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::PowerPC64LE
56            | Arch::SpirV
57            | Arch::Other(_)) => bug!("unsupported target architecture for malloc: `{arch}`"),
58        };
59        // The C standard only requires sufficient alignment for any *type* with size less than or
60        // equal to the size requested. Types one can define in standard C seem to never have an alignment
61        // bigger than their size. So if the size is 2, then only alignment 2 is guaranteed, even if
62        // `max_fundamental_align` is bigger.
63        // This matches what some real-world implementations do, see e.g.
64        // - https://github.com/jemalloc/jemalloc/issues/1533
65        // - https://github.com/llvm/llvm-project/issues/53540
66        // - https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2293.htm
67        if size >= max_fundamental_align {
68            return Align::from_bytes(max_fundamental_align).unwrap();
69        }
70        // C doesn't have zero-sized types, so presumably nothing is guaranteed here.
71        if size == 0 {
72            return Align::ONE;
73        }
74        // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
75        fn prev_power_of_two(x: u64) -> u64 {
76            let next_pow2 = x.next_power_of_two();
77            if next_pow2 == x {
78                // x *is* a power of two, just use that.
79                x
80            } else {
81                // x is between two powers, so next = 2*prev.
82                next_pow2 / 2
83            }
84        }
85        Align::from_bytes(prev_power_of_two(size)).unwrap()
86    }
87
88    /// Check some basic requirements for this allocation request:
89    /// non-zero size, power-of-two alignment.
90    fn check_rust_alloc_request(&self, size: u64, align: u64) -> InterpResult<'tcx> {
91        let this = self.eval_context_ref();
92        if size == 0 {
93            throw_ub_format!("creating allocation with size 0");
94        }
95        if size > this.max_size_of_val().bytes() {
96            throw_ub_format!("creating an allocation larger than half the address space");
97        }
98        if let Err(e) = Align::from_bytes(align) {
99            match e {
100                AlignFromBytesError::TooLarge(_) => {
101                    throw_unsup_format!(
102                        "creating allocation with alignment {align} exceeding rustc's maximum \
103                         supported value"
104                    );
105                }
106                AlignFromBytesError::NotPowerOfTwo(_) => {
107                    throw_ub_format!("creating allocation with non-power-of-two alignment {align}");
108                }
109            }
110        }
111
112        interp_ok(())
113    }
114
115    fn rust_special_allocator_method(
116        &mut self,
117        method: SpecialAllocatorMethod,
118        link_name: Symbol,
119        abi: &FnAbi<'tcx, Ty<'tcx>>,
120        args: &[OpTy<'tcx>],
121        dest: &PlaceTy<'tcx>,
122    ) -> InterpResult<'tcx> {
123        let this = self.eval_context_mut();
124
125        match method {
126            SpecialAllocatorMethod::Alloc | SpecialAllocatorMethod::AllocZeroed => {
127                let [size, align] =
128                    this.check_shim_sig_lenient(abi, CanonAbi::Rust, link_name, args)?;
129                let size = this.read_target_usize(size)?;
130                let align = this.read_target_usize(align)?;
131
132                this.check_rust_alloc_request(size, align)?;
133
134                let ptr = this.allocate_ptr(
135                    Size::from_bytes(size),
136                    Align::from_bytes(align).unwrap(),
137                    MiriMemoryKind::Rust.into(),
138                    if matches!(method, SpecialAllocatorMethod::AllocZeroed) {
139                        AllocInit::Zero
140                    } else {
141                        AllocInit::Uninit
142                    },
143                )?;
144
145                this.write_pointer(ptr, dest)
146            }
147            SpecialAllocatorMethod::Dealloc => {
148                let [ptr, old_size, align] =
149                    this.check_shim_sig_lenient(abi, CanonAbi::Rust, link_name, args)?;
150                let ptr = this.read_pointer(ptr)?;
151                let old_size = this.read_target_usize(old_size)?;
152                let align = this.read_target_usize(align)?;
153
154                // No need to check old_size/align; we anyway check that they match the allocation.
155                this.deallocate_ptr(
156                    ptr,
157                    Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
158                    MiriMemoryKind::Rust.into(),
159                )
160            }
161            SpecialAllocatorMethod::Realloc => {
162                let [ptr, old_size, align, new_size] =
163                    this.check_shim_sig_lenient(abi, CanonAbi::Rust, link_name, args)?;
164                let ptr = this.read_pointer(ptr)?;
165                let old_size = this.read_target_usize(old_size)?;
166                let align = this.read_target_usize(align)?;
167                let new_size = this.read_target_usize(new_size)?;
168                // No need to check old_size; we anyway check that they match the allocation.
169
170                this.check_rust_alloc_request(new_size, align)?;
171
172                let align = Align::from_bytes(align).unwrap();
173                let new_ptr = this.reallocate_ptr(
174                    ptr,
175                    Some((Size::from_bytes(old_size), align)),
176                    Size::from_bytes(new_size),
177                    align,
178                    MiriMemoryKind::Rust.into(),
179                    AllocInit::Uninit,
180                )?;
181                this.write_pointer(new_ptr, dest)
182            }
183        }
184    }
185
186    fn malloc(&mut self, size: u64, init: AllocInit) -> InterpResult<'tcx, Pointer> {
187        let this = self.eval_context_mut();
188        let align = this.malloc_align(size);
189        let ptr =
190            this.allocate_ptr(Size::from_bytes(size), align, MiriMemoryKind::C.into(), init)?;
191        interp_ok(ptr.into())
192    }
193
194    fn posix_memalign(
195        &mut self,
196        memptr: &OpTy<'tcx>,
197        align: &OpTy<'tcx>,
198        size: &OpTy<'tcx>,
199    ) -> InterpResult<'tcx, Scalar> {
200        let this = self.eval_context_mut();
201        let memptr = this.deref_pointer_as(memptr, this.machine.layouts.mut_raw_ptr)?;
202        let align = this.read_target_usize(align)?;
203        let size = this.read_target_usize(size)?;
204
205        // Align must be power of 2, and also at least ptr-sized (POSIX rules).
206        // But failure to adhere to this is not UB, it's an error condition.
207        if !align.is_power_of_two() || align < this.pointer_size().bytes() {
208            interp_ok(this.eval_libc("EINVAL"))
209        } else {
210            let ptr = this.allocate_ptr(
211                Size::from_bytes(size),
212                Align::from_bytes(align).unwrap(),
213                MiriMemoryKind::C.into(),
214                AllocInit::Uninit,
215            )?;
216            this.write_pointer(ptr, &memptr)?;
217            interp_ok(Scalar::from_i32(0))
218        }
219    }
220
221    fn free(&mut self, ptr: Pointer) -> InterpResult<'tcx> {
222        let this = self.eval_context_mut();
223        if !this.ptr_is_null(ptr)? {
224            this.deallocate_ptr(ptr, None, MiriMemoryKind::C.into())?;
225        }
226        interp_ok(())
227    }
228
229    fn realloc(&mut self, old_ptr: Pointer, new_size: u64) -> InterpResult<'tcx, Pointer> {
230        let this = self.eval_context_mut();
231        let new_align = this.malloc_align(new_size);
232        if this.ptr_is_null(old_ptr)? {
233            // Here we must behave like `malloc`.
234            self.malloc(new_size, AllocInit::Uninit)
235        } else {
236            if new_size == 0 {
237                // C, in their infinite wisdom, made this UB.
238                // <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2464.pdf>
239                throw_ub_format!("`realloc` with a size of zero");
240            } else {
241                let new_ptr = this.reallocate_ptr(
242                    old_ptr,
243                    None,
244                    Size::from_bytes(new_size),
245                    new_align,
246                    MiriMemoryKind::C.into(),
247                    AllocInit::Uninit,
248                )?;
249                interp_ok(new_ptr.into())
250            }
251        }
252    }
253
254    fn aligned_alloc(
255        &mut self,
256        align: &OpTy<'tcx>,
257        size: &OpTy<'tcx>,
258    ) -> InterpResult<'tcx, Pointer> {
259        let this = self.eval_context_mut();
260        let align = this.read_target_usize(align)?;
261        let size = this.read_target_usize(size)?;
262
263        // Alignment must be a power of 2, and "supported by the implementation".
264        // We decide that "supported by the implementation" means that the
265        // size must be a multiple of the alignment. (This restriction seems common
266        // enough that it is stated on <https://en.cppreference.com/w/c/memory/aligned_alloc>
267        // as a general rule, but the actual standard has no such rule.)
268        // If any of these are violated, we have to return NULL.
269        // All fundamental alignments must be supported.
270        //
271        // macOS and Illumos are buggy in that they require the alignment
272        // to be at least the size of a pointer, so they do not support all fundamental
273        // alignments. We do not emulate those platform bugs.
274        //
275        // Linux also sets errno to EINVAL, but that's non-standard behavior that we do not
276        // emulate.
277        // FreeBSD says some of these cases are UB but that's violating the C standard.
278        // http://en.cppreference.com/w/cpp/memory/c/aligned_alloc
279        // Linux: https://linux.die.net/man/3/aligned_alloc
280        // FreeBSD: https://man.freebsd.org/cgi/man.cgi?query=aligned_alloc&apropos=0&sektion=3&manpath=FreeBSD+9-current&format=html
281        match size.checked_rem(align) {
282            Some(0) if align.is_power_of_two() => {
283                let align = align.max(this.malloc_align(size).bytes());
284                let ptr = this.allocate_ptr(
285                    Size::from_bytes(size),
286                    Align::from_bytes(align).unwrap(),
287                    MiriMemoryKind::C.into(),
288                    AllocInit::Uninit,
289                )?;
290                interp_ok(ptr.into())
291            }
292            _ => interp_ok(Pointer::null()),
293        }
294    }
295}
miri/shims/alloc.rs

miri/shims/
alloc.rs
