src/shims/foreign_items.rs

   1 use std::{collections::hash_map::Entry, 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::MiriEvalContext<'mir, 'tcx> {}
  41 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'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                         (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1))
 168                     }),
 169                 ) {
 170                     // We can ignore `_export_info` here: we are a Rust crate, and everything is exported
 171                     // from a Rust crate.
 172                     for &(symbol, _export_info) in tcx.exported_symbols(cnum) {
 173                         if let ExportedSymbol::NonGeneric(def_id) = symbol {
 174                             let attrs = tcx.codegen_fn_attrs(def_id);
 175                             let symbol_name = if let Some(export_name) = attrs.export_name {
 176                                 export_name
 177                             } else if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
 178                                 tcx.item_name(def_id)
 179                             } else {
 180                                 // Skip over items without an explicitly defined symbol name.
 181                                 continue;
 182                             };
 183                             if symbol_name == link_name {
 184                                 if let Some((original_instance, original_cnum)) = instance_and_crate
 185                                 {
 186                                     // Make sure we are consistent wrt what is 'first' and 'second'.
 187                                     let original_span =
 188                                         tcx.def_span(original_instance.def_id()).data();
 189                                     let span = tcx.def_span(def_id).data();
 190                                     if original_span < span {
 191                                         throw_machine_stop!(
 192                                             TerminationInfo::MultipleSymbolDefinitions {
 193                                                 link_name,
 194                                                 first: original_span,
 195                                                 first_crate: tcx.crate_name(original_cnum),
 196                                                 second: span,
 197                                                 second_crate: tcx.crate_name(cnum),
 198                                             }
 199                                         );
 200                                     } else {
 201                                         throw_machine_stop!(
 202                                             TerminationInfo::MultipleSymbolDefinitions {
 203                                                 link_name,
 204                                                 first: span,
 205                                                 first_crate: tcx.crate_name(cnum),
 206                                                 second: original_span,
 207                                                 second_crate: tcx.crate_name(original_cnum),
 208                                             }
 209                                         );
 210                                     }
 211                                 }
 212                                 if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 213                                     throw_ub_format!(
 214                                         "attempt to call an exported symbol that is not defined as a function"
 215                                     );
 216                                 }
 217                                 instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 218                             }
 219                         }
 220                     }
 221                 }
 222
 223                 e.insert(instance_and_crate.map(|ic| ic.0))
 224             }
 225         };
 226         match instance {
 227             None => Ok(None), // no symbol with this name
 228             Some(instance) => Ok(Some((this.load_mir(instance.def, None)?, instance))),
 229         }
 230     }
 231
 232     /// Emulates calling a foreign item, failing if the item is not supported.
 233     /// This function will handle `goto_block` if needed.
 234     /// Returns Ok(None) if the foreign item was completely handled
 235     /// by this function.
 236     /// Returns Ok(Some(body)) if processing the foreign item
 237     /// is delegated to another function.
 238     fn emulate_foreign_item(
 239         &mut self,
 240         def_id: DefId,
 241         abi: Abi,
 242         args: &[OpTy<'tcx, Provenance>],
 243         dest: &PlaceTy<'tcx, Provenance>,
 244         ret: Option<mir::BasicBlock>,
 245         unwind: StackPopUnwind,
 246     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 247         let this = self.eval_context_mut();
 248         let link_name = this.item_link_name(def_id);
 249         let tcx = this.tcx.tcx;
 250
 251         // First: functions that diverge.
 252         let ret = match ret {
 253             None =>
 254                 match link_name.as_str() {
 255                     "miri_start_panic" => {
 256                         // `check_shim` happens inside `handle_miri_start_panic`.
 257                         this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 258                         return Ok(None);
 259                     }
 260                     // This matches calls to the foreign item `panic_impl`.
 261                     // The implementation is provided by the function with the `#[panic_handler]` attribute.
 262                     "panic_impl" => {
 263                         // We don't use `check_shim` here because we are just forwarding to the lang
 264                         // item. Argument count checking will be performed when the returned `Body` is
 265                         // called.
 266                         this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 267                         let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 268                         let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 269                         return Ok(Some((
 270                             this.load_mir(panic_impl_instance.def, None)?,
 271                             panic_impl_instance,
 272                         )));
 273                     }
 274                     #[rustfmt::skip]
 275                     | "exit"
 276                     | "ExitProcess"
 277                     => {
 278                         let exp_abi = if link_name.as_str() == "exit" {
 279                             Abi::C { unwind: false }
 280                         } else {
 281                             Abi::System { unwind: false }
 282                         };
 283                         let [code] = this.check_shim(abi, exp_abi, link_name, args)?;
 284                         // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 285                         let code = this.read_scalar(code)?.to_i32()?;
 286                         throw_machine_stop!(TerminationInfo::Exit(code.into()));
 287                     }
 288                     "abort" => {
 289                         let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 290                         throw_machine_stop!(TerminationInfo::Abort(
 291                             "the program aborted execution".to_owned()
 292                         ))
 293                     }
 294                     _ => {
 295                         if let Some(body) = this.lookup_exported_symbol(link_name)? {
 296                             return Ok(Some(body));
 297                         }
 298                         this.handle_unsupported(format!(
 299                             "can't call (diverging) foreign function: {}",
 300                             link_name
 301                         ))?;
 302                         return Ok(None);
 303                     }
 304                 },
 305             Some(p) => p,
 306         };
 307
 308         // Second: functions that return immediately.
 309         match this.emulate_foreign_item_by_name(link_name, abi, args, dest)? {
 310             EmulateByNameResult::NeedsJumping => {
 311                 trace!("{:?}", this.dump_place(**dest));
 312                 this.go_to_block(ret);
 313             }
 314             EmulateByNameResult::AlreadyJumped => (),
 315             EmulateByNameResult::MirBody(mir, instance) => return Ok(Some((mir, instance))),
 316             EmulateByNameResult::NotSupported => {
 317                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 318                     return Ok(Some(body));
 319                 }
 320
 321                 this.handle_unsupported(format!("can't call foreign function: {}", link_name))?;
 322                 return Ok(None);
 323             }
 324         }
 325
 326         Ok(None)
 327     }
 328
 329     /// Emulates calling the internal __rust_* allocator functions
 330     fn emulate_allocator(
 331         &mut self,
 332         symbol: Symbol,
 333         default: impl FnOnce(&mut MiriEvalContext<'mir, 'tcx>) -> InterpResult<'tcx>,
 334     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 335         let this = self.eval_context_mut();
 336
 337         let allocator_kind = if let Some(allocator_kind) = this.tcx.allocator_kind(()) {
 338             allocator_kind
 339         } else {
 340             // in real code, this symbol does not exist without an allocator
 341             return Ok(EmulateByNameResult::NotSupported);
 342         };
 343
 344         match allocator_kind {
 345             AllocatorKind::Global => {
 346                 let (body, instance) = this
 347                     .lookup_exported_symbol(symbol)?
 348                     .expect("symbol should be present if there is a global allocator");
 349
 350                 Ok(EmulateByNameResult::MirBody(body, instance))
 351             }
 352             AllocatorKind::Default => {
 353                 default(this)?;
 354                 Ok(EmulateByNameResult::NeedsJumping)
 355             }
 356         }
 357     }
 358
 359     /// Emulates calling a foreign item using its name.
 360     fn emulate_foreign_item_by_name(
 361         &mut self,
 362         link_name: Symbol,
 363         abi: Abi,
 364         args: &[OpTy<'tcx, Provenance>],
 365         dest: &PlaceTy<'tcx, Provenance>,
 366     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 367         let this = self.eval_context_mut();
 368
 369         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 370         // shim, add it to the corresponding submodule.
 371         match link_name.as_str() {
 372             // Miri-specific extern functions
 373             "miri_static_root" => {
 374                 let [ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 375                 let ptr = this.read_pointer(ptr)?;
 376                 let (alloc_id, offset, _) = this.ptr_get_alloc_id(ptr)?;
 377                 if offset != Size::ZERO {
 378                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 379                 }
 380                 this.machine.static_roots.push(alloc_id);
 381             }
 382
 383             // Obtains the size of a Miri backtrace. See the README for details.
 384             "miri_backtrace_size" => {
 385                 this.handle_miri_backtrace_size(abi, link_name, args, dest)?;
 386             }
 387
 388             // Obtains a Miri backtrace. See the README for details.
 389             "miri_get_backtrace" => {
 390                 // `check_shim` happens inside `handle_miri_get_backtrace`.
 391                 this.handle_miri_get_backtrace(abi, link_name, args, dest)?;
 392             }
 393
 394             // Resolves a Miri backtrace frame. See the README for details.
 395             "miri_resolve_frame" => {
 396                 // `check_shim` happens inside `handle_miri_resolve_frame`.
 397                 this.handle_miri_resolve_frame(abi, link_name, args, dest)?;
 398             }
 399
 400             // Writes the function and file names of a Miri backtrace frame into a user provided buffer. See the README for details.
 401             "miri_resolve_frame_names" => {
 402                 this.handle_miri_resolve_frame_names(abi, link_name, args)?;
 403             }
 404
 405             // Standard C allocation
 406             "malloc" => {
 407                 let [size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 408                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 409                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C)?;
 410                 this.write_pointer(res, dest)?;
 411             }
 412             "calloc" => {
 413                 let [items, len] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 414                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 415                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 416                 let size =
 417                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 418                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C)?;
 419                 this.write_pointer(res, dest)?;
 420             }
 421             "free" => {
 422                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 423                 let ptr = this.read_pointer(ptr)?;
 424                 this.free(ptr, MiriMemoryKind::C)?;
 425             }
 426             "realloc" => {
 427                 let [old_ptr, new_size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 428                 let old_ptr = this.read_pointer(old_ptr)?;
 429                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 430                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 431                 this.write_pointer(res, dest)?;
 432             }
 433
 434             // Rust allocation
 435             "__rust_alloc" => {
 436                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 437                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 438                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 439
 440                 return this.emulate_allocator(Symbol::intern("__rg_alloc"), |this| {
 441                     Self::check_alloc_request(size, align)?;
 442
 443                     let ptr = this.allocate_ptr(
 444                         Size::from_bytes(size),
 445                         Align::from_bytes(align).unwrap(),
 446                         MiriMemoryKind::Rust.into(),
 447                     )?;
 448
 449                     this.write_pointer(ptr, dest)
 450                 });
 451             }
 452             "__rust_alloc_zeroed" => {
 453                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 454                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 455                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 456
 457                 return this.emulate_allocator(Symbol::intern("__rg_alloc_zeroed"), |this| {
 458                     Self::check_alloc_request(size, align)?;
 459
 460                     let ptr = this.allocate_ptr(
 461                         Size::from_bytes(size),
 462                         Align::from_bytes(align).unwrap(),
 463                         MiriMemoryKind::Rust.into(),
 464                     )?;
 465
 466                     // We just allocated this, the access is definitely in-bounds.
 467                     this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 468                     this.write_pointer(ptr, dest)
 469                 });
 470             }
 471             "__rust_dealloc" => {
 472                 let [ptr, old_size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 473                 let ptr = this.read_pointer(ptr)?;
 474                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 475                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 476
 477                 return this.emulate_allocator(Symbol::intern("__rg_dealloc"), |this| {
 478                     // No need to check old_size/align; we anyway check that they match the allocation.
 479                     this.deallocate_ptr(
 480                         ptr,
 481                         Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 482                         MiriMemoryKind::Rust.into(),
 483                     )
 484                 });
 485             }
 486             "__rust_realloc" => {
 487                 let [ptr, old_size, align, new_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 488                 let ptr = this.read_pointer(ptr)?;
 489                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 490                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 491                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 492                 // No need to check old_size; we anyway check that they match the allocation.
 493
 494                 return this.emulate_allocator(Symbol::intern("__rg_realloc"), |this| {
 495                     Self::check_alloc_request(new_size, align)?;
 496
 497                     let align = Align::from_bytes(align).unwrap();
 498                     let new_ptr = this.reallocate_ptr(
 499                         ptr,
 500                         Some((Size::from_bytes(old_size), align)),
 501                         Size::from_bytes(new_size),
 502                         align,
 503                         MiriMemoryKind::Rust.into(),
 504                     )?;
 505                     this.write_pointer(new_ptr, dest)
 506                 });
 507             }
 508
 509             // C memory handling functions
 510             "memcmp" => {
 511                 let [left, right, n] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 512                 let left = this.read_pointer(left)?;
 513                 let right = this.read_pointer(right)?;
 514                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 515
 516                 let result = {
 517                     let left_bytes = this.read_bytes_ptr(left, n)?;
 518                     let right_bytes = this.read_bytes_ptr(right, n)?;
 519
 520                     use std::cmp::Ordering::*;
 521                     match left_bytes.cmp(right_bytes) {
 522                         Less => -1i32,
 523                         Equal => 0,
 524                         Greater => 1,
 525                     }
 526                 };
 527
 528                 this.write_scalar(Scalar::from_i32(result), dest)?;
 529             }
 530             "memrchr" => {
 531                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 532                 let ptr = this.read_pointer(ptr)?;
 533                 let val = this.read_scalar(val)?.to_i32()?;
 534                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 535                 // The docs say val is "interpreted as unsigned char".
 536                 #[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
 537                 let val = val as u8;
 538
 539                 if let Some(idx) = this
 540                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 541                     .iter()
 542                     .rev()
 543                     .position(|&c| c == val)
 544                 {
 545                     let new_ptr = ptr.offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 546                     this.write_pointer(new_ptr, dest)?;
 547                 } else {
 548                     this.write_null(dest)?;
 549                 }
 550             }
 551             "memchr" => {
 552                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 553                 let ptr = this.read_pointer(ptr)?;
 554                 let val = this.read_scalar(val)?.to_i32()?;
 555                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 556                 // The docs say val is "interpreted as unsigned char".
 557                 #[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
 558                 let val = val as u8;
 559
 560                 let idx = this
 561                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 562                     .iter()
 563                     .position(|&c| c == val);
 564                 if let Some(idx) = idx {
 565                     let new_ptr = ptr.offset(Size::from_bytes(idx as u64), this)?;
 566                     this.write_pointer(new_ptr, dest)?;
 567                 } else {
 568                     this.write_null(dest)?;
 569                 }
 570             }
 571             "strlen" => {
 572                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 573                 let ptr = this.read_pointer(ptr)?;
 574                 let n = this.read_c_str(ptr)?.len();
 575                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 576             }
 577
 578             // math functions
 579             #[rustfmt::skip]
 580             | "cbrtf"
 581             | "coshf"
 582             | "sinhf"
 583             | "tanf"
 584             | "acosf"
 585             | "asinf"
 586             | "atanf"
 587             => {
 588                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 589                 // FIXME: Using host floats.
 590                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 591                 let f = match link_name.as_str() {
 592                     "cbrtf" => f.cbrt(),
 593                     "coshf" => f.cosh(),
 594                     "sinhf" => f.sinh(),
 595                     "tanf" => f.tan(),
 596                     "acosf" => f.acos(),
 597                     "asinf" => f.asin(),
 598                     "atanf" => f.atan(),
 599                     _ => bug!(),
 600                 };
 601                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 602             }
 603             #[rustfmt::skip]
 604             | "_hypotf"
 605             | "hypotf"
 606             | "atan2f"
 607             => {
 608                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 609                 // underscore case for windows, here and below
 610                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 611                 // FIXME: Using host floats.
 612                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 613                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 614                 let n = match link_name.as_str() {
 615                     "_hypotf" | "hypotf" => f1.hypot(f2),
 616                     "atan2f" => f1.atan2(f2),
 617                     _ => bug!(),
 618                 };
 619                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 620             }
 621             #[rustfmt::skip]
 622             | "cbrt"
 623             | "cosh"
 624             | "sinh"
 625             | "tan"
 626             | "acos"
 627             | "asin"
 628             | "atan"
 629             => {
 630                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 631                 // FIXME: Using host floats.
 632                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 633                 let f = match link_name.as_str() {
 634                     "cbrt" => f.cbrt(),
 635                     "cosh" => f.cosh(),
 636                     "sinh" => f.sinh(),
 637                     "tan" => f.tan(),
 638                     "acos" => f.acos(),
 639                     "asin" => f.asin(),
 640                     "atan" => f.atan(),
 641                     _ => bug!(),
 642                 };
 643                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 644             }
 645             #[rustfmt::skip]
 646             | "_hypot"
 647             | "hypot"
 648             | "atan2"
 649             => {
 650                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 651                 // FIXME: Using host floats.
 652                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 653                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 654                 let n = match link_name.as_str() {
 655                     "_hypot" | "hypot" => f1.hypot(f2),
 656                     "atan2" => f1.atan2(f2),
 657                     _ => bug!(),
 658                 };
 659                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 660             }
 661             #[rustfmt::skip]
 662             | "_ldexp"
 663             | "ldexp"
 664             | "scalbn"
 665             => {
 666                 let [x, exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 667                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 668                 let x = this.read_scalar(x)?.to_f64()?;
 669                 let exp = this.read_scalar(exp)?.to_i32()?;
 670
 671                 // Saturating cast to i16. Even those are outside the valid exponent range to
 672                 // `scalbn` below will do its over/underflow handling.
 673                 let exp = if exp > i32::from(i16::MAX) {
 674                     i16::MAX
 675                 } else if exp < i32::from(i16::MIN) {
 676                     i16::MIN
 677                 } else {
 678                     exp.try_into().unwrap()
 679                 };
 680
 681                 let res = x.scalbn(exp);
 682                 this.write_scalar(Scalar::from_f64(res), dest)?;
 683             }
 684
 685             // Architecture-specific shims
 686             "llvm.x86.addcarry.64" if this.tcx.sess.target.arch == "x86_64" => {
 687                 // Computes u8+u64+u64, returning tuple (u8,u64) comprising the output carry and truncated sum.
 688                 let [c_in, a, b] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 689                 let c_in = this.read_scalar(c_in)?.to_u8()?;
 690                 let a = this.read_scalar(a)?.to_u64()?;
 691                 let b = this.read_scalar(b)?.to_u64()?;
 692
 693                 let wide_sum = u128::from(c_in) + u128::from(a) + u128::from(b);
 694                 let (c_out, sum) = ((wide_sum >> 64).truncate::<u8>(), wide_sum.truncate::<u64>());
 695
 696                 let c_out_field = this.place_field(dest, 0)?;
 697                 this.write_scalar(Scalar::from_u8(c_out), &c_out_field)?;
 698                 let sum_field = this.place_field(dest, 1)?;
 699                 this.write_scalar(Scalar::from_u64(sum), &sum_field)?;
 700             }
 701             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 702                 let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 703                 this.yield_active_thread();
 704             }
 705             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 706                 let [arg] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 707                 let arg = this.read_scalar(arg)?.to_i32()?;
 708                 match arg {
 709                     15 => { // SY ("full system scope")
 710                         this.yield_active_thread();
 711                     }
 712                     _ => {
 713                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 714                     }
 715                 }
 716             }
 717
 718             // Platform-specific shims
 719             _ => match this.tcx.sess.target.os.as_ref() {
 720                 target if target_os_is_unix(target) => return shims::unix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest),
 721                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest),
 722                 target => throw_unsup_format!("the target `{}` is not supported", target),
 723             }
 724         };
 725
 726         // We only fall through to here if we did *not* hit the `_` arm above,
 727         // i.e., if we actually emulated the function.
 728         Ok(EmulateByNameResult::NeedsJumping)
 729     }
 730
 731     /// Check some basic requirements for this allocation request:
 732     /// non-zero size, power-of-two alignment.
 733     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 734         if size == 0 {
 735             throw_ub_format!("creating allocation with size 0");
 736         }
 737         if !align.is_power_of_two() {
 738             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 739         }
 740         Ok(())
 741     }
 742 }