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@xxxxxxxxxx> Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
