miri/shims/unix/
mem.rs

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1//! This is an incomplete implementation of mmap/munmap which is restricted in order to be
2//! implementable on top of the existing memory system. The point of these function as-written is
3//! to allow memory allocators written entirely in Rust to be executed by Miri. This implementation
4//! does not support other uses of mmap such as file mappings.
5//!
6//! mmap/munmap behave a lot like alloc/dealloc, and for simple use they are exactly
7//! equivalent. That is the only part we support: no MAP_FIXED or MAP_SHARED or anything
8//! else that goes beyond a basic allocation API.
9//!
10//! Note that in addition to only supporting malloc-like calls to mmap, we only support free-like
11//! calls to munmap, but for a very different reason. In principle, according to the man pages, it
12//! is possible to unmap arbitrary regions of address space. But in a high-level language like Rust
13//! this amounts to partial deallocation, which LLVM does not support. So any attempt to call our
14//! munmap shim which would partially unmap a region of address space previously mapped by mmap will
15//! report UB.
16
17use rustc_abi::Size;
18
19use crate::*;
20
21impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
22pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
23    fn mmap(
24        &mut self,
25        addr: &OpTy<'tcx>,
26        length: &OpTy<'tcx>,
27        prot: &OpTy<'tcx>,
28        flags: &OpTy<'tcx>,
29        fd: &OpTy<'tcx>,
30        offset: i128,
31    ) -> InterpResult<'tcx, Scalar> {
32        let this = self.eval_context_mut();
33
34        // We do not support MAP_FIXED, so the addr argument is always ignored (except for the MacOS hack)
35        let addr = this.read_target_usize(addr)?;
36        let length = this.read_target_usize(length)?;
37        let prot = this.read_scalar(prot)?.to_i32()?;
38        let flags = this.read_scalar(flags)?.to_i32()?;
39        let fd = this.read_scalar(fd)?.to_i32()?;
40
41        let map_private = this.eval_libc_i32("MAP_PRIVATE");
42        let map_anonymous = this.eval_libc_i32("MAP_ANONYMOUS");
43        let map_shared = this.eval_libc_i32("MAP_SHARED");
44        let map_fixed = this.eval_libc_i32("MAP_FIXED");
45
46        // This is a horrible hack, but on MacOS and Solarish the guard page mechanism uses mmap
47        // in a way we do not support. We just give it the return value it expects.
48        if this.frame_in_std()
49            && matches!(&*this.tcx.sess.target.os, "macos" | "solaris" | "illumos")
50            && (flags & map_fixed) != 0
51        {
52            return interp_ok(Scalar::from_maybe_pointer(Pointer::from_addr_invalid(addr), this));
53        }
54
55        let prot_read = this.eval_libc_i32("PROT_READ");
56        let prot_write = this.eval_libc_i32("PROT_WRITE");
57
58        // First, we do some basic argument validation as required by mmap
59        if (flags & (map_private | map_shared)).count_ones() != 1 {
60            this.set_last_error(LibcError("EINVAL"))?;
61            return interp_ok(this.eval_libc("MAP_FAILED"));
62        }
63        if length == 0 {
64            this.set_last_error(LibcError("EINVAL"))?;
65            return interp_ok(this.eval_libc("MAP_FAILED"));
66        }
67
68        // If a user tries to map a file, we want to loudly inform them that this is not going
69        // to work. It is possible that POSIX gives us enough leeway to return an error, but the
70        // outcome for the user (I need to add cfg(miri)) is the same, just more frustrating.
71        if fd != -1 {
72            throw_unsup_format!("Miri does not support file-backed memory mappings");
73        }
74
75        // Miri doesn't support MAP_FIXED.
76        if flags & map_fixed != 0 {
77            throw_unsup_format!(
78                "Miri does not support calls to mmap with MAP_FIXED as part of the flags argument",
79            );
80        }
81
82        // Miri doesn't support protections other than PROT_READ|PROT_WRITE.
83        if prot != prot_read | prot_write {
84            throw_unsup_format!(
85                "Miri does not support calls to mmap with protections other than \
86                 PROT_READ|PROT_WRITE",
87            );
88        }
89
90        // Miri does not support shared mappings, or any of the other extensions that for example
91        // Linux has added to the flags arguments.
92        if flags != map_private | map_anonymous {
93            throw_unsup_format!(
94                "Miri only supports calls to mmap which set the flags argument to \
95                 MAP_PRIVATE|MAP_ANONYMOUS",
96            );
97        }
98
99        // This is only used for file mappings, which we don't support anyway.
100        if offset != 0 {
101            throw_unsup_format!("Miri does not support non-zero offsets to mmap");
102        }
103
104        let align = this.machine.page_align();
105        let Some(map_length) = length.checked_next_multiple_of(this.machine.page_size) else {
106            this.set_last_error(LibcError("EINVAL"))?;
107            return interp_ok(this.eval_libc("MAP_FAILED"));
108        };
109        if map_length > this.target_usize_max() {
110            this.set_last_error(LibcError("EINVAL"))?;
111            return interp_ok(this.eval_libc("MAP_FAILED"));
112        }
113
114        let ptr = this.allocate_ptr(
115            Size::from_bytes(map_length),
116            align,
117            MiriMemoryKind::Mmap.into(),
118            // mmap guarantees new mappings are zero-init.
119            AllocInit::Zero,
120        )?;
121
122        interp_ok(Scalar::from_pointer(ptr, this))
123    }
124
125    fn munmap(&mut self, addr: &OpTy<'tcx>, length: &OpTy<'tcx>) -> InterpResult<'tcx, Scalar> {
126        let this = self.eval_context_mut();
127
128        let addr = this.read_pointer(addr)?;
129        let length = this.read_target_usize(length)?;
130
131        // addr must be a multiple of the page size, but apart from that munmap is just implemented
132        // as a dealloc.
133        #[expect(clippy::arithmetic_side_effects)] // PAGE_SIZE is nonzero
134        if addr.addr().bytes() % this.machine.page_size != 0 {
135            return this.set_last_error_and_return_i32(LibcError("EINVAL"));
136        }
137
138        let Some(length) = length.checked_next_multiple_of(this.machine.page_size) else {
139            return this.set_last_error_and_return_i32(LibcError("EINVAL"));
140        };
141        if length > this.target_usize_max() {
142            this.set_last_error(LibcError("EINVAL"))?;
143            return interp_ok(this.eval_libc("MAP_FAILED"));
144        }
145
146        let length = Size::from_bytes(length);
147        this.deallocate_ptr(
148            addr,
149            Some((length, this.machine.page_align())),
150            MemoryKind::Machine(MiriMemoryKind::Mmap),
151        )?;
152
153        interp_ok(Scalar::from_i32(0))
154    }
155}
miri/shims/unix/mem.rs

miri/shims/unix/
mem.rs
