedsJumping => {
 313                 trace!("{:?}", this.dump_place(**dest));
 314                 this.go_to_block(ret);
 315             }
 316             EmulateByNameResult::AlreadyJumped => (),
 317             EmulateByNameResult::MirBody(mir, instance) => return Ok(Some((mir, instance))),
 318             EmulateByNameResult::NotSupported => {
 319                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 320                     return Ok(Some(body));
 321                 }
 322
 323                 this.handle_unsupported(format!("can't call foreign function: {link_name}"))?;
 324                 return Ok(None);
 325             }
 326         }
 327
 328         Ok(None)
 329     }
 330
 331     /// Emulates calling the internal __rust_* allocator functions
 332     fn emulate_allocator(
 333         &mut self,
 334         symbol: Symbol,
 335         default: impl FnOnce(&mut MiriInterpCx<'mir, 'tcx>) -> InterpResult<'tcx>,
 336     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 337         let this = self.eval_context_mut();
 338
 339         let allocator_kind = if let Some(allocator_kind) = this.tcx.allocator_kind(()) {
 340             allocator_kind
 341         } else {
 342             // in real code, this symbol does not exist without an allocator
 343             return Ok(EmulateByNameResult::NotSupported);
 344         };
 345
 346         match allocator_kind {
 347             AllocatorKind::Global => {
 348                 let (body, instance) = this
 349                     .lookup_exported_symbol(symbol)?
 350                     .expect("symbol should be present if there is a global allocator");
 351
 352                 Ok(EmulateByNameResult::MirBody(body, instance))
 353             }
 354             AllocatorKind::Default => {
 355                 default(this)?;
 356                 Ok(EmulateByNameResult::NeedsJumping)
 357             }
 358         }
 359     }
 360
 361     /// Emulates calling a foreign item using its name.
 362     fn emulate_foreign_item_by_name(
 363         &mut self,
 364         link_name: Symbol,
 365         abi: Abi,
 366         args: &[OpTy<'tcx, Provenance>],
 367         dest: &PlaceTy<'tcx, Provenance>,
 368     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 369         let this = self.eval_context_mut();
 370
 371         // First deal with any external C functions in linked .so file.
 372         #[cfg(target_os = "linux")]
 373         if this.machine.external_so_lib.as_ref().is_some() {
 374             use crate::shims::ffi_support::EvalContextExt as _;
 375             // An Ok(false) here means that the function being called was not exported
 376             // by the specified `.so` file; we should continue and check if it corresponds to
 377             // a provided shim.
 378             if this.call_external_c_fct(link_name, dest, args)? {
 379                 return Ok(EmulateByNameResult::NeedsJumping);
 380             }
 381         }
 382
 383         // When adding a new shim, you should follow the following pattern:
 384         // ```
 385         // "shim_name" => {
 386         //     let [arg1, arg2, arg3] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 387         //     let result = this.shim_name(arg1, arg2, arg3)?;
 388         //     this.write_scalar(result, dest)?;
 389         // }
 390         // ```
 391         // and then define `shim_name` as a helper function in an extension trait in a suitable file
 392         // (see e.g. `unix/fs.rs`):
 393         // ```
 394         // fn shim_name(
 395         //     &mut self,
 396         //     arg1: &OpTy<'tcx, Provenance>,
 397         //     arg2: &OpTy<'tcx, Provenance>,
 398         //     arg3: &OpTy<'tcx, Provenance>)
 399         // -> InterpResult<'tcx, Scalar<Provenance>> {
 400         //     let this = self.eval_context_mut();
 401         //
 402         //     // First thing: load all the arguments. Details depend on the shim.
 403         //     let arg1 = this.read_scalar(arg1)?.to_u32()?;
 404         //     let arg2 = this.read_pointer(arg2)?; // when you need to work with the pointer directly
 405         //     let arg3 = this.deref_operand(arg3)?; // when you want to load/store through the pointer at its declared type
 406         //
 407         //     // ...
 408         //
 409         //     Ok(Scalar::from_u32(42))
 410         // }
 411         // ```
 412         // You might find existing shims not following this pattern, most
 413         // likely because they predate it or because for some reason they cannot be made to fit.
 414
 415         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 416         // shim, add it to the corresponding submodule.
 417         match link_name.as_str() {
 418             // Miri-specific extern functions
 419             "miri_get_alloc_id" => {
 420                 let [ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 421                 let ptr = this.read_pointer(ptr)?;
 422                 let (alloc_id, _, _) = this.ptr_get_alloc_id(ptr).map_err(|_e| {
 423                     err_machine_stop!(TerminationInfo::Abort(
 424                         format!("pointer passed to miri_get_alloc_id must not be dangling, got {ptr:?}")
 425                     ))
 426                 })?;
 427                 this.write_scalar(Scalar::from_u64(alloc_id.0.get()), dest)?;
 428             }
 429             "miri_print_borrow_stacks" => {
 430                 let [id] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 431                 let id = this.read_scalar(id)?.to_u64()?;
 432                 if let Some(id) = std::num::NonZeroU64::new(id) {
 433                     this.print_stacks(AllocId(id))?;
 434                 }
 435             }
 436             "miri_static_root" => {
 437                 let [ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 438                 let ptr = this.read_pointer(ptr)?;
 439                 let (alloc_id, offset, _) = this.ptr_get_alloc_id(ptr)?;
 440                 if offset != Size::ZERO {
 441                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 442                 }
 443                 this.machine.static_roots.push(alloc_id);
 444             }
 445
 446             // Obtains the size of a Miri backtrace. See the README for details.
 447             "miri_backtrace_size" => {
 448                 this.handle_miri_backtrace_size(abi, link_name, args, dest)?;
 449             }
 450
 451             // Obtains a Miri backtrace. See the README for details.
 452             "miri_get_backtrace" => {
 453                 // `check_shim` happens inside `handle_miri_get_backtrace`.
 454                 this.handle_miri_get_backtrace(abi, link_name, args, dest)?;
 455             }
 456
 457             // Resolves a Miri backtrace frame. See the README for details.
 458             "miri_resolve_frame" => {
 459                 // `check_shim` happens inside `handle_miri_resolve_frame`.
 460                 this.handle_miri_resolve_frame(abi, link_name, args, dest)?;
 461             }
 462
 463             // Writes the function and file names of a Miri backtrace frame into a user provided buffer. See the README for details.
 464             "miri_resolve_frame_names" => {
 465                 this.handle_miri_resolve_frame_names(abi, link_name, args)?;
 466             }
 467
 468             // Writes some bytes to the interpreter's stdout/stderr. See the
 469             // README for details.
 470             "miri_write_to_stdout" | "miri_write_to_stderr" => {
 471                 let [bytes] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 472                 let (ptr, len) = this.read_immediate(bytes)?.to_scalar_pair();
 473                 let ptr = ptr.to_pointer(this)?;
 474                 let len = len.to_machine_usize(this)?;
 475                 let msg = this.read_bytes_ptr_strip_provenance(ptr, Size::from_bytes(len))?;
 476
 477                 // Note: we're ignoring errors writing to host stdout/stderr.
 478                 let _ignore = match link_name.as_str() {
 479                     "miri_write_to_stdout" => std::io::stdout().write_all(msg),
 480                     "miri_write_to_stderr" => std::io::stderr().write_all(msg),
 481                     _ => unreachable!(),
 482                 };
 483             }
 484
 485             // Standard C allocation
 486             "malloc" => {
 487                 let [size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 488                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 489                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C)?;
 490                 this.write_pointer(res, dest)?;
 491             }
 492             "calloc" => {
 493                 let [items, len] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 494                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 495                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 496                 let size =
 497                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 498                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C)?;
 499                 this.write_pointer(res, dest)?;
 500             }
 501             "free" => {
 502                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 503                 let ptr = this.read_pointer(ptr)?;
 504                 this.free(ptr, MiriMemoryKind::C)?;
 505             }
 506             "realloc" => {
 507                 let [old_ptr, new_size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 508                 let old_ptr = this.read_pointer(old_ptr)?;
 509                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 510                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 511                 this.write_pointer(res, dest)?;
 512             }
 513
 514             // Rust allocation
 515             "__rust_alloc" | "miri_alloc" => {
 516                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 517                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 518                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 519
 520                 let default = |this: &mut MiriInterpCx<'mir, 'tcx>| {
 521                     Self::check_alloc_request(size, align)?;
 522
 523                     let memory_kind = match link_name.as_str() {
 524                         "__rust_alloc" => MiriMemoryKind::Rust,
 525                         "miri_alloc" => MiriMemoryKind::Miri,
 526                         _ => unreachable!(),
 527                     };
 528
 529                     let ptr = this.allocate_ptr(
 530                         Size::from_bytes(size),
 531                         Align::from_bytes(align).unwrap(),
 532                         memory_kind.into(),
 533                     )?;
 534
 535                     this.write_pointer(ptr, dest)
 536                 };
 537
 538                 match link_name.as_str() {
 539                     "__rust_alloc" => return this.emulate_allocator(Symbol::intern("__rg_alloc"), default),
 540                     "miri_alloc" => {
 541                         default(this)?;
 542                         return Ok(EmulateByNameResult::NeedsJumping);
 543                     },
 544                     _ => unreachable!(),
 545                 }
 546             }
 547             "__rust_alloc_zeroed" => {
 548                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 549                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 550                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 551
 552                 return this.emulate_allocator(Symbol::intern("__rg_alloc_zeroed"), |this| {
 553                     Self::check_alloc_request(size, align)?;
 554
 555                     let ptr = this.allocate_ptr(
 556                         Size::from_bytes(size),
 557                         Align::from_bytes(align).unwrap(),
 558                         MiriMemoryKind::Rust.into(),
 559                     )?;
 560
 561                     // We just allocated this, the access is definitely in-bounds.
 562                     this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 563                     this.write_pointer(ptr, dest)
 564                 });
 565             }
 566             "__rust_dealloc" | "miri_dealloc" => {
 567                 let [ptr, old_size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 568                 let ptr = this.read_pointer(ptr)?;
 569                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 570                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 571
 572                 let default = |this: &mut MiriInterpCx<'mir, 'tcx>| {
 573                     let memory_kind = match link_name.as_str() {
 574                         "__rust_dealloc" => MiriMemoryKind::Rust,
 575                         "miri_dealloc" => MiriMemoryKind::Miri,
 576                         _ => unreachable!(),
 577                     };
 578
 579                     // No need to check old_size/align; we anyway check that they match the allocation.
 580                     this.deallocate_ptr(
 581                         ptr,
 582                         Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 583                         memory_kind.into(),
 584                     )
 585                 };
 586
 587                 match link_name.as_str() {
 588                     "__rust_dealloc" => return this.emulate_allocator(Symbol::intern("__rg_dealloc"), default),
 589                     "miri_dealloc" => {
 590                         default(this)?;
 591                         return Ok(EmulateByNameResult::NeedsJumping);
 592                     }
 593                     _ => unreachable!(),
 594                 }
 595             }
 596             "__rust_realloc" => {
 597                 let [ptr, old_size, align, new_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 598                 let ptr = this.read_pointer(ptr)?;
 599                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 600                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 601                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 602                 // No need to check old_size; we anyway check that they match the allocation.
 603
 604                 return this.emulate_allocator(Symbol::intern("__rg_realloc"), |this| {
 605                     Self::check_alloc_request(new_size, align)?;
 606
 607                     let align = Align::from_bytes(align).unwrap();
 608                     let new_ptr = this.reallocate_ptr(
 609                         ptr,
 610                         Some((Size::from_bytes(old_size), align)),
 611                         Size::from_bytes(new_size),
 612                         align,
 613                         MiriMemoryKind::Rust.into(),
 614                     )?;
 615                     this.write_pointer(new_ptr, dest)
 616                 });
 617             }
 618
 619             // C memory handling functions
 620             "memcmp" => {
 621                 let [left, right, n] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 622                 let left = this.read_pointer(left)?;
 623                 let right = this.read_pointer(right)?;
 624                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 625
 626                 let result = {
 627                     let left_bytes = this.read_bytes_ptr_strip_provenance(left, n)?;
 628                     let right_bytes = this.read_bytes_ptr_strip_provenance(right, n)?;
 629
 630                     use std::cmp::Ordering::*;
 631                     match left_bytes.cmp(right_bytes) {
 632                         Less => -1i32,
 633                         Equal => 0,
 634                         Greater => 1,
 635                     }
 636                 };
 637
 638                 this.write_scalar(Scalar::from_i32(result), dest)?;
 639             }
 640             "memrchr" => {
 641                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 642                 let ptr = this.read_pointer(ptr)?;
 643                 let val = this.read_scalar(val)?.to_i32()?;
 644                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 645                 // The docs say val is "interpreted as unsigned char".
 646                 #[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
 647                 let val = val as u8;
 648
 649                 if let Some(idx) = this
 650                     .read_bytes_ptr_strip_provenance(ptr, Size::from_bytes(num))?
 651                     .iter()
 652                     .rev()
 653                     .position(|&c| c == val)
 654                 {
 655                     let idx = u64::try_from(idx).unwrap();
 656                     #[allow(clippy::integer_arithmetic)] // idx < num, so this never wraps
 657                     let new_ptr = ptr.offset(Size::from_bytes(num - idx - 1), this)?;
 658                     this.write_pointer(new_ptr, dest)?;
 659                 } else {
 660                     this.write_null(dest)?;
 661                 }
 662             }
 663             "memchr" => {
 664                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 665                 let ptr = this.read_pointer(ptr)?;
 666                 let val = this.read_scalar(val)?.to_i32()?;
 667                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 668                 // The docs say val is "interpreted as unsigned char".
 669                 #[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
 670                 let val = val as u8;
 671
 672                 let idx = this
 673                     .read_bytes_ptr_strip_provenance(ptr, Size::from_bytes(num))?
 674                     .iter()
 675                     .position(|&c| c == val);
 676                 if let Some(idx) = idx {
 677                     let new_ptr = ptr.offset(Size::from_bytes(idx as u64), this)?;
 678                     this.write_pointer(new_ptr, dest)?;
 679                 } else {
 680                     this.write_null(dest)?;
 681                 }
 682             }
 683             "strlen" => {
 684                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 685                 let ptr = this.read_pointer(ptr)?;
 686                 let n = this.read_c_str(ptr)?.len();
 687                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 688             }
 689
 690             // math functions (note that there are also intrinsics for some other functions)
 691             #[rustfmt::skip]
 692             | "cbrtf"
 693             | "coshf"
 694             | "sinhf"
 695             | "tanf"
 696             | "tanhf"
 697             | "acosf"
 698             | "asinf"
 699             | "atanf"
 700             | "log1pf"
 701             | "expm1f"
 702             => {
 703                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 704                 // FIXME: Using host floats.
 705                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 706                 let res = match link_name.as_str() {
 707                     "cbrtf" => f.cbrt(),
 708                     "coshf" => f.cosh(),
 709                     "sinhf" => f.sinh(),
 710                     "tanf" => f.tan(),
 711                     "tanhf" => f.tanh(),
 712                     "acosf" => f.acos(),
 713                     "asinf" => f.asin(),
 714                     "atanf" => f.atan(),
 715                     "log1pf" => f.ln_1p(),
 716                     "expm1f" => f.exp_m1(),
 717                     _ => bug!(),
 718                 };
 719                 this.write_scalar(Scalar::from_u32(res.to_bits()), dest)?;
 720             }
 721             #[rustfmt::skip]
 722             | "_hypotf"
 723             | "hypotf"
 724             | "atan2f"
 725             | "fdimf"
 726             => {
 727                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 728                 // underscore case for windows, here and below
 729                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 730                 // FIXME: Using host floats.
 731                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 732                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 733                 let res = match link_name.as_str() {
 734                     "_hypotf" | "hypotf" => f1.hypot(f2),
 735                     "atan2f" => f1.atan2(f2),
 736                     #[allow(deprecated)]
 737                     "fdimf" => f1.abs_sub(f2),
 738                     _ => bug!(),
 739                 };
 740                 this.write_scalar(Scalar::from_u32(res.to_bits()), dest)?;
 741             }
 742             #[rustfmt::skip]
 743             | "cbrt"
 744             | "cosh"
 745             | "sinh"
 746             | "tan"
 747             | "tanh"
 748             | "acos"
 749             | "asin"
 750             | "atan"
 751             | "log1p"
 752             | "expm1"
 753             => {
 754                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 755                 // FIXME: Using host floats.
 756                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 757                 let res = match link_name.as_str() {
 758                     "cbrt" => f.cbrt(),
 759                     "cosh" => f.cosh(),
 760                     "sinh" => f.sinh(),
 761                     "tan" => f.tan(),
 762                     "tanh" => f.tanh(),
 763                     "acos" => f.acos(),
 764                     "asin" => f.asin(),
 765                     "atan" => f.atan(),
 766                     "log1p" => f.ln_1p(),
 767                     "expm1" => f.exp_m1(),
 768                     _ => bug!(),
 769                 };
 770                 this.write_scalar(Scalar::from_u64(res.to_bits()), dest)?;
 771             }
 772             #[rustfmt::skip]
 773             | "_hypot"
 774             | "hypot"
 775             | "atan2"
 776             | "fdim"
 777             => {
 778                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 779                 // FIXME: Using host floats.
 780                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 781                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 782                 let res = match link_name.as_str() {
 783                     "_hypot" | "hypot" => f1.hypot(f2),
 784                     "atan2" => f1.atan2(f2),
 785                     #[allow(deprecated)]
 786                     "fdim" => f1.abs_sub(f2),
 787                     _ => bug!(),
 788                 };
 789                 this.write_scalar(Scalar::from_u64(res.to_bits()), dest)?;
 790             }
 791             #[rustfmt::skip]
 792             | "_ldexp"
 793             | "ldexp"
 794             | "scalbn"
 795             => {
 796                 let [x, exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 797                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 798                 let x = this.read_scalar(x)?.to_f64()?;
 799                 let exp = this.read_scalar(exp)?.to_i32()?;
 800
 801                 // Saturating cast to i16. Even those are outside the valid exponent range so
 802                 // `scalbn` below will do its over/underflow handling.
 803                 let exp = if exp > i32::from(i16::MAX) {
 804                     i16::MAX
 805                 } else if exp < i32::from(i16::MIN) {
 806                     i16::MIN
 807                 } else {
 808                     exp.try_into().unwrap()
 809                 };
 810
 811                 let res = x.scalbn(exp);
 812                 this.write_scalar(Scalar::from_f64(res), dest)?;
 813             }
 814
 815             // Architecture-specific shims
 816             "llvm.x86.addcarry.64" if this.tcx.sess.target.arch == "x86_64" => {
 817                 // Computes u8+u64+u64, returning tuple (u8,u64) comprising the output carry and truncated sum.
 818                 let [c_in, a, b] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 819                 let c_in = this.read_scalar(c_in)?.to_u8()?;
 820                 let a = this.read_scalar(a)?.to_u64()?;
 821                 let b = this.read_scalar(b)?.to_u64()?;
 822
 823                 #[allow(clippy::integer_arithmetic)] // adding two u64 and a u8 cannot wrap in a u128
 824                 let wide_sum = u128::from(c_in) + u128::from(a) + u128::from(b);
 825                 #[allow(clippy::integer_arithmetic)] // it's a u128, we can shift by 64
 826                 let (c_out, sum) = ((wide_sum >> 64).truncate::<u8>(), wide_sum.truncate::<u64>());
 827
 828                 let c_out_field = this.place_field(dest, 0)?;
 829                 this.write_scalar(Scalar::from_u8(c_out), &c_out_field)?;
 830                 let sum_field = this.place_field(dest, 1)?;
 831                 this.write_scalar(Scalar::from_u64(sum), &sum_field)?;
 832             }
 833             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 834                 let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 835                 this.yield_active_thread();
 836             }
 837             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 838                 let [arg] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 839                 let arg = this.read_scalar(arg)?.to_i32()?;
 840                 match arg {
 841                     15 => { // SY ("full system scope")
 842                         this.yield_active_thread();
 843                     }
 844                     _ => {
 845                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 846                     }
 847                 }
 848             }
 849
 850             // Platform-specific shims
 851             _ => match this.tcx.sess.target.os.as_ref() {
 852                 target if target_os_is_unix(target) => return shims::unix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest),
 853                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest),
 854                 target => throw_unsup_format!("the target `{}` is not supported", target),
 855             }
 856         };
 857         // We only fall through to here if we did *not* hit the `_` arm above,
 858         // i.e., if we actually emulated the function with one of the shims.
 859         Ok(EmulateByNameResult::NeedsJumping)
 860     }
 861
 862     /// Check some basic requirements for this allocation request:
 863     /// non-zero size, power-of-two alignment.
 864     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 865         if size == 0 {
 866             throw_ub_format!("creating allocation with size 0");
 867         }
 868         if !align.is_power_of_two() {
 869             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 870         }
 871         Ok(())
 872     }
 873 }