1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156

//! This is an incomplete implementation of mmap/munmap which is restricted in order to be
//! implementable on top of the existing memory system. The point of these function as-written is
//! to allow memory allocators written entirely in Rust to be executed by Miri. This implementation
//! does not support other uses of mmap such as file mappings.
//!
//! mmap/munmap behave a lot like alloc/dealloc, and for simple use they are exactly
//! equivalent. That is the only part we support: no MAP_FIXED or MAP_SHARED or anything
//! else that goes beyond a basic allocation API.
//!
//! Note that in addition to only supporting malloc-like calls to mmap, we only support free-like
//! calls to munmap, but for a very different reason. In principle, according to the man pages, it
//! is possible to unmap arbitrary regions of address space. But in a high-level language like Rust
//! this amounts to partial deallocation, which LLVM does not support. So any attempt to call our
//! munmap shim which would partily unmap a region of address space previously mapped by mmap will
//! report UB.

use crate::*;
use rustc_target::abi::Size;

impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
    fn mmap(
        &mut self,
        addr: &OpTy<'tcx, Provenance>,
        length: &OpTy<'tcx, Provenance>,
        prot: &OpTy<'tcx, Provenance>,
        flags: &OpTy<'tcx, Provenance>,
        fd: &OpTy<'tcx, Provenance>,
        offset: i128,
    ) -> InterpResult<'tcx, Scalar<Provenance>> {
        let this = self.eval_context_mut();

        // We do not support MAP_FIXED, so the addr argument is always ignored (except for the MacOS hack)
        let addr = this.read_target_usize(addr)?;
        let length = this.read_target_usize(length)?;
        let prot = this.read_scalar(prot)?.to_i32()?;
        let flags = this.read_scalar(flags)?.to_i32()?;
        let fd = this.read_scalar(fd)?.to_i32()?;

        let map_private = this.eval_libc_i32("MAP_PRIVATE");
        let map_anonymous = this.eval_libc_i32("MAP_ANONYMOUS");
        let map_shared = this.eval_libc_i32("MAP_SHARED");
        let map_fixed = this.eval_libc_i32("MAP_FIXED");

        // This is a horrible hack, but on MacOS the guard page mechanism uses mmap
        // in a way we do not support. We just give it the return value it expects.
        if this.frame_in_std() && this.tcx.sess.target.os == "macos" && (flags & map_fixed) != 0 {
            return Ok(Scalar::from_maybe_pointer(Pointer::from_addr_invalid(addr), this));
        }

        let prot_read = this.eval_libc_i32("PROT_READ");
        let prot_write = this.eval_libc_i32("PROT_WRITE");

        // First, we do some basic argument validation as required by mmap
        if (flags & (map_private | map_shared)).count_ones() != 1 {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(this.eval_libc("MAP_FAILED"));
        }
        if length == 0 {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(this.eval_libc("MAP_FAILED"));
        }

        // If a user tries to map a file, we want to loudly inform them that this is not going
        // to work. It is possible that POSIX gives us enough leeway to return an error, but the
        // outcome for the user (I need to add cfg(miri)) is the same, just more frustrating.
        if fd != -1 {
            throw_unsup_format!("Miri does not support file-backed memory mappings");
        }

        // POSIX says:
        // [ENOTSUP]
        // * MAP_FIXED or MAP_PRIVATE was specified in the flags argument and the implementation
        // does not support this functionality.
        // * The implementation does not support the combination of accesses requested in the
        // prot argument.
        //
        // Miri doesn't support MAP_FIXED or any any protections other than PROT_READ|PROT_WRITE.
        if flags & map_fixed != 0 || prot != prot_read | prot_write {
            this.set_last_error(this.eval_libc("ENOTSUP"))?;
            return Ok(this.eval_libc("MAP_FAILED"));
        }

        // Miri does not support shared mappings, or any of the other extensions that for example
        // Linux has added to the flags arguments.
        if flags != map_private | map_anonymous {
            throw_unsup_format!(
                "Miri only supports calls to mmap which set the flags argument to MAP_PRIVATE|MAP_ANONYMOUS"
            );
        }

        // This is only used for file mappings, which we don't support anyway.
        if offset != 0 {
            throw_unsup_format!("Miri does not support non-zero offsets to mmap");
        }

        let align = this.machine.page_align();
        let Some(map_length) = length.checked_next_multiple_of(this.machine.page_size) else {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(this.eval_libc("MAP_FAILED"));
        };
        if map_length > this.target_usize_max() {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(this.eval_libc("MAP_FAILED"));
        }

        let ptr =
            this.allocate_ptr(Size::from_bytes(map_length), align, MiriMemoryKind::Mmap.into())?;
        // We just allocated this, the access is definitely in-bounds and fits into our address space.
        // mmap guarantees new mappings are zero-init.
        this.write_bytes_ptr(
            ptr.into(),
            std::iter::repeat(0u8).take(usize::try_from(map_length).unwrap()),
        )
        .unwrap();

        Ok(Scalar::from_pointer(ptr, this))
    }

    fn munmap(
        &mut self,
        addr: &OpTy<'tcx, Provenance>,
        length: &OpTy<'tcx, Provenance>,
    ) -> InterpResult<'tcx, Scalar<Provenance>> {
        let this = self.eval_context_mut();

        let addr = this.read_pointer(addr)?;
        let length = this.read_target_usize(length)?;

        // addr must be a multiple of the page size, but apart from that munmap is just implemented
        // as a dealloc.
        #[allow(clippy::arithmetic_side_effects)] // PAGE_SIZE is nonzero
        if addr.addr().bytes() % this.machine.page_size != 0 {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(Scalar::from_i32(-1));
        }

        let Some(length) = length.checked_next_multiple_of(this.machine.page_size) else {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(Scalar::from_i32(-1));
        };
        if length > this.target_usize_max() {
            this.set_last_error(this.eval_libc("EINVAL"))?;
            return Ok(this.eval_libc("MAP_FAILED"));
        }

        let length = Size::from_bytes(length);
        this.deallocate_ptr(
            addr,
            Some((length, this.machine.page_align())),
            MemoryKind::Machine(MiriMemoryKind::Mmap),
        )?;

        Ok(Scalar::from_i32(0))
    }
}