On Thu, Aug 8, 2024 at 1:40 AM Andrii Nakryiko <andrii@xxxxxxxxxx> 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>
[...]
> +static int freader_get_folio(struct freader *r, loff_t file_off)
> +{
> + /* check if we can just reuse current folio */
> + if (r->folio && file_off >= r->folio_off &&
> + file_off < r->folio_off + folio_size(r->folio))
> + return 0;
> +
> + freader_put_folio(r);
> +
> + r->folio = filemap_get_folio(r->mapping, file_off >> PAGE_SHIFT);
> + if (IS_ERR(r->folio) || !folio_test_uptodate(r->folio)) {
Can you also fix the uptodate stuff in patch 1, or in another
stable-backportable patch? (I guess alternatively we can fix it with a
custom patch in stable after it's been fixed in mainline, but that
feels kinda hacky to me.)
