Re: [PATCH v4 bpf-next 02/10] lib/buildid: add single folio-based file reader abstraction

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Aug 07, 2024 at 04:40:21PM GMT, Andrii Nakryiko wrote:
> Add freader abstraction that transparently manages fetching and local
> mapping of the underlying file page(s) and provides a simple direct data
> access interface.
> 
> freader_fetch() is the only and single interface necessary. It accepts
> file offset and desired number of bytes that should be accessed, and
> will return a kernel mapped pointer that caller can use to dereference
> data up to requested size. Requested size can't be bigger than the size
> of the extra buffer provided during initialization (because, worst case,
> all requested data has to be copied into it, so it's better to flag
> wrongly sized buffer unconditionally, regardless if requested data range
> is crossing page boundaries or not).
> 
> If folio is not paged in, or some of the conditions are not satisfied,
> NULL is returned and more detailed error code can be accessed through
> freader->err field. This approach makes the usage of freader_fetch()
> cleaner.
> 
> To accommodate accessing file data that crosses folio boundaries, user
> has to provide an extra buffer that will be used to make a local copy,
> if necessary. This is done to maintain a simple linear pointer data
> access interface.
> 
> We switch existing build ID parsing logic to it, without changing or
> lifting any of the existing constraints, yet. This will be done
> separately.
> 
> Given existing code was written with the assumption that it's always
> working with a single (first) page of the underlying ELF file, logic
> passes direct pointers around, which doesn't really work well with
> freader approach and would be limiting when removing the single page (folio)
> limitation. So we adjust all the logic to work in terms of file offsets.
> 
> There is also a memory buffer-based version (freader_init_from_mem())
> for cases when desired data is already available in kernel memory. This
> is used for parsing vmlinux's own build ID note. In this mode assumption
> is that provided data starts at "file offset" zero, which works great
> when parsing ELF notes sections, as all the parsing logic is relative to
> note section's start.
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>