Re: [PATCHES 00/12] pahole: Support more rust tags and references to dwz alternate debug files

On Fri Aug 7, 2026 at 7:53 PM BST, Arnaldo Carvalho de Melo wrote:
> On Fri, Aug 07, 2026 at 07:34:10PM +0200, Miguel Ojeda wrote:
>> On Fri, Aug 7, 2026 at 2:43 PM Arnaldo Carvalho de Melo <[email protected]> wrote:
>> >
>> > Adding Miguel to the CC list, Miguel we're trying to improve BTF support
>> > for Rust, this series handles multiple enabling fixes/improvements that
>> > gets us to the next level, i.e. we can load everything from DWARF, now
>> 
>> Thanks for the Cc! Cc'ing rust-for-linux and Gary.
>> 
>> > its time to try to map it to existing BTF kinds and propose BTF
>> > extensions when we can't use existing encodings.
>> 
>> Sounds cool :)
>> 
>> > Inferring from "does any member have a niche layout" means
>> > reverse-engineering rustc's layout algorithm, which is explicitly
>> > unstable and unspecified. It'd break silently on a compiler bump, with
>> > no way to tell from the DWARF that you got it wrong.
>> 
>> Yeah, the LLM is correct here: the Rust compiler gives very few
>> guarantees for the default `repr`, i.e. in common cases it has the
>> freedom to use whatever layout it wants (i.e. even for essentially
>> equivalent types in the same compilation unit) -- the official docs
>> are at:
>> 
>>   https://doc.rust-lang.org/reference/type-layout.html
>> 
>> >     u8                  __discriminant;      /*  0  1 */
>> 
>> For the niche cases like `Option<NonZero<u32>>`, what would be printed?
>
> Can you think about one such niche case that is present in the kernel
> rust .o files right now? I tried finding the `Option<NonZero<u32>>` to
> look at its DWARF but couldn't find one.

For direct use of `Option<NonZero<..>>` we only have `sriov_get_totalvfs` which
is `Option<NonZero<u16>>`.

However, we have many case of `Result<(), Error>` where `Error` is `NonZeroI32`,
so `Ok(())` is mapped to 0.

Best,
Gary