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::*;
  26
  27 /// Returned by `emulate_foreign_item_by_name`.
  28 pub enum EmulateByNameResult<'mir, 'tcx> {
  29     /// The caller is expected to jump to the return block.
  30     NeedsJumping,
  31     /// Jumping has already been taken care of.
  32     AlreadyJumped,
  33     /// A MIR body has been found for the function
  34     MirBody(&'mir mir::Body<'tcx>, ty::Instance<'tcx>),
  35     /// The item is not supported.
  36     NotSupported,
  37 }
  38
  39 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  40 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  41     /// Returns the minimum alignment for the target architecture for allocations of the given size.
  42     fn min_align(&self, size: u64, kind: MiriMemoryKind) -> Align {
  43         let this = self.eval_context_ref();
  44         // List taken from `libstd/sys_common/alloc.rs`.
  45         let min_align = match this.tcx.sess.target.arch.as_ref() {
  46             "x86" | "arm" | "mips" | "powerpc" | "powerpc64" | "asmjs" | "wasm32" => 8,
  47             "x86_64" | "aarch64" | "mips64" | "s390x" | "sparc64" => 16,
  48             arch => bug!("Unsupported target architecture: {}", arch),
  49         };
  50         // Windows always aligns, even small allocations.
  51         // Source: <https://support.microsoft.com/en-us/help/286470/how-to-use-pageheap-exe-in-windows-xp-windows-2000-and-windows-server>
  52         // But jemalloc does not, so for the C heap we only align if the allocation is sufficiently big.
  53         if kind == MiriMemoryKind::WinHeap || size >= min_align {
  54             return Align::from_bytes(min_align).unwrap();
  55         }
  56         // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
  57         fn prev_power_of_two(x: u64) -> u64 {
  58             let next_pow2 = x.next_power_of_two();
  59             if next_pow2 == x {
  60                 // x *is* a power of two, just use that.
  61                 x
  62             } else {
  63                 // x is between two powers, so next = 2*prev.
  64                 next_pow2 / 2
  65             }
  66         }
  67         Align::from_bytes(prev_power_of_two(size)).unwrap()
  68     }
  69
  70     fn malloc(
  71         &mut self,
  72         size: u64,
  73         zero_init: bool,
  74         kind: MiriMemoryKind,
  75     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
  76         let this = self.eval_context_mut();
  77         if size == 0 {
  78             Ok(Pointer::null())
  79         } else {
  80             let align = this.min_align(size, kind);
  81             let ptr = this.allocate_ptr(Size::from_bytes(size), align, kind.into())?;
  82             if zero_init {
  83                 // We just allocated this, the access is definitely in-bounds.
  84                 this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(size as usize)).unwrap();
  85             }
  86             Ok(ptr.into())
  87         }
  88     }
  89
  90     fn free(&mut self, ptr: Pointer<Option<Tag>>, kind: MiriMemoryKind) -> InterpResult<'tcx> {
  91         let this = self.eval_context_mut();
  92         if !this.ptr_is_null(ptr)? {
  93             this.deallocate_ptr(ptr, None, kind.into())?;
  94         }
  95         Ok(())
  96     }
  97
  98     fn realloc(
  99         &mut self,
 100         old_ptr: Pointer<Option<Tag>>,
 101         new_size: u64,
 102         kind: MiriMemoryKind,
 103     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
 104         let this = self.eval_context_mut();
 105         let new_align = this.min_align(new_size, kind);
 106         if this.ptr_is_null(old_ptr)? {
 107             if new_size == 0 {
 108                 Ok(Pointer::null())
 109             } else {
 110                 let new_ptr =
 111                     this.allocate_ptr(Size::from_bytes(new_size), new_align, kind.into())?;
 112                 Ok(new_ptr.into())
 113             }
 114         } else {
 115             if new_size == 0 {
 116                 this.deallocate_ptr(old_ptr, None, kind.into())?;
 117                 Ok(Pointer::null())
 118             } else {
 119                 let new_ptr = this.reallocate_ptr(
 120                     old_ptr,
 121                     None,
 122                     Size::from_bytes(new_size),
 123                     new_align,
 124                     kind.into(),
 125                 )?;
 126                 Ok(new_ptr.into())
 127             }
 128         }
 129     }
 130
 131     /// Lookup the body of a function that has `link_name` as the symbol name.
 132     fn lookup_exported_symbol(
 133         &mut self,
 134         link_name: Symbol,
 135     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 136         let this = self.eval_context_mut();
 137         let tcx = this.tcx.tcx;
 138
 139         // If the result was cached, just return it.
 140         // (Cannot use `or_insert` since the code below might have to throw an error.)
 141         let entry = this.machine.exported_symbols_cache.entry(link_name);
 142         let instance = *match entry {
 143             Entry::Occupied(e) => e.into_mut(),
 144             Entry::Vacant(e) => {
 145                 // Find it if it was not cached.
 146                 let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum)> = None;
 147                 // `dependency_formats` includes all the transitive informations needed to link a crate,
 148                 // which is what we need here since we need to dig out `exported_symbols` from all transitive
 149                 // dependencies.
 150                 let dependency_formats = tcx.dependency_formats(());
 151                 let dependency_format = dependency_formats
 152                     .iter()
 153                     .find(|(crate_type, _)| *crate_type == CrateType::Executable)
 154                     .expect("interpreting a non-executable crate");
 155                 for cnum in iter::once(LOCAL_CRATE).chain(
 156                     dependency_format.1.iter().enumerate().filter_map(|(num, &linkage)| {
 157                         (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1))
 158                     }),
 159                 ) {
 160                     // We can ignore `_export_level` here: we are a Rust crate, and everything is exported
 161                     // from a Rust crate.
 162                     for &(symbol, _export_level) in tcx.exported_symbols(cnum) {
 163                         if let ExportedSymbol::NonGeneric(def_id) = symbol {
 164                             let attrs = tcx.codegen_fn_attrs(def_id);
 165                             let symbol_name = if let Some(export_name) = attrs.export_name {
 166                                 export_name
 167                             } else if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
 168                                 tcx.item_name(def_id)
 169                             } else {
 170                                 // Skip over items without an explicitly defined symbol name.
 171                                 continue;
 172                             };
 173                             if symbol_name == link_name {
 174                                 if let Some((original_instance, original_cnum)) = instance_and_crate
 175                                 {
 176                                     // Make sure we are consistent wrt what is 'first' and 'second'.
 177                                     let original_span =
 178                                         tcx.def_span(original_instance.def_id()).data();
 179                                     let span = tcx.def_span(def_id).data();
 180                                     if original_span < span {
 181                                         throw_machine_stop!(
 182                                             TerminationInfo::MultipleSymbolDefinitions {
 183                                                 link_name,
 184                                                 first: original_span,
 185                                                 first_crate: tcx.crate_name(original_cnum),
 186                                                 second: span,
 187                                                 second_crate: tcx.crate_name(cnum),
 188                                             }
 189                                         );
 190                                     } else {
 191                                         throw_machine_stop!(
 192                                             TerminationInfo::MultipleSymbolDefinitions {
 193                                                 link_name,
 194                                                 first: span,
 195                                                 first_crate: tcx.crate_name(cnum),
 196                                                 second: original_span,
 197                                                 second_crate: tcx.crate_name(original_cnum),
 198                                             }
 199                                         );
 200                                     }
 201                                 }
 202                                 if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 203                                     throw_ub_format!(
 204                                         "attempt to call an exported symbol that is not defined as a function"
 205                                     );
 206                                 }
 207                                 instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 208                             }
 209                         }
 210                     }
 211                 }
 212
 213                 e.insert(instance_and_crate.map(|ic| ic.0))
 214             }
 215         };
 216         match instance {
 217             None => Ok(None), // no symbol with this name
 218             Some(instance) => Ok(Some((this.load_mir(instance.def, None)?, instance))),
 219         }
 220     }
 221
 222     /// Emulates calling a foreign item, failing if the item is not supported.
 223     /// This function will handle `goto_block` if needed.
 224     /// Returns Ok(None) if the foreign item was completely handled
 225     /// by this function.
 226     /// Returns Ok(Some(body)) if processing the foreign item
 227     /// is delegated to another function.
 228     fn emulate_foreign_item(
 229         &mut self,
 230         def_id: DefId,
 231         abi: Abi,
 232         args: &[OpTy<'tcx, Tag>],
 233         ret: Option<(&PlaceTy<'tcx, Tag>, mir::BasicBlock)>,
 234         unwind: StackPopUnwind,
 235     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 236         let this = self.eval_context_mut();
 237         let attrs = this.tcx.get_attrs(def_id);
 238         let link_name = this
 239             .tcx
 240             .sess
 241             .first_attr_value_str_by_name(&attrs, sym::link_name)
 242             .unwrap_or_else(|| this.tcx.item_name(def_id));
 243         let tcx = this.tcx.tcx;
 244
 245         // First: functions that diverge.
 246         let (dest, ret) = match ret {
 247             None =>
 248                 match &*link_name.as_str() {
 249                     "miri_start_panic" => {
 250                         // `check_shim` happens inside `handle_miri_start_panic`.
 251                         this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 252                         return Ok(None);
 253                     }
 254                     // This matches calls to the foreign item `panic_impl`.
 255                     // The implementation is provided by the function with the `#[panic_handler]` attribute.
 256                     "panic_impl" => {
 257                         // We don't use `check_shim` here because we are just forwarding to the lang
 258                         // item. Argument count checking will be performed when the returned `Body` is
 259                         // called.
 260                         this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 261                         let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 262                         let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 263                         return Ok(Some((
 264                             &*this.load_mir(panic_impl_instance.def, None)?,
 265                             panic_impl_instance,
 266                         )));
 267                     }
 268                     #[rustfmt::skip]
 269                     | "exit"
 270                     | "ExitProcess"
 271                     => {
 272                         let exp_abi = if link_name.as_str() == "exit" {
 273                             Abi::C { unwind: false }
 274                         } else {
 275                             Abi::System { unwind: false }
 276                         };
 277                         let &[ref code] = this.check_shim(abi, exp_abi, link_name, args)?;
 278                         // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 279                         let code = this.read_scalar(code)?.to_i32()?;
 280                         throw_machine_stop!(TerminationInfo::Exit(code.into()));
 281                     }
 282                     "abort" => {
 283                         let &[] =
 284                             this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 285                         throw_machine_stop!(TerminationInfo::Abort(
 286                             "the program aborted execution".to_owned()
 287                         ))
 288                     }
 289                     _ => {
 290                         if let Some(body) = this.lookup_exported_symbol(link_name)? {
 291                             return Ok(Some(body));
 292                         }
 293                         this.handle_unsupported(format!(
 294                             "can't call (diverging) foreign function: {}",
 295                             link_name
 296                         ))?;
 297                         return Ok(None);
 298                     }
 299                 },
 300             Some(p) => p,
 301         };
 302
 303         // Second: functions that return.
 304         match this.emulate_foreign_item_by_name(link_name, abi, args, dest, ret)? {
 305             EmulateByNameResult::NeedsJumping => {
 306                 trace!("{:?}", this.dump_place(**dest));
 307                 this.go_to_block(ret);
 308             }
 309             EmulateByNameResult::AlreadyJumped => (),
 310             EmulateByNameResult::MirBody(mir, instance) => return Ok(Some((mir, instance))),
 311             EmulateByNameResult::NotSupported => {
 312                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 313                     return Ok(Some(body));
 314                 }
 315
 316                 this.handle_unsupported(format!("can't call foreign function: {}", link_name))?;
 317                 return Ok(None);
 318             }
 319         }
 320
 321         Ok(None)
 322     }
 323
 324     /// Emulates calling the internal __rust_* allocator functions
 325     fn emulate_allocator(
 326         &mut self,
 327         symbol: Symbol,
 328         default: impl FnOnce(&mut MiriEvalContext<'mir, 'tcx>) -> InterpResult<'tcx>,
 329     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 330         let this = self.eval_context_mut();
 331
 332         let allocator_kind = if let Some(allocator_kind) = this.tcx.allocator_kind(()) {
 333             allocator_kind
 334         } else {
 335             // in real code, this symbol does not exist without an allocator
 336             return Ok(EmulateByNameResult::NotSupported);
 337         };
 338
 339         match allocator_kind {
 340             AllocatorKind::Global => {
 341                 let (body, instance) = this
 342                     .lookup_exported_symbol(symbol)?
 343                     .expect("symbol should be present if there is a global allocator");
 344
 345                 Ok(EmulateByNameResult::MirBody(body, instance))
 346             }
 347             AllocatorKind::Default => {
 348                 default(this)?;
 349                 Ok(EmulateByNameResult::NeedsJumping)
 350             }
 351         }
 352     }
 353
 354     /// Emulates calling a foreign item using its name.
 355     fn emulate_foreign_item_by_name(
 356         &mut self,
 357         link_name: Symbol,
 358         abi: Abi,
 359         args: &[OpTy<'tcx, Tag>],
 360         dest: &PlaceTy<'tcx, Tag>,
 361         ret: mir::BasicBlock,
 362     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 363         let this = self.eval_context_mut();
 364
 365         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 366         // shim, add it to the corresponding submodule.
 367         match &*link_name.as_str() {
 368             // Miri-specific extern functions
 369             "miri_static_root" => {
 370                 let &[ref ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 371                 let ptr = this.read_pointer(ptr)?;
 372                 let (alloc_id, offset, _) = this.ptr_get_alloc_id(ptr)?;
 373                 if offset != Size::ZERO {
 374                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 375                 }
 376                 this.machine.static_roots.push(alloc_id);
 377             }
 378
 379             // Obtains the size of a Miri backtrace. See the README for details.
 380             "miri_backtrace_size" => {
 381                 this.handle_miri_backtrace_size(abi, link_name, args, dest)?;
 382             }
 383
 384             // Obtains a Miri backtrace. See the README for details.
 385             "miri_get_backtrace" => {
 386                 // `check_shim` happens inside `handle_miri_get_backtrace`.
 387                 this.handle_miri_get_backtrace(abi, link_name, args, dest)?;
 388             }
 389
 390             // Resolves a Miri backtrace frame. See the README for details.
 391             "miri_resolve_frame" => {
 392                 // `check_shim` happens inside `handle_miri_resolve_frame`.
 393                 this.handle_miri_resolve_frame(abi, link_name, args, dest)?;
 394             }
 395
 396             // Writes the function and file names of a Miri backtrace frame into a user provided buffer. See the README for details.
 397             "miri_resolve_frame_names" => {
 398                 this.handle_miri_resolve_frame_names(abi, link_name, args)?;
 399             }
 400
 401             // Standard C allocation
 402             "malloc" => {
 403                 let &[ref size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 404                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 405                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C)?;
 406                 this.write_pointer(res, dest)?;
 407             }
 408             "calloc" => {
 409                 let &[ref items, ref len] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 410                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 411                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 412                 let size =
 413                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 414                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C)?;
 415                 this.write_pointer(res, dest)?;
 416             }
 417             "free" => {
 418                 let &[ref ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 419                 let ptr = this.read_pointer(ptr)?;
 420                 this.free(ptr, MiriMemoryKind::C)?;
 421             }
 422             "realloc" => {
 423                 let &[ref old_ptr, ref new_size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 424                 let old_ptr = this.read_pointer(old_ptr)?;
 425                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 426                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 427                 this.write_pointer(res, dest)?;
 428             }
 429
 430             // Rust allocation
 431             "__rust_alloc" => {
 432                 let &[ref size, ref align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 433                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 434                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 435
 436                 return this.emulate_allocator(Symbol::intern("__rg_alloc"), |this| {
 437                     Self::check_alloc_request(size, align)?;
 438
 439                     let ptr = this.allocate_ptr(
 440                         Size::from_bytes(size),
 441                         Align::from_bytes(align).unwrap(),
 442                         MiriMemoryKind::Rust.into(),
 443                     )?;
 444
 445                     this.write_pointer(ptr, dest)
 446                 });
 447             }
 448             "__rust_alloc_zeroed" => {
 449                 let &[ref size, ref align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 450                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 451                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 452
 453                 return this.emulate_allocator(Symbol::intern("__rg_alloc_zeroed"), |this| {
 454                     Self::check_alloc_request(size, align)?;
 455
 456                     let ptr = this.allocate_ptr(
 457                         Size::from_bytes(size),
 458                         Align::from_bytes(align).unwrap(),
 459                         MiriMemoryKind::Rust.into(),
 460                     )?;
 461
 462                     // We just allocated this, the access is definitely in-bounds.
 463                     this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 464                     this.write_pointer(ptr, dest)
 465                 });
 466             }
 467             "__rust_dealloc" => {
 468                 let &[ref ptr, ref old_size, ref align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 469                 let ptr = this.read_pointer(ptr)?;
 470                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 471                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 472
 473                 return this.emulate_allocator(Symbol::intern("__rg_dealloc"), |this| {
 474                     // No need to check old_size/align; we anyway check that they match the allocation.
 475                     this.deallocate_ptr(
 476                         ptr,
 477                         Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 478                         MiriMemoryKind::Rust.into(),
 479                     )
 480                 });
 481             }
 482             "__rust_realloc" => {
 483                 let &[ref ptr, ref old_size, ref align, ref new_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 484                 let ptr = this.read_pointer(ptr)?;
 485                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 486                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 487                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 488                 // No need to check old_size; we anyway check that they match the allocation.
 489
 490                 return this.emulate_allocator(Symbol::intern("__rg_realloc"), |this| {
 491                     Self::check_alloc_request(new_size, align)?;
 492
 493                     let align = Align::from_bytes(align).unwrap();
 494                     let new_ptr = this.reallocate_ptr(
 495                         ptr,
 496                         Some((Size::from_bytes(old_size), align)),
 497                         Size::from_bytes(new_size),
 498                         align,
 499                         MiriMemoryKind::Rust.into(),
 500                     )?;
 501                     this.write_pointer(new_ptr, dest)
 502                 });
 503             }
 504
 505             // C memory handling functions
 506             "memcmp" => {
 507                 let &[ref left, ref right, ref n] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 508                 let left = this.read_pointer(left)?;
 509                 let right = this.read_pointer(right)?;
 510                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 511
 512                 let result = {
 513                     let left_bytes = this.read_bytes_ptr(left, n)?;
 514                     let right_bytes = this.read_bytes_ptr(right, n)?;
 515
 516                     use std::cmp::Ordering::*;
 517                     match left_bytes.cmp(right_bytes) {
 518                         Less => -1i32,
 519                         Equal => 0,
 520                         Greater => 1,
 521                     }
 522                 };
 523
 524                 this.write_scalar(Scalar::from_i32(result), dest)?;
 525             }
 526             "memrchr" => {
 527                 let &[ref ptr, ref val, ref num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 528                 let ptr = this.read_pointer(ptr)?;
 529                 let val = this.read_scalar(val)?.to_i32()? as u8;
 530                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 531                 if let Some(idx) = this
 532                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 533                     .iter()
 534                     .rev()
 535                     .position(|&c| c == val)
 536                 {
 537                     let new_ptr = ptr.offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 538                     this.write_pointer(new_ptr, dest)?;
 539                 } else {
 540                     this.write_null(dest)?;
 541                 }
 542             }
 543             "memchr" => {
 544                 let &[ref ptr, ref val, ref num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 545                 let ptr = this.read_pointer(ptr)?;
 546                 let val = this.read_scalar(val)?.to_i32()? as u8;
 547                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 548                 let idx = this
 549                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 550                     .iter()
 551                     .position(|&c| c == val);
 552                 if let Some(idx) = idx {
 553                     let new_ptr = ptr.offset(Size::from_bytes(idx as u64), this)?;
 554                     this.write_pointer(new_ptr, dest)?;
 555                 } else {
 556                     this.write_null(dest)?;
 557                 }
 558             }
 559             "strlen" => {
 560                 let &[ref ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 561                 let ptr = this.read_pointer(ptr)?;
 562                 let n = this.read_c_str(ptr)?.len();
 563                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 564             }
 565
 566             // math functions
 567             #[rustfmt::skip]
 568             | "cbrtf"
 569             | "coshf"
 570             | "sinhf"
 571             | "tanf"
 572             | "acosf"
 573             | "asinf"
 574             | "atanf"
 575             => {
 576                 let &[ref f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 577                 // FIXME: Using host floats.
 578                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 579                 let f = match &*link_name.as_str() {
 580                     "cbrtf" => f.cbrt(),
 581                     "coshf" => f.cosh(),
 582                     "sinhf" => f.sinh(),
 583                     "tanf" => f.tan(),
 584                     "acosf" => f.acos(),
 585                     "asinf" => f.asin(),
 586                     "atanf" => f.atan(),
 587                     _ => bug!(),
 588                 };
 589                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 590             }
 591             #[rustfmt::skip]
 592             | "_hypotf"
 593             | "hypotf"
 594             | "atan2f"
 595             => {
 596                 let &[ref f1, ref f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 597                 // underscore case for windows, here and below
 598                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 599                 // FIXME: Using host floats.
 600                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 601                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 602                 let n = match &*link_name.as_str() {
 603                     "_hypotf" | "hypotf" => f1.hypot(f2),
 604                     "atan2f" => f1.atan2(f2),
 605                     _ => bug!(),
 606                 };
 607                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 608             }
 609             #[rustfmt::skip]
 610             | "cbrt"
 611             | "cosh"
 612             | "sinh"
 613             | "tan"
 614             | "acos"
 615             | "asin"
 616             | "atan"
 617             => {
 618                 let &[ref f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 619                 // FIXME: Using host floats.
 620                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 621                 let f = match &*link_name.as_str() {
 622                     "cbrt" => f.cbrt(),
 623                     "cosh" => f.cosh(),
 624                     "sinh" => f.sinh(),
 625                     "tan" => f.tan(),
 626                     "acos" => f.acos(),
 627                     "asin" => f.asin(),
 628                     "atan" => f.atan(),
 629                     _ => bug!(),
 630                 };
 631                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 632             }
 633             #[rustfmt::skip]
 634             | "_hypot"
 635             | "hypot"
 636             | "atan2"
 637             => {
 638                 let &[ref f1, ref f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 639                 // FIXME: Using host floats.
 640                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 641                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 642                 let n = match &*link_name.as_str() {
 643                     "_hypot" | "hypot" => f1.hypot(f2),
 644                     "atan2" => f1.atan2(f2),
 645                     _ => bug!(),
 646                 };
 647                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 648             }
 649             #[rustfmt::skip]
 650             | "_ldexp"
 651             | "ldexp"
 652             | "scalbn"
 653             => {
 654                 let &[ref x, ref exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 655                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 656                 let x = this.read_scalar(x)?.to_f64()?;
 657                 let exp = this.read_scalar(exp)?.to_i32()?;
 658
 659                 // Saturating cast to i16. Even those are outside the valid exponent range to
 660                 // `scalbn` below will do its over/underflow handling.
 661                 let exp = if exp > i32::from(i16::MAX) {
 662                     i16::MAX
 663                 } else if exp < i32::from(i16::MIN) {
 664                     i16::MIN
 665                 } else {
 666                     exp.try_into().unwrap()
 667                 };
 668
 669                 let res = x.scalbn(exp);
 670                 this.write_scalar(Scalar::from_f64(res), dest)?;
 671             }
 672
 673             // Architecture-specific shims
 674             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 675                 let &[] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 676                 this.yield_active_thread();
 677             }
 678             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 679                 let &[ref arg] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 680                 let arg = this.read_scalar(arg)?.to_i32()?;
 681                 match arg {
 682                     15 => { // SY ("full system scope")
 683                         this.yield_active_thread();
 684                     }
 685                     _ => {
 686                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 687                     }
 688                 }
 689             }
 690
 691             // Platform-specific shims
 692             _ => match this.tcx.sess.target.os.as_ref() {
 693                 "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 694                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 695                 target => throw_unsup_format!("the target `{}` is not supported", target),
 696             }
 697         };
 698
 699         // We only fall through to here if we did *not* hit the `_` arm above,
 700         // i.e., if we actually emulated the function.
 701         Ok(EmulateByNameResult::NeedsJumping)
 702     }
 703
 704     /// Check some basic requirements for this allocation request:
 705     /// non-zero size, power-of-two alignment.
 706     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 707         if size == 0 {
 708             throw_ub_format!("creating allocation with size 0");
 709         }
 710         if !align.is_power_of_two() {
 711             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 712         }
 713         Ok(())
 714     }
 715 }