[dm-devel] block: fix a crash when bio_for_each_folio_all iterates over an empty bio

[Mike Snitzer]

On Mon, Apr 17 2023 at  3:11P -0400,
Mikulas Patocka <mpatocka at redhat.com> wrote:

> If we use bio_for_each_folio_all on an empty bio, it will access the first
> bio vector unconditionally (it is uninitialized) and it may crash
> depending on the uninitialized data.
> 
> This patch fixes it by checking the parameter "i" against "bio->bi_vcnt"
> and returning NULL fi->folio if it is out of range.
> 
> The patch also drops the test "if (fi->_i + 1 < bio->bi_vcnt)" from
> bio_next_folio because the same condition is already being checked in
> bio_first_folio.
>
> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

This fix is a prereq for this dm-crypt patch to use folios:
https://patchwork.kernel.org/project/dm-devel/patch/alpine.LRH.2.21.2302161619430.5436@file01.intranet.prod.int.rdu2.redhat.com/
Mikulas explained why an empty bio is possible here:
https://listman.redhat.com/archives/dm-devel/2023-April/053916.html

Reviewed-by: Mike Snitzer <snitzer at kernel.org>