Re: [PATCH 09/31] gpu: nova-core: factor out a section_name_eq() function

[John Hubbard <jhubbard@nvidia.com>]

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