src/shims/foreign_items.rs

   1 use std::{
   2     collections::hash_map::Entry,
   3     convert::{TryFrom, TryInto},
   4     iter,
   5 };
   6
   7 use log::trace;
   8
   9 use rustc_apfloat::Float;
  10 use rustc_ast::expand::allocator::AllocatorKind;
  11 use rustc_hir::{
  12     def::DefKind,
  13     def_id::{CrateNum, DefId, LOCAL_CRATE},
  14 };
  15 use rustc_middle::middle::{
  16     codegen_fn_attrs::CodegenFnAttrFlags, dependency_format::Linkage,
  17     exported_symbols::ExportedSymbol,
  18 };
  19 use rustc_middle::mir;
  20 use rustc_middle::ty;
  21 use rustc_session::config::CrateType;
  22 use rustc_span::{symbol::sym, Symbol};
  23 use rustc_target::{
  24     abi::{Align, Size},
  25     spec::abi::Abi,
  26 };
  27
  28 use super::backtrace::EvalContextExt as _;
  29 use crate::*;
  30
  31 /// Returned by `emulate_foreign_item_by_name`.
  32 pub enum EmulateByNameResult<'mir, 'tcx> {
  33     /// The caller is expected to jump to the return block.
  34     NeedsJumping,
  35     /// Jumping has already been taken care of.
  36     AlreadyJumped,
  37     /// A MIR body has been found for the function
  38     MirBody(&'mir mir::Body<'tcx>, ty::Instance<'tcx>),
  39     /// The item is not supported.
  40     NotSupported,
  41 }
  42
  43 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  44 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  45     /// Returns the minimum alignment for the target architecture for allocations of the given size.
  46     fn min_align(&self, size: u64, kind: MiriMemoryKind) -> Align {
  47         let this = self.eval_context_ref();
  48         // List taken from `libstd/sys_common/alloc.rs`.
  49         let min_align = match this.tcx.sess.target.arch.as_ref() {
  50             "x86" | "arm" | "mips" | "powerpc" | "powerpc64" | "asmjs" | "wasm32" => 8,
  51             "x86_64" | "aarch64" | "mips64" | "s390x" | "sparc64" => 16,
  52             arch => bug!("Unsupported target architecture: {}", arch),
  53         };
  54         // Windows always aligns, even small allocations.
  55         // Source: <https://support.microsoft.com/en-us/help/286470/how-to-use-pageheap-exe-in-windows-xp-windows-2000-and-windows-server>
  56         // But jemalloc does not, so for the C heap we only align if the allocation is sufficiently big.
  57         if kind == MiriMemoryKind::WinHeap || size >= min_align {
  58             return Align::from_bytes(min_align).unwrap();
  59         }
  60         // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
  61         fn prev_power_of_two(x: u64) -> u64 {
  62             let next_pow2 = x.next_power_of_two();
  63             if next_pow2 == x {
  64                 // x *is* a power of two, just use that.
  65                 x
  66             } else {
  67                 // x is between two powers, so next = 2*prev.
  68                 next_pow2 / 2
  69             }
  70         }
  71         Align::from_bytes(prev_power_of_two(size)).unwrap()
  72     }
  73
  74     fn malloc(
  75         &mut self,
  76         size: u64,
  77         zero_init: bool,
  78         kind: MiriMemoryKind,
  79     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
  80         let this = self.eval_context_mut();
  81         if size == 0 {
  82             Ok(Pointer::null())
  83         } else {
  84             let align = this.min_align(size, kind);
  85             let ptr = this.allocate_ptr(Size::from_bytes(size), align, kind.into())?;
  86             if zero_init {
  87                 // We just allocated this, the access is definitely in-bounds.
  88                 this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(size as usize)).unwrap();
  89             }
  90             Ok(ptr.into())
  91         }
  92     }
  93
  94     fn free(&mut self, ptr: Pointer<Option<Tag>>, kind: MiriMemoryKind) -> InterpResult<'tcx> {
  95         let this = self.eval_context_mut();
  96         if !this.ptr_is_null(ptr)? {
  97             this.deallocate_ptr(ptr, None, kind.into())?;
  98         }
  99         Ok(())
 100     }
 101
 102     fn realloc(
 103         &mut self,
 104         old_ptr: Pointer<Option<Tag>>,
 105         new_size: u64,
 106         kind: MiriMemoryKind,
 107     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
 108         let this = self.eval_context_mut();
 109         let new_align = this.min_align(new_size, kind);
 110         if this.ptr_is_null(old_ptr)? {
 111             if new_size == 0 {
 112                 Ok(Pointer::null())
 113             } else {
 114                 let new_ptr =
 115                     this.allocate_ptr(Size::from_bytes(new_size), new_align, kind.into())?;
 116                 Ok(new_ptr.into())
 117             }
 118         } else {
 119             if new_size == 0 {
 120                 this.deallocate_ptr(old_ptr, None, kind.into())?;
 121                 Ok(Pointer::null())
 122             } else {
 123                 let new_ptr = this.reallocate_ptr(
 124                     old_ptr,
 125                     None,
 126                     Size::from_bytes(new_size),
 127                     new_align,
 128                     kind.into(),
 129                 )?;
 130                 Ok(new_ptr.into())
 131             }
 132         }
 133     }
 134
 135     /// Lookup the body of a function that has `link_name` as the symbol name.
 136     fn lookup_exported_symbol(
 137         &mut self,
 138         link_name: Symbol,
 139     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 140         let this = self.eval_context_mut();
 141         let tcx = this.tcx.tcx;
 142
 143         // If the result was cached, just return it.
 144         // (Cannot use `or_insert` since the code below might have to throw an error.)
 145         let entry = this.machine.exported_symbols_cache.entry(link_name);
 146         let instance = *match entry {
 147             Entry::Occupied(e) => e.into_mut(),
 148             Entry::Vacant(e) => {
 149                 // Find it if it was not cached.
 150                 let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum)> = None;
 151                 // `dependency_formats` includes all the transitive informations needed to link a crate,
 152                 // which is what we need here since we need to dig out `exported_symbols` from all transitive
 153                 // dependencies.
 154                 let dependency_formats = tcx.dependency_formats(());
 155                 let dependency_format = dependency_formats
 156                     .iter()
 157                     .find(|(crate_type, _)| *crate_type == CrateType::Executable)
 158                     .expect("interpreting a non-executable crate");
 159                 for cnum in iter::once(LOCAL_CRATE).chain(
 160                     dependency_format.1.iter().enumerate().filter_map(|(num, &linkage)| {
 161                         (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1))
 162                     }),
 163                 ) {
 164                     // We can ignore `_export_level` here: we are a Rust crate, and everything is exported
 165                     // from a Rust crate.
 166                     for &(symbol, _export_level) in tcx.exported_symbols(cnum) {
 167                         if let ExportedSymbol::NonGeneric(def_id) = symbol {
 168                             let attrs = tcx.codegen_fn_attrs(def_id);
 169                             let symbol_name = if let Some(export_name) = attrs.export_name {
 170                                 export_name
 171                             } else if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
 172                                 tcx.item_name(def_id)
 173                             } else {
 174                                 // Skip over items without an explicitly defined symbol name.
 175                                 continue;
 176                             };
 177                             if symbol_name == link_name {
 178                                 if let Some((original_instance, original_cnum)) = instance_and_crate
 179                                 {
 180                                     // Make sure we are consistent wrt what is 'first' and 'second'.
 181                                     let original_span =
 182                                         tcx.def_span(original_instance.def_id()).data();
 183                                     let span = tcx.def_span(def_id).data();
 184                                     if original_span < span {
 185                                         throw_machine_stop!(
 186                                             TerminationInfo::MultipleSymbolDefinitions {
 187                                                 link_name,
 188                                                 first: original_span,
 189                                                 first_crate: tcx.crate_name(original_cnum),
 190                                                 second: span,
 191                                                 second_crate: tcx.crate_name(cnum),
 192                                             }
 193                                         );
 194                                     } else {
 195                                         throw_machine_stop!(
 196                                             TerminationInfo::MultipleSymbolDefinitions {
 197                                                 link_name,
 198                                                 first: span,
 199                                                 first_crate: tcx.crate_name(cnum),
 200                                                 second: original_span,
 201                                                 second_crate: tcx.crate_name(original_cnum),
 202                                             }
 203                                         );
 204                                     }
 205                                 }
 206                                 if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 207                                     throw_ub_format!(
 208                                         "attempt to call an exported symbol that is not defined as a function"
 209                                     );
 210                                 }
 211                                 instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 212                             }
 213                         }
 214                     }
 215                 }
 216
 217                 e.insert(instance_and_crate.map(|ic| ic.0))
 218             }
 219         };
 220         match instance {
 221             None => Ok(None), // no symbol with this name
 222             Some(instance) => Ok(Some((this.load_mir(instance.def, None)?, instance))),
 223         }
 224     }
 225
 226     /// Emulates calling a foreign item, failing if the item is not supported.
 227     /// This function will handle `goto_block` if needed.
 228     /// Returns Ok(None) if the foreign item was completely handled
 229     /// by this function.
 230     /// Returns Ok(Some(body)) if processing the foreign item
 231     /// is delegated to another function.
 232     fn emulate_foreign_item(
 233         &mut self,
 234         def_id: DefId,
 235         abi: Abi,
 236         args: &[OpTy<'tcx, Tag>],
 237         ret: Option<(&PlaceTy<'tcx, Tag>, mir::BasicBlock)>,
 238         unwind: StackPopUnwind,
 239     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 240         let this = self.eval_context_mut();
 241         let attrs = this.tcx.get_attrs(def_id);
 242         let link_name = this
 243             .tcx
 244             .sess
 245             .first_attr_value_str_by_name(&attrs, sym::link_name)
 246             .unwrap_or_else(|| this.tcx.item_name(def_id));
 247         let tcx = this.tcx.tcx;
 248
 249         // First: functions that diverge.
 250         let (dest, ret) = match ret {
 251             None =>
 252                 match &*link_name.as_str() {
 253                     "miri_start_panic" => {
 254                         // `check_shim` happens inside `handle_miri_start_panic`.
 255                         this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 256                         return Ok(None);
 257                     }
 258                     // This matches calls to the foreign item `panic_impl`.
 259                     // The implementation is provided by the function with the `#[panic_handler]` attribute.
 260                     "panic_impl" => {
 261                         // We don't use `check_shim` here because we are just forwarding to the lang
 262                         // item. Argument count checking will be performed when the returned `Body` is
 263                         // called.
 264                         this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 265                         let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 266                         let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 267                         return Ok(Some((
 268                             &*this.load_mir(panic_impl_instance.def, None)?,
 269                             panic_impl_instance,
 270                         )));
 271                     }
 272                     #[rustfmt::skip]
 273                     | "exit"
 274                     | "ExitProcess"
 275                     => {
 276                         let exp_abi = if link_name.as_str() == "exit" {
 277                             Abi::C { unwind: false }
 278                         } else {
 279                             Abi::System { unwind: false }
 280                         };
 281                         let &[ref code] = this.check_shim(abi, exp_abi, link_name, args)?;
 282                         // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 283                         let code = this.read_scalar(code)?.to_i32()?;
 284                         throw_machine_stop!(TerminationInfo::Exit(code.into()));
 285                     }
 286                     "abort" => {
 287                         let &[] =
 288                             this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 289                         throw_machine_stop!(TerminationInfo::Abort(
 290                             "the program aborted execution".to_owned()
 291                         ))
 292                     }
 293                     _ => {
 294                         if let Some(body) = this.lookup_exported_symbol(link_name)? {
 295                             return Ok(Some(body));
 296                         }
 297                         this.handle_unsupported(format!(
 298                             "can't call (diverging) foreign function: {}",
 299                             link_name
 300                         ))?;
 301                         return Ok(None);
 302                     }
 303                 },
 304             Some(p) => p,
 305         };
 306
 307         // Second: functions that return.
 308         match this.emulate_foreign_item_by_name(link_name, abi, args, dest, ret)? {
 309             EmulateByNameResult::NeedsJumping => {
 310                 trace!("{:?}", this.dump_place(**dest));
 311                 this.go_to_block(ret);
 312             }
 313             EmulateByNameResult::AlreadyJumped => (),
 314             EmulateByNameResult::MirBody(mir, instance) => return Ok(Some((mir, instance))),
 315             EmulateByNameResult::NotSupported => {
 316                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 317                     return Ok(Some(body));
 318                 }
 319
 320                 this.handle_unsupported(format!("can't call foreign function: {}", link_name))?;
 321                 return Ok(None);
 322             }
 323         }
 324
 325         Ok(None)
 326     }
 327
 328     /// Emulates calling the internal __rust_* allocator functions
 329     fn emulate_allocator(
 330         &mut self,
 331         symbol: Symbol,
 332         default: impl FnOnce(&mut MiriEvalContext<'mir, 'tcx>) -> InterpResult<'tcx>,
 333     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 334         let this = self.eval_context_mut();
 335
 336         let allocator_kind = if let Some(allocator_kind) = this.tcx.allocator_kind(()) {
 337             allocator_kind
 338         } else {
 339             // in real code, this symbol does not exist without an allocator
 340             return Ok(EmulateByNameResult::NotSupported);
 341         };
 342
 343         match allocator_kind {
 344             AllocatorKind::Global => {
 345                 let (body, instance) = this
 346                     .lookup_exported_symbol(symbol)?
 347                     .expect("symbol should be present if there is a global allocator");
 348
 349                 Ok(EmulateByNameResult::MirBody(body, instance))
 350             }
 351             AllocatorKind::Default => {
 352                 default(this)?;
 353                 Ok(EmulateByNameResult::NeedsJumping)
 354             }
 355         }
 356     }
 357
 358     /// Emulates calling a foreign item using its name.
 359     fn emulate_foreign_item_by_name(
 360         &mut self,
 361         link_name: Symbol,
 362         abi: Abi,
 363         args: &[OpTy<'tcx, Tag>],
 364         dest: &PlaceTy<'tcx, Tag>,
 365         ret: mir::BasicBlock,
 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 &[ref 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 &[ref 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 &[ref items, ref 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 &[ref 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 &[ref old_ptr, ref 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 &[ref size, ref 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 &[ref size, ref 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 &[ref ptr, ref old_size, ref 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 &[ref ptr, ref old_size, ref align, ref 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 &[ref left, ref right, ref 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 &[ref ptr, ref val, ref 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()? as u8;
 534                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 535                 if let Some(idx) = this
 536                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 537                     .iter()
 538                     .rev()
 539                     .position(|&c| c == val)
 540                 {
 541                     let new_ptr = ptr.offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 542                     this.write_pointer(new_ptr, dest)?;
 543                 } else {
 544                     this.write_null(dest)?;
 545                 }
 546             }
 547             "memchr" => {
 548                 let &[ref ptr, ref val, ref num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 549                 let ptr = this.read_pointer(ptr)?;
 550                 let val = this.read_scalar(val)?.to_i32()? as u8;
 551                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 552                 let idx = this
 553                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 554                     .iter()
 555                     .position(|&c| c == val);
 556                 if let Some(idx) = idx {
 557                     let new_ptr = ptr.offset(Size::from_bytes(idx as u64), this)?;
 558                     this.write_pointer(new_ptr, dest)?;
 559                 } else {
 560                     this.write_null(dest)?;
 561                 }
 562             }
 563             "strlen" => {
 564                 let &[ref ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 565                 let ptr = this.read_pointer(ptr)?;
 566                 let n = this.read_c_str(ptr)?.len();
 567                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 568             }
 569
 570             // math functions
 571             #[rustfmt::skip]
 572             | "cbrtf"
 573             | "coshf"
 574             | "sinhf"
 575             | "tanf"
 576             | "acosf"
 577             | "asinf"
 578             | "atanf"
 579             => {
 580                 let &[ref f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 581                 // FIXME: Using host floats.
 582                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 583                 let f = match &*link_name.as_str() {
 584                     "cbrtf" => f.cbrt(),
 585                     "coshf" => f.cosh(),
 586                     "sinhf" => f.sinh(),
 587                     "tanf" => f.tan(),
 588                     "acosf" => f.acos(),
 589                     "asinf" => f.asin(),
 590                     "atanf" => f.atan(),
 591                     _ => bug!(),
 592                 };
 593                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 594             }
 595             #[rustfmt::skip]
 596             | "_hypotf"
 597             | "hypotf"
 598             | "atan2f"
 599             => {
 600                 let &[ref f1, ref f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 601                 // underscore case for windows, here and below
 602                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 603                 // FIXME: Using host floats.
 604                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 605                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 606                 let n = match &*link_name.as_str() {
 607                     "_hypotf" | "hypotf" => f1.hypot(f2),
 608                     "atan2f" => f1.atan2(f2),
 609                     _ => bug!(),
 610                 };
 611                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 612             }
 613             #[rustfmt::skip]
 614             | "cbrt"
 615             | "cosh"
 616             | "sinh"
 617             | "tan"
 618             | "acos"
 619             | "asin"
 620             | "atan"
 621             => {
 622                 let &[ref f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 623                 // FIXME: Using host floats.
 624                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 625                 let f = match &*link_name.as_str() {
 626                     "cbrt" => f.cbrt(),
 627                     "cosh" => f.cosh(),
 628                     "sinh" => f.sinh(),
 629                     "tan" => f.tan(),
 630                     "acos" => f.acos(),
 631                     "asin" => f.asin(),
 632                     "atan" => f.atan(),
 633                     _ => bug!(),
 634                 };
 635                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 636             }
 637             #[rustfmt::skip]
 638             | "_hypot"
 639             | "hypot"
 640             | "atan2"
 641             => {
 642                 let &[ref f1, ref f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 643                 // FIXME: Using host floats.
 644                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 645                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 646                 let n = match &*link_name.as_str() {
 647                     "_hypot" | "hypot" => f1.hypot(f2),
 648                     "atan2" => f1.atan2(f2),
 649                     _ => bug!(),
 650                 };
 651                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 652             }
 653             #[rustfmt::skip]
 654             | "_ldexp"
 655             | "ldexp"
 656             | "scalbn"
 657             => {
 658                 let &[ref x, ref exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 659                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 660                 let x = this.read_scalar(x)?.to_f64()?;
 661                 let exp = this.read_scalar(exp)?.to_i32()?;
 662
 663                 // Saturating cast to i16. Even those are outside the valid exponent range to
 664                 // `scalbn` below will do its over/underflow handling.
 665                 let exp = if exp > i32::from(i16::MAX) {
 666                     i16::MAX
 667                 } else if exp < i32::from(i16::MIN) {
 668                     i16::MIN
 669                 } else {
 670                     exp.try_into().unwrap()
 671                 };
 672
 673                 let res = x.scalbn(exp);
 674                 this.write_scalar(Scalar::from_f64(res), dest)?;
 675             }
 676
 677             // Architecture-specific shims
 678             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 679                 let &[] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 680                 this.yield_active_thread();
 681             }
 682             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 683                 let &[ref arg] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 684                 let arg = this.read_scalar(arg)?.to_i32()?;
 685                 match arg {
 686                     15 => { // SY ("full system scope")
 687                         this.yield_active_thread();
 688                     }
 689                     _ => {
 690                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 691                     }
 692                 }
 693             }
 694
 695             // Platform-specific shims
 696             _ => match this.tcx.sess.target.os.as_ref() {
 697                 "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 698                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 699                 target => throw_unsup_format!("the target `{}` is not supported", target),
 700             }
 701         };
 702
 703         // We only fall through to here if we did *not* hit the `_` arm above,
 704         // i.e., if we actually emulated the function.
 705         Ok(EmulateByNameResult::NeedsJumping)
 706     }
 707
 708     /// Check some basic requirements for this allocation request:
 709     /// non-zero size, power-of-two alignment.
 710     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 711         if size == 0 {
 712             throw_ub_format!("creating allocation with size 0");
 713         }
 714         if !align.is_power_of_two() {
 715             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 716         }
 717         Ok(())
 718     }
 719 }