miri/shims/
alloc.rs

1use rustc_abi::{Align, AlignFromBytesError, CanonAbi, Size};
2use rustc_ast::expand::allocator::SpecialAllocatorMethod;
3use rustc_middle::ty::Ty;
4use rustc_span::Symbol;
5use rustc_target::callconv::FnAbi;
6use rustc_target::spec::{Arch, Os};
7
8use crate::*;
9
10impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
11pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
12    /// Returns the alignment that `malloc` would guarantee for requests of the given size.
13    fn malloc_align(&self, size: u64) -> Align {
14        let this = self.eval_context_ref();
15        // The C standard says: "The pointer returned if the allocation succeeds is suitably aligned
16        // so that it may be assigned to a pointer to any type of object with a fundamental
17        // alignment requirement and size less than or equal to the size requested."
18        // So first we need to figure out what the limits are for "fundamental alignment".
19        // This is given by `alignof(max_align_t)`. The following list is taken from
20        // `library/std/src/sys/alloc/mod.rs` (where this is called `MIN_ALIGN`) and should
21        // be kept in sync.
22        let os = &this.tcx.sess.target.os;
23        let max_fundamental_align = match &this.tcx.sess.target.arch {
24            Arch::RiscV32 if matches!(os, Os::EspIdf | Os::Zkvm) => 4,
25            Arch::Xtensa if matches!(os, Os::EspIdf) => 4,
26            Arch::X86
27            | Arch::Arm
28            | Arch::M68k
29            | Arch::CSky
30            | Arch::LoongArch32
31            | Arch::Mips
32            | Arch::Mips32r6
33            | Arch::PowerPC
34            | Arch::PowerPC64
35            | Arch::Sparc
36            | Arch::Wasm32
37            | Arch::Hexagon
38            | Arch::RiscV32
39            | Arch::Xtensa => 8,
40            Arch::X86_64
41            | Arch::AArch64
42            | Arch::Arm64EC
43            | Arch::LoongArch64
44            | Arch::Mips64
45            | Arch::Mips64r6
46            | Arch::S390x
47            | Arch::Sparc64
48            | Arch::RiscV64
49            | Arch::Wasm64 => 16,
50            arch @ (Arch::AmdGpu
51            | Arch::Avr
52            | Arch::Bpf
53            | Arch::Msp430
54            | Arch::Nvptx64
55            | Arch::SpirV
56            | Arch::Other(_)) => bug!("unsupported target architecture for malloc: `{arch}`"),
57        };
58        // The C standard only requires sufficient alignment for any *type* with size less than or
59        // equal to the size requested. Types one can define in standard C seem to never have an alignment
60        // bigger than their size. So if the size is 2, then only alignment 2 is guaranteed, even if
61        // `max_fundamental_align` is bigger.
62        // This matches what some real-world implementations do, see e.g.
63        // - https://github.com/jemalloc/jemalloc/issues/1533
64        // - https://github.com/llvm/llvm-project/issues/53540
65        // - https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2293.htm
66        if size >= max_fundamental_align {
67            return Align::from_bytes(max_fundamental_align).unwrap();
68        }
69        // C doesn't have zero-sized types, so presumably nothing is guaranteed here.
70        if size == 0 {
71            return Align::ONE;
72        }
73        // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
74        fn prev_power_of_two(x: u64) -> u64 {
75            let next_pow2 = x.next_power_of_two();
76            if next_pow2 == x {
77                // x *is* a power of two, just use that.
78                x
79            } else {
80                // x is between two powers, so next = 2*prev.
81                next_pow2 / 2
82            }
83        }
84        Align::from_bytes(prev_power_of_two(size)).unwrap()
85    }
86
87    /// Check some basic requirements for this allocation request:
88    /// non-zero size, power-of-two alignment.
89    fn check_rust_alloc_request(&self, size: u64, align: u64) -> InterpResult<'tcx> {
90        let this = self.eval_context_ref();
91        if size == 0 {
92            throw_ub_format!("creating allocation with size 0");
93        }
94        if size > this.max_size_of_val().bytes() {
95            throw_ub_format!("creating an allocation larger than half the address space");
96        }
97        if let Err(e) = Align::from_bytes(align) {
98            match e {
99                AlignFromBytesError::TooLarge(_) => {
100                    throw_unsup_format!(
101                        "creating allocation with alignment {align} exceeding rustc's maximum \
102                         supported value"
103                    );
104                }
105                AlignFromBytesError::NotPowerOfTwo(_) => {
106                    throw_ub_format!("creating allocation with non-power-of-two alignment {align}");
107                }
108            }
109        }
110
111        interp_ok(())
112    }
113
114    fn rust_special_allocator_method(
115        &mut self,
116        method: SpecialAllocatorMethod,
117        link_name: Symbol,
118        abi: &FnAbi<'tcx, Ty<'tcx>>,
119        args: &[OpTy<'tcx>],
120        dest: &PlaceTy<'tcx>,
121    ) -> InterpResult<'tcx> {
122        let this = self.eval_context_mut();
123
124        match method {
125            SpecialAllocatorMethod::Alloc | SpecialAllocatorMethod::AllocZeroed => {
126                let [size, align] =
127                    this.check_shim_sig_lenient(abi, CanonAbi::Rust, link_name, args)?;
128                let size = this.read_target_usize(size)?;
129                let align = this.read_target_usize(align)?;
130
131                this.check_rust_alloc_request(size, align)?;
132
133                let ptr = this.allocate_ptr(
134                    Size::from_bytes(size),
135                    Align::from_bytes(align).unwrap(),
136                    MiriMemoryKind::Rust.into(),
137                    if matches!(method, SpecialAllocatorMethod::AllocZeroed) {
138                        AllocInit::Zero
139                    } else {
140                        AllocInit::Uninit
141                    },
142                )?;
143
144                this.write_pointer(ptr, dest)
145            }
146            SpecialAllocatorMethod::Dealloc => {
147                let [ptr, old_size, align] =
148                    this.check_shim_sig_lenient(abi, CanonAbi::Rust, link_name, args)?;
149                let ptr = this.read_pointer(ptr)?;
150                let old_size = this.read_target_usize(old_size)?;
151                let align = this.read_target_usize(align)?;
152
153                // No need to check old_size/align; we anyway check that they match the allocation.
154                this.deallocate_ptr(
155                    ptr,
156                    Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
157                    MiriMemoryKind::Rust.into(),
158                )
159            }
160            SpecialAllocatorMethod::Realloc => {
161                let [ptr, old_size, align, new_size] =
162                    this.check_shim_sig_lenient(abi, CanonAbi::Rust, link_name, args)?;
163                let ptr = this.read_pointer(ptr)?;
164                let old_size = this.read_target_usize(old_size)?;
165                let align = this.read_target_usize(align)?;
166                let new_size = this.read_target_usize(new_size)?;
167                // No need to check old_size; we anyway check that they match the allocation.
168
169                this.check_rust_alloc_request(new_size, align)?;
170
171                let align = Align::from_bytes(align).unwrap();
172                let new_ptr = this.reallocate_ptr(
173                    ptr,
174                    Some((Size::from_bytes(old_size), align)),
175                    Size::from_bytes(new_size),
176                    align,
177                    MiriMemoryKind::Rust.into(),
178                    AllocInit::Uninit,
179                )?;
180                this.write_pointer(new_ptr, dest)
181            }
182        }
183    }
184
185    fn malloc(&mut self, size: u64, init: AllocInit) -> InterpResult<'tcx, Pointer> {
186        let this = self.eval_context_mut();
187        let align = this.malloc_align(size);
188        let ptr =
189            this.allocate_ptr(Size::from_bytes(size), align, MiriMemoryKind::C.into(), init)?;
190        interp_ok(ptr.into())
191    }
192
193    fn posix_memalign(
194        &mut self,
195        memptr: &OpTy<'tcx>,
196        align: &OpTy<'tcx>,
197        size: &OpTy<'tcx>,
198    ) -> InterpResult<'tcx, Scalar> {
199        let this = self.eval_context_mut();
200        let memptr = this.deref_pointer_as(memptr, this.machine.layouts.mut_raw_ptr)?;
201        let align = this.read_target_usize(align)?;
202        let size = this.read_target_usize(size)?;
203
204        // Align must be power of 2, and also at least ptr-sized (POSIX rules).
205        // But failure to adhere to this is not UB, it's an error condition.
206        if !align.is_power_of_two() || align < this.pointer_size().bytes() {
207            interp_ok(this.eval_libc("EINVAL"))
208        } else {
209            let ptr = this.allocate_ptr(
210                Size::from_bytes(size),
211                Align::from_bytes(align).unwrap(),
212                MiriMemoryKind::C.into(),
213                AllocInit::Uninit,
214            )?;
215            this.write_pointer(ptr, &memptr)?;
216            interp_ok(Scalar::from_i32(0))
217        }
218    }
219
220    fn free(&mut self, ptr: Pointer) -> InterpResult<'tcx> {
221        let this = self.eval_context_mut();
222        if !this.ptr_is_null(ptr)? {
223            this.deallocate_ptr(ptr, None, MiriMemoryKind::C.into())?;
224        }
225        interp_ok(())
226    }
227
228    fn realloc(&mut self, old_ptr: Pointer, new_size: u64) -> InterpResult<'tcx, Pointer> {
229        let this = self.eval_context_mut();
230        let new_align = this.malloc_align(new_size);
231        if this.ptr_is_null(old_ptr)? {
232            // Here we must behave like `malloc`.
233            self.malloc(new_size, AllocInit::Uninit)
234        } else {
235            if new_size == 0 {
236                // C, in their infinite wisdom, made this UB.
237                // <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2464.pdf>
238                throw_ub_format!("`realloc` with a size of zero");
239            } else {
240                let new_ptr = this.reallocate_ptr(
241                    old_ptr,
242                    None,
243                    Size::from_bytes(new_size),
244                    new_align,
245                    MiriMemoryKind::C.into(),
246                    AllocInit::Uninit,
247                )?;
248                interp_ok(new_ptr.into())
249            }
250        }
251    }
252
253    fn aligned_alloc(
254        &mut self,
255        align: &OpTy<'tcx>,
256        size: &OpTy<'tcx>,
257    ) -> InterpResult<'tcx, Pointer> {
258        let this = self.eval_context_mut();
259        let align = this.read_target_usize(align)?;
260        let size = this.read_target_usize(size)?;
261
262        // Alignment must be a power of 2, and "supported by the implementation".
263        // We decide that "supported by the implementation" means that the
264        // size must be a multiple of the alignment. (This restriction seems common
265        // enough that it is stated on <https://en.cppreference.com/w/c/memory/aligned_alloc>
266        // as a general rule, but the actual standard has no such rule.)
267        // If any of these are violated, we have to return NULL.
268        // All fundamental alignments must be supported.
269        //
270        // macOS and Illumos are buggy in that they require the alignment
271        // to be at least the size of a pointer, so they do not support all fundamental
272        // alignments. We do not emulate those platform bugs.
273        //
274        // Linux also sets errno to EINVAL, but that's non-standard behavior that we do not
275        // emulate.
276        // FreeBSD says some of these cases are UB but that's violating the C standard.
277        // http://en.cppreference.com/w/cpp/memory/c/aligned_alloc
278        // Linux: https://linux.die.net/man/3/aligned_alloc
279        // FreeBSD: https://man.freebsd.org/cgi/man.cgi?query=aligned_alloc&apropos=0&sektion=3&manpath=FreeBSD+9-current&format=html
280        match size.checked_rem(align) {
281            Some(0) if align.is_power_of_two() => {
282                let align = align.max(this.malloc_align(size).bytes());
283                let ptr = this.allocate_ptr(
284                    Size::from_bytes(size),
285                    Align::from_bytes(align).unwrap(),
286                    MiriMemoryKind::C.into(),
287                    AllocInit::Uninit,
288                )?;
289                interp_ok(ptr.into())
290            }
291            _ => interp_ok(Pointer::null()),
292        }
293    }
294}
miri/shims/alloc.rs

miri/shims/
alloc.rs
