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::sym, 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;
  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<Tag>>> {
  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.
  86                 this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(size as usize)).unwrap();
  87             }
  88             Ok(ptr.into())
  89         }
  90     }
  91
  92     fn free(&mut self, ptr: Pointer<Option<Tag>>, kind: MiriMemoryKind) -> InterpResult<'tcx> {
  93         let this = self.eval_context_mut();
  94         if !this.ptr_is_null(ptr)? {
  95             this.deallocate_ptr(ptr, None, kind.into())?;
  96         }
  97         Ok(())
  98     }
  99
 100     fn realloc(
 101         &mut self,
 102         old_ptr: Pointer<Option<Tag>>,
 103         new_size: u64,
 104         kind: MiriMemoryKind,
 105     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
 106         let this = self.eval_context_mut();
 107         let new_align = this.min_align(new_size, kind);
 108         if this.ptr_is_null(old_ptr)? {
 109             if new_size == 0 {
 110                 Ok(Pointer::null())
 111             } else {
 112                 let new_ptr =
 113                     this.allocate_ptr(Size::from_bytes(new_size), new_align, kind.into())?;
 114                 Ok(new_ptr.into())
 115             }
 116         } else {
 117             if new_size == 0 {
 118                 this.deallocate_ptr(old_ptr, None, kind.into())?;
 119                 Ok(Pointer::null())
 120             } else {
 121                 let new_ptr = this.reallocate_ptr(
 122                     old_ptr,
 123                     None,
 124                     Size::from_bytes(new_size),
 125                     new_align,
 126                     kind.into(),
 127                 )?;
 128                 Ok(new_ptr.into())
 129             }
 130         }
 131     }
 132
 133     /// Lookup the body of a function that has `link_name` as the symbol name.
 134     fn lookup_exported_symbol(
 135         &mut self,
 136         link_name: Symbol,
 137     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 138         let this = self.eval_context_mut();
 139         let tcx = this.tcx.tcx;
 140
 141         // If the result was cached, just return it.
 142         // (Cannot use `or_insert` since the code below might have to throw an error.)
 143         let entry = this.machine.exported_symbols_cache.entry(link_name);
 144         let instance = *match entry {
 145             Entry::Occupied(e) => e.into_mut(),
 146             Entry::Vacant(e) => {
 147                 // Find it if it was not cached.
 148                 let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum)> = None;
 149                 // `dependency_formats` includes all the transitive informations needed to link a crate,
 150                 // which is what we need here since we need to dig out `exported_symbols` from all transitive
 151                 // dependencies.
 152                 let dependency_formats = tcx.dependency_formats(());
 153                 let dependency_format = dependency_formats
 154                     .iter()
 155                     .find(|(crate_type, _)| *crate_type == CrateType::Executable)
 156                     .expect("interpreting a non-executable crate");
 157                 for cnum in iter::once(LOCAL_CRATE).chain(
 158                     dependency_format.1.iter().enumerate().filter_map(|(num, &linkage)| {
 159                         (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1))
 160                     }),
 161                 ) {
 162                     // We can ignore `_export_info` here: we are a Rust crate, and everything is exported
 163                     // from a Rust crate.
 164                     for &(symbol, _export_info) in tcx.exported_symbols(cnum) {
 165                         if let ExportedSymbol::NonGeneric(def_id) = symbol {
 166                             let attrs = tcx.codegen_fn_attrs(def_id);
 167                             let symbol_name = if let Some(export_name) = attrs.export_name {
 168                                 export_name
 169                             } else if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
 170                                 tcx.item_name(def_id)
 171                             } else {
 172                                 // Skip over items without an explicitly defined symbol name.
 173                                 continue;
 174                             };
 175                             if symbol_name == link_name {
 176                                 if let Some((original_instance, original_cnum)) = instance_and_crate
 177                                 {
 178                                     // Make sure we are consistent wrt what is 'first' and 'second'.
 179                                     let original_span =
 180                                         tcx.def_span(original_instance.def_id()).data();
 181                                     let span = tcx.def_span(def_id).data();
 182                                     if original_span < span {
 183                                         throw_machine_stop!(
 184                                             TerminationInfo::MultipleSymbolDefinitions {
 185                                                 link_name,
 186                                                 first: original_span,
 187                                                 first_crate: tcx.crate_name(original_cnum),
 188                                                 second: span,
 189                                                 second_crate: tcx.crate_name(cnum),
 190                                             }
 191                                         );
 192                                     } else {
 193                                         throw_machine_stop!(
 194                                             TerminationInfo::MultipleSymbolDefinitions {
 195                                                 link_name,
 196                                                 first: span,
 197                                                 first_crate: tcx.crate_name(cnum),
 198                                                 second: original_span,
 199                                                 second_crate: tcx.crate_name(original_cnum),
 200                                             }
 201                                         );
 202                                     }
 203                                 }
 204                                 if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 205                                     throw_ub_format!(
 206                                         "attempt to call an exported symbol that is not defined as a function"
 207                                     );
 208                                 }
 209                                 instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 210                             }
 211                         }
 212                     }
 213                 }
 214
 215                 e.insert(instance_and_crate.map(|ic| ic.0))
 216             }
 217         };
 218         match instance {
 219             None => Ok(None), // no symbol with this name
 220             Some(instance) => Ok(Some((this.load_mir(instance.def, None)?, instance))),
 221         }
 222     }
 223
 224     /// Emulates calling a foreign item, failing if the item is not supported.
 225     /// This function will handle `goto_block` if needed.
 226     /// Returns Ok(None) if the foreign item was completely handled
 227     /// by this function.
 228     /// Returns Ok(Some(body)) if processing the foreign item
 229     /// is delegated to another function.
 230     fn emulate_foreign_item(
 231         &mut self,
 232         def_id: DefId,
 233         abi: Abi,
 234         args: &[OpTy<'tcx, Tag>],
 235         ret: Option<(&PlaceTy<'tcx, Tag>, mir::BasicBlock)>,
 236         unwind: StackPopUnwind,
 237     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 238         let this = self.eval_context_mut();
 239         let attrs = this.tcx.get_attrs(def_id);
 240         let link_name = this
 241             .tcx
 242             .sess
 243             .first_attr_value_str_by_name(attrs, sym::link_name)
 244             .unwrap_or_else(|| this.tcx.item_name(def_id));
 245         let tcx = this.tcx.tcx;
 246
 247         // First: functions that diverge.
 248         let (dest, ret) = match ret {
 249             None =>
 250                 match &*link_name.as_str() {
 251                     "miri_start_panic" => {
 252                         // `check_shim` happens inside `handle_miri_start_panic`.
 253                         this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 254                         return Ok(None);
 255                     }
 256                     // This matches calls to the foreign item `panic_impl`.
 257                     // The implementation is provided by the function with the `#[panic_handler]` attribute.
 258                     "panic_impl" => {
 259                         // We don't use `check_shim` here because we are just forwarding to the lang
 260                         // item. Argument count checking will be performed when the returned `Body` is
 261                         // called.
 262                         this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 263                         let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 264                         let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 265                         return Ok(Some((
 266                             &*this.load_mir(panic_impl_instance.def, None)?,
 267                             panic_impl_instance,
 268                         )));
 269                     }
 270                     #[rustfmt::skip]
 271                     | "exit"
 272                     | "ExitProcess"
 273                     => {
 274                         let exp_abi = if link_name.as_str() == "exit" {
 275                             Abi::C { unwind: false }
 276                         } else {
 277                             Abi::System { unwind: false }
 278                         };
 279                         let [code] = this.check_shim(abi, exp_abi, link_name, args)?;
 280                         // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 281                         let code = this.read_scalar(code)?.to_i32()?;
 282                         throw_machine_stop!(TerminationInfo::Exit(code.into()));
 283                     }
 284                     "abort" => {
 285                         let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 286                         throw_machine_stop!(TerminationInfo::Abort(
 287                             "the program aborted execution".to_owned()
 288                         ))
 289                     }
 290                     _ => {
 291                         if let Some(body) = this.lookup_exported_symbol(link_name)? {
 292                             return Ok(Some(body));
 293                         }
 294                         this.handle_unsupported(format!(
 295                             "can't call (diverging) foreign function: {}",
 296                             link_name
 297                         ))?;
 298                         return Ok(None);
 299                     }
 300                 },
 301             Some(p) => p,
 302         };
 303
 304         // Second: functions that return.
 305         match this.emulate_foreign_item_by_name(link_name, abi, args, dest, ret)? {
 306             EmulateByNameResult::NeedsJumping => {
 307                 trace!("{:?}", this.dump_place(**dest));
 308                 this.go_to_block(ret);
 309             }
 310             EmulateByNameResult::AlreadyJumped => (),
 311             EmulateByNameResult::MirBody(mir, instance) => return Ok(Some((mir, instance))),
 312             EmulateByNameResult::NotSupported => {
 313                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 314                     return Ok(Some(body));
 315                 }
 316
 317                 this.handle_unsupported(format!("can't call foreign function: {}", link_name))?;
 318                 return Ok(None);
 319             }
 320         }
 321
 322         Ok(None)
 323     }
 324
 325     /// Emulates calling the internal __rust_* allocator functions
 326     fn emulate_allocator(
 327         &mut self,
 328         symbol: Symbol,
 329         default: impl FnOnce(&mut MiriEvalContext<'mir, 'tcx>) -> InterpResult<'tcx>,
 330     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 331         let this = self.eval_context_mut();
 332
 333         let allocator_kind = if let Some(allocator_kind) = this.tcx.allocator_kind(()) {
 334             allocator_kind
 335         } else {
 336             // in real code, this symbol does not exist without an allocator
 337             return Ok(EmulateByNameResult::NotSupported);
 338         };
 339
 340         match allocator_kind {
 341             AllocatorKind::Global => {
 342                 let (body, instance) = this
 343                     .lookup_exported_symbol(symbol)?
 344                     .expect("symbol should be present if there is a global allocator");
 345
 346                 Ok(EmulateByNameResult::MirBody(body, instance))
 347             }
 348             AllocatorKind::Default => {
 349                 default(this)?;
 350                 Ok(EmulateByNameResult::NeedsJumping)
 351             }
 352         }
 353     }
 354
 355     /// Emulates calling a foreign item using its name.
 356     fn emulate_foreign_item_by_name(
 357         &mut self,
 358         link_name: Symbol,
 359         abi: Abi,
 360         args: &[OpTy<'tcx, Tag>],
 361         dest: &PlaceTy<'tcx, Tag>,
 362         ret: mir::BasicBlock,
 363     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 364         let this = self.eval_context_mut();
 365
 366         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 367         // shim, add it to the corresponding submodule.
 368         match &*link_name.as_str() {
 369             // Miri-specific extern functions
 370             "miri_static_root" => {
 371                 let [ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 372                 let ptr = this.read_pointer(ptr)?;
 373                 let (alloc_id, offset, _) = this.ptr_get_alloc_id(ptr)?;
 374                 if offset != Size::ZERO {
 375                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 376                 }
 377                 this.machine.static_roots.push(alloc_id);
 378             }
 379
 380             // Obtains the size of a Miri backtrace. See the README for details.
 381             "miri_backtrace_size" => {
 382                 this.handle_miri_backtrace_size(abi, link_name, args, dest)?;
 383             }
 384
 385             // Obtains a Miri backtrace. See the README for details.
 386             "miri_get_backtrace" => {
 387                 // `check_shim` happens inside `handle_miri_get_backtrace`.
 388                 this.handle_miri_get_backtrace(abi, link_name, args, dest)?;
 389             }
 390
 391             // Resolves a Miri backtrace frame. See the README for details.
 392             "miri_resolve_frame" => {
 393                 // `check_shim` happens inside `handle_miri_resolve_frame`.
 394                 this.handle_miri_resolve_frame(abi, link_name, args, dest)?;
 395             }
 396
 397             // Writes the function and file names of a Miri backtrace frame into a user provided buffer. See the README for details.
 398             "miri_resolve_frame_names" => {
 399                 this.handle_miri_resolve_frame_names(abi, link_name, args)?;
 400             }
 401
 402             // Standard C allocation
 403             "malloc" => {
 404                 let [size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 405                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 406                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C)?;
 407                 this.write_pointer(res, dest)?;
 408             }
 409             "calloc" => {
 410                 let [items, len] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 411                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 412                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 413                 let size =
 414                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 415                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C)?;
 416                 this.write_pointer(res, dest)?;
 417             }
 418             "free" => {
 419                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 420                 let ptr = this.read_pointer(ptr)?;
 421                 this.free(ptr, MiriMemoryKind::C)?;
 422             }
 423             "realloc" => {
 424                 let [old_ptr, new_size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 425                 let old_ptr = this.read_pointer(old_ptr)?;
 426                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 427                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 428                 this.write_pointer(res, dest)?;
 429             }
 430
 431             // Rust allocation
 432             "__rust_alloc" => {
 433                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 434                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 435                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 436
 437                 return this.emulate_allocator(Symbol::intern("__rg_alloc"), |this| {
 438                     Self::check_alloc_request(size, align)?;
 439
 440                     let ptr = this.allocate_ptr(
 441                         Size::from_bytes(size),
 442                         Align::from_bytes(align).unwrap(),
 443                         MiriMemoryKind::Rust.into(),
 444                     )?;
 445
 446                     this.write_pointer(ptr, dest)
 447                 });
 448             }
 449             "__rust_alloc_zeroed" => {
 450                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 451                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 452                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 453
 454                 return this.emulate_allocator(Symbol::intern("__rg_alloc_zeroed"), |this| {
 455                     Self::check_alloc_request(size, align)?;
 456
 457                     let ptr = this.allocate_ptr(
 458                         Size::from_bytes(size),
 459                         Align::from_bytes(align).unwrap(),
 460                         MiriMemoryKind::Rust.into(),
 461                     )?;
 462
 463                     // We just allocated this, the access is definitely in-bounds.
 464                     this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 465                     this.write_pointer(ptr, dest)
 466                 });
 467             }
 468             "__rust_dealloc" => {
 469                 let [ptr, old_size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 470                 let ptr = this.read_pointer(ptr)?;
 471                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 472                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 473
 474                 return this.emulate_allocator(Symbol::intern("__rg_dealloc"), |this| {
 475                     // No need to check old_size/align; we anyway check that they match the allocation.
 476                     this.deallocate_ptr(
 477                         ptr,
 478                         Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 479                         MiriMemoryKind::Rust.into(),
 480                     )
 481                 });
 482             }
 483             "__rust_realloc" => {
 484                 let [ptr, old_size, align, new_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 485                 let ptr = this.read_pointer(ptr)?;
 486                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 487                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 488                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 489                 // No need to check old_size; we anyway check that they match the allocation.
 490
 491                 return this.emulate_allocator(Symbol::intern("__rg_realloc"), |this| {
 492                     Self::check_alloc_request(new_size, align)?;
 493
 494                     let align = Align::from_bytes(align).unwrap();
 495                     let new_ptr = this.reallocate_ptr(
 496                         ptr,
 497                         Some((Size::from_bytes(old_size), align)),
 498                         Size::from_bytes(new_size),
 499                         align,
 500                         MiriMemoryKind::Rust.into(),
 501                     )?;
 502                     this.write_pointer(new_ptr, dest)
 503                 });
 504             }
 505
 506             // C memory handling functions
 507             "memcmp" => {
 508                 let [left, right, n] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 509                 let left = this.read_pointer(left)?;
 510                 let right = this.read_pointer(right)?;
 511                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 512
 513                 let result = {
 514                     let left_bytes = this.read_bytes_ptr(left, n)?;
 515                     let right_bytes = this.read_bytes_ptr(right, n)?;
 516
 517                     use std::cmp::Ordering::*;
 518                     match left_bytes.cmp(right_bytes) {
 519                         Less => -1i32,
 520                         Equal => 0,
 521                         Greater => 1,
 522                     }
 523                 };
 524
 525                 this.write_scalar(Scalar::from_i32(result), dest)?;
 526             }
 527             "memrchr" => {
 528                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 529                 let ptr = this.read_pointer(ptr)?;
 530                 let val = this.read_scalar(val)?.to_i32()? as u8;
 531                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 532                 if let Some(idx) = this
 533                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 534                     .iter()
 535                     .rev()
 536                     .position(|&c| c == val)
 537                 {
 538                     let new_ptr = ptr.offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 539                     this.write_pointer(new_ptr, dest)?;
 540                 } else {
 541                     this.write_null(dest)?;
 542                 }
 543             }
 544             "memchr" => {
 545                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 546                 let ptr = this.read_pointer(ptr)?;
 547                 let val = this.read_scalar(val)?.to_i32()? as u8;
 548                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 549                 let idx = this
 550                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 551                     .iter()
 552                     .position(|&c| c == val);
 553                 if let Some(idx) = idx {
 554                     let new_ptr = ptr.offset(Size::from_bytes(idx as u64), this)?;
 555                     this.write_pointer(new_ptr, dest)?;
 556                 } else {
 557                     this.write_null(dest)?;
 558                 }
 559             }
 560             "strlen" => {
 561                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 562                 let ptr = this.read_pointer(ptr)?;
 563                 let n = this.read_c_str(ptr)?.len();
 564                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 565             }
 566
 567             // math functions
 568             #[rustfmt::skip]
 569             | "cbrtf"
 570             | "coshf"
 571             | "sinhf"
 572             | "tanf"
 573             | "acosf"
 574             | "asinf"
 575             | "atanf"
 576             => {
 577                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 578                 // FIXME: Using host floats.
 579                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 580                 let f = match &*link_name.as_str() {
 581                     "cbrtf" => f.cbrt(),
 582                     "coshf" => f.cosh(),
 583                     "sinhf" => f.sinh(),
 584                     "tanf" => f.tan(),
 585                     "acosf" => f.acos(),
 586                     "asinf" => f.asin(),
 587                     "atanf" => f.atan(),
 588                     _ => bug!(),
 589                 };
 590                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 591             }
 592             #[rustfmt::skip]
 593             | "_hypotf"
 594             | "hypotf"
 595             | "atan2f"
 596             => {
 597                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 598                 // underscore case for windows, here and below
 599                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 600                 // FIXME: Using host floats.
 601                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 602                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 603                 let n = match &*link_name.as_str() {
 604                     "_hypotf" | "hypotf" => f1.hypot(f2),
 605                     "atan2f" => f1.atan2(f2),
 606                     _ => bug!(),
 607                 };
 608                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 609             }
 610             #[rustfmt::skip]
 611             | "cbrt"
 612             | "cosh"
 613             | "sinh"
 614             | "tan"
 615             | "acos"
 616             | "asin"
 617             | "atan"
 618             => {
 619                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 620                 // FIXME: Using host floats.
 621                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 622                 let f = match &*link_name.as_str() {
 623                     "cbrt" => f.cbrt(),
 624                     "cosh" => f.cosh(),
 625                     "sinh" => f.sinh(),
 626                     "tan" => f.tan(),
 627                     "acos" => f.acos(),
 628                     "asin" => f.asin(),
 629                     "atan" => f.atan(),
 630                     _ => bug!(),
 631                 };
 632                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 633             }
 634             #[rustfmt::skip]
 635             | "_hypot"
 636             | "hypot"
 637             | "atan2"
 638             => {
 639                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 640                 // FIXME: Using host floats.
 641                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 642                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 643                 let n = match &*link_name.as_str() {
 644                     "_hypot" | "hypot" => f1.hypot(f2),
 645                     "atan2" => f1.atan2(f2),
 646                     _ => bug!(),
 647                 };
 648                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 649             }
 650             #[rustfmt::skip]
 651             | "_ldexp"
 652             | "ldexp"
 653             | "scalbn"
 654             => {
 655                 let [x, exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 656                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 657                 let x = this.read_scalar(x)?.to_f64()?;
 658                 let exp = this.read_scalar(exp)?.to_i32()?;
 659
 660                 // Saturating cast to i16. Even those are outside the valid exponent range to
 661                 // `scalbn` below will do its over/underflow handling.
 662                 let exp = if exp > i32::from(i16::MAX) {
 663                     i16::MAX
 664                 } else if exp < i32::from(i16::MIN) {
 665                     i16::MIN
 666                 } else {
 667                     exp.try_into().unwrap()
 668                 };
 669
 670                 let res = x.scalbn(exp);
 671                 this.write_scalar(Scalar::from_f64(res), dest)?;
 672             }
 673
 674             // Architecture-specific shims
 675             "llvm.x86.addcarry.64" if this.tcx.sess.target.arch == "x86_64" => {
 676                 // Computes u8+u64+u64, returning tuple (u8,u64) comprising the output carry and truncated sum.
 677                 let [c_in, a, b] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 678                 let c_in = this.read_scalar(c_in)?.to_u8()?;
 679                 let a = this.read_scalar(a)?.to_u64()?;
 680                 let b = this.read_scalar(b)?.to_u64()?;
 681
 682                 let wide_sum = u128::from(c_in) + u128::from(a) + u128::from(b);
 683                 let (c_out, sum) = ((wide_sum >> 64).truncate::<u8>(), wide_sum.truncate::<u64>());
 684
 685                 let c_out_field = this.place_field(dest, 0)?;
 686                 this.write_scalar(Scalar::from_u8(c_out), &c_out_field)?;
 687                 let sum_field = this.place_field(dest, 1)?;
 688                 this.write_scalar(Scalar::from_u64(sum), &sum_field)?;
 689             }
 690             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 691                 let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 692                 this.yield_active_thread();
 693             }
 694             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 695                 let [arg] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 696                 let arg = this.read_scalar(arg)?.to_i32()?;
 697                 match arg {
 698                     15 => { // SY ("full system scope")
 699                         this.yield_active_thread();
 700                     }
 701                     _ => {
 702                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 703                     }
 704                 }
 705             }
 706
 707             // Platform-specific shims
 708             _ => match this.tcx.sess.target.os.as_ref() {
 709                 "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 710                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 711                 target => throw_unsup_format!("the target `{}` is not supported", target),
 712             }
 713         };
 714
 715         // We only fall through to here if we did *not* hit the `_` arm above,
 716         // i.e., if we actually emulated the function.
 717         Ok(EmulateByNameResult::NeedsJumping)
 718     }
 719
 720     /// Check some basic requirements for this allocation request:
 721     /// non-zero size, power-of-two alignment.
 722     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 723         if size == 0 {
 724             throw_ub_format!("creating allocation with size 0");
 725         }
 726         if !align.is_power_of_two() {
 727             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 728         }
 729         Ok(())
 730     }
 731 }