std/backtrace/src/symbolize/gimli/
libs_macos.rs

1#![allow(deprecated)]
2
3use super::mystd::ffi::OsStr;
4use super::mystd::os::unix::prelude::*;
5use super::mystd::prelude::v1::*;
6use super::{Library, LibrarySegment};
7use core::convert::TryInto;
8use core::ffi::CStr;
9use core::mem;
10
11// FIXME: replace with ptr::from_ref once MSRV is high enough
12#[inline(always)]
13#[must_use]
14const fn ptr_from_ref<T: ?Sized>(r: &T) -> *const T {
15    r
16}
17
18pub(super) fn native_libraries() -> Vec<Library> {
19    let mut ret = Vec::new();
20    let images = unsafe { libc::_dyld_image_count() };
21    for i in 0..images {
22        ret.extend(native_library(i));
23    }
24    return ret;
25}
26
27fn native_library(i: u32) -> Option<Library> {
28    use object::macho;
29    use object::read::macho::{MachHeader, Segment};
30    use object::NativeEndian;
31
32    // Fetch the name of this library which corresponds to the path of
33    // where to load it as well.
34    let name = unsafe {
35        let name = libc::_dyld_get_image_name(i);
36        if name.is_null() {
37            return None;
38        }
39        CStr::from_ptr(name)
40    };
41
42    // Load the image header of this library and delegate to `object` to
43    // parse all the load commands so we can figure out all the segments
44    // involved here.
45    let (mut load_commands, endian) = unsafe {
46        let header = libc::_dyld_get_image_header(i);
47        if header.is_null() {
48            return None;
49        }
50        match (*header).magic {
51            macho::MH_MAGIC => {
52                let endian = NativeEndian;
53                let header = &*header.cast::<macho::MachHeader32<NativeEndian>>();
54                let data = core::slice::from_raw_parts(
55                    ptr_from_ref(header).cast::<u8>(),
56                    mem::size_of_val(header) + header.sizeofcmds.get(endian) as usize,
57                );
58                (header.load_commands(endian, data, 0).ok()?, endian)
59            }
60            macho::MH_MAGIC_64 => {
61                let endian = NativeEndian;
62                let header = &*header.cast::<macho::MachHeader64<NativeEndian>>();
63                let data = core::slice::from_raw_parts(
64                    ptr_from_ref(header).cast::<u8>(),
65                    mem::size_of_val(header) + header.sizeofcmds.get(endian) as usize,
66                );
67                (header.load_commands(endian, data, 0).ok()?, endian)
68            }
69            _ => return None,
70        }
71    };
72
73    // Iterate over the segments and register known regions for segments
74    // that we find. Additionally record information bout text segments
75    // for processing later, see comments below.
76    let mut segments = Vec::new();
77    let mut first_text = 0;
78    let mut text_fileoff_zero = false;
79    while let Some(cmd) = load_commands.next().ok()? {
80        if let Some((seg, _)) = cmd.segment_32().ok()? {
81            if seg.name() == b"__TEXT" {
82                first_text = segments.len();
83                if seg.fileoff(endian) == 0 && seg.filesize(endian) > 0 {
84                    text_fileoff_zero = true;
85                }
86            }
87            segments.push(LibrarySegment {
88                len: seg.vmsize(endian).try_into().ok()?,
89                stated_virtual_memory_address: seg.vmaddr(endian).try_into().ok()?,
90            });
91        }
92        if let Some((seg, _)) = cmd.segment_64().ok()? {
93            if seg.name() == b"__TEXT" {
94                first_text = segments.len();
95                if seg.fileoff(endian) == 0 && seg.filesize(endian) > 0 {
96                    text_fileoff_zero = true;
97                }
98            }
99            segments.push(LibrarySegment {
100                len: seg.vmsize(endian).try_into().ok()?,
101                stated_virtual_memory_address: seg.vmaddr(endian).try_into().ok()?,
102            });
103        }
104    }
105
106    // Determine the "slide" for this library which ends up being the
107    // bias we use to figure out where in memory objects are loaded.
108    // This is a bit of a weird computation though and is the result of
109    // trying a few things in the wild and seeing what sticks.
110    //
111    // The general idea is that the `bias` plus a segment's
112    // `stated_virtual_memory_address` is going to be where in the
113    // actual address space the segment resides. The other thing we rely
114    // on though is that a real address minus the `bias` is the index to
115    // look up in the symbol table and debuginfo.
116    //
117    // It turns out, though, that for system loaded libraries these
118    // calculations are incorrect. For native executables, however, it
119    // appears correct. Lifting some logic from LLDB's source it has
120    // some special-casing for the first `__TEXT` section loaded from
121    // file offset 0 with a nonzero size. For whatever reason when this
122    // is present it appears to mean that the symbol table is relative
123    // to just the vmaddr slide for the library. If it's *not* present
124    // then the symbol table is relative to the vmaddr slide plus the
125    // segment's stated address.
126    //
127    // To handle this situation if we *don't* find a text section at
128    // file offset zero then we increase the bias by the first text
129    // sections's stated address and decrease all stated addresses by
130    // that amount as well. That way the symbol table is always appears
131    // relative to the library's bias amount. This appears to have the
132    // right results for symbolizing via the symbol table.
133    //
134    // Honestly I'm not entirely sure whether this is right or if
135    // there's something else that should indicate how to do this. For
136    // now though this seems to work well enough (?) and we should
137    // always be able to tweak this over time if necessary.
138    //
139    // For some more information see #318
140    let mut slide = unsafe { libc::_dyld_get_image_vmaddr_slide(i) as usize };
141    if !text_fileoff_zero {
142        let adjust = segments[first_text].stated_virtual_memory_address;
143        for segment in segments.iter_mut() {
144            segment.stated_virtual_memory_address -= adjust;
145        }
146        slide += adjust;
147    }
148
149    Some(Library {
150        name: OsStr::from_bytes(name.to_bytes()).to_owned(),
151        segments,
152        bias: slide,
153    })
154}