Some ELF files do have segments with size zero, at offsets that are
invalid / out of bounds. This commit changes the default `ReadRef`
implementation for `&[u8]` to permit reads that are out of bounds, if
the requested length is zero, by returning `&[]`.
Examples of such files are debug files created with `objcopy --only-keep-debug`,
for example
`/usr/lib/debug/.build-id/bd/dd2eaf3326ffce6d173666b5f3e62a376e123a.debug`
in package `libgedit-gfls-1-0-dbgsym_0.2.1-2_arm64.deb`:
```
~ cp /usr/lib/debug/.build-id/bd/dd2eaf3326ffce6d173666b5f3e62a376e123a.debug /tmp/bad_file.elf
~ r2 /tmp/bad_file.elf
[0x0000027c]> iI~binsz
binsz 64184
[0x0000027c]> iSS
[Segments]
nth paddr size vaddr vsize perm type name
―――――――――――――――――――――――――――――――――――――――――――――――――――――――
0 0x00000000 0x27c 0x00000000 0x55e0 -r-x MAP LOAD0
1 0x0000fab8 0x0 0x0001fab8 0x5c0 -rw- MAP LOAD1
2 0x0000fab8 0x0 0x0001fb28 0x240 -rw- MAP DYNAMIC
[...]
8 0x0000fab8 0x0 0x0001fab8 0x548 -r-- MAP GNU_RELRO
```
This file has multiple segments starting at `0xfab8` (the end of the
file), with a physical size of `0`, while the file size is also `0xfab8`
(64184).
The current `ReadRef` implementation for `&[u8]` causes
`ProgramHeader::data` to fail for them, causing e.g. `ElfSegment::bytes`
to also fail.
While a caller could handle this error, or one could provide their own
type implementing `ReadRef` this way, I think it makes sense to do this
by default, since no data is *actually* being read out of bounds, but
with the current implementation we still try to index out of bounds and
then error.
Notably with this change this also matches the implementation of
`ReadRef` for `ReadCache` which already has a similar check implemented
for `read_bytes_at`.
When using compression, debug sections in Mach-O produced by the go
compiler have a __zdebug_ section name prefix, and the section data has
the same format as GNU .zdebug_ compression for ELF.
Support these section names in `Object::section_by_name`, and support
the compressed section data in `ObjectSection::compressed_data`.
This commit extracts the GNU-style section compression logic from the
read::elf::section to a module underneath read, and then uses it also
in read::macho.
Timo von Hartz
ajwerner
Philip Craig
Guillaume Girol