On 1/13/26 5:57 AM, Gary Guo wrote:
> On Wed Dec 3, 2025 at 5:59 AM GMT, John Hubbard wrote:
...
> What I would do is to provide a helper function to be obtain a NUL-terminated
> string from ELF:
>
> fn elf_str(elf: &[u8], offset: u64) -> Option<&str> {
> // Note that you have a more efficient `from_bytes_until_nul`, you don't
> // need to iterate yourself!
>
> CStr::from_bytes_until_nul(elf.get(usize::try_from(idx)?..)).ok()?.to_str().ok()
> }
>
> and then you can do
>
> strtab_offset.checked_add(name_offest.into()).and_then(|idx| elf_str(elf,
> idx)).is_some_and(|s| s == target)
>
OK, will do. (I keep thinking we have found all of the older places
that should be updated to use CStr::from_bytes_until_nul(), but still
not there yet.)
>
>> +
>> /// Tries to extract section with name `name` from the ELF64 image
>> `elf`, and returns it.
>> pub(super) fn elf64_section<'a, 'b>(elf: &'a [u8], name: &'b str) ->
>> Option<&'a [u8]> {
>> let hdr = &elf
>> @@ -298,26 +316,7 @@ pub(super) fn elf64_section<'a, 'b>(elf: &'a [u8],
>> name: &'b str) -> Option<&'a
>> return false;
>> };
>>
>> - let Some(name_idx) = strhdr
>> - .0
>> - .sh_offset
>> - .checked_add(u64::from(hdr.0.sh_name))
>
> I think the change is making the code hide the error when ELF is malformed.
> The
> old code fails early which is arguably better?
OK, so something like this would be easier to debug, but I'm not
sure if it is as Rust-idiomatic as it should be?
// Find the section which name matches `name` and return it.
shdr_iter.find_map(|sh_bytes| {
let sh = S::from_bytes(sh_bytes)?;
// Compute the name offset; fail early if the ELF is malformed.
let Some(name_offset) =
strhdr.offset().checked_add(u64::from(sh.name())) else {
return None;
};
// Get section name; skip if we can't read it.
let Some(section_name) = elf_str(elf, name_offset) else {
return None;
};
// Check if the section name matches.
if section_name != name {
return None;
}
let start = usize::try_from(sh.offset()).ok()?;
let end = usize::try_from(sh.size())
.ok()
.and_then(|sz| start.checked_add(sz))?;
elf.get(start..end)
})
>
> Best,
> Gary
>
>> - .and_then(|idx| usize::try_from(idx).ok())
>> - else {
>> - return false;
>> - };
>> -
>> - // Get the start of the name.
>> - elf.get(name_idx..)
>> - // Stop at the first `0`.
>> - .and_then(|nstr| nstr.get(0..=nstr.iter().position(|b| *b
>> == 0)?))
>> - // Convert into CStr. This should never fail because of the
>> line above.
>> - .and_then(|nstr| CStr::from_bytes_with_nul(nstr).ok())
>> - // Convert into str.
>> - .and_then(|c_str| c_str.to_str().ok())
>> - // Check that the name matches.
>> - .map(|str| str == name)
>> - .unwrap_or(false)
>> + section_name_eq(elf, strhdr.0.sh_offset, hdr.0.sh_name, name)
>> })
>> // Return the slice containing the section.
>> .and_then(|sh| {
>
thanks,
--
John Hubbard
