On Mon, Aug 12, 2024 at 02:03:44PM +0200, Thorsten Blum wrote:
> On 12. Aug 2024, at 12:54, Johannes Thumshirn <johannes.thumsh...@wdc.com>
> wrote:
> > On 12.08.24 12:37, Thorsten Blum wrote:
> >> Add the __counted_by compiler attribute to the flexible array member
> >> stripes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> >> CONFIG_FORTIFY_SOURCE.
> >>
> >> Signed-off-by: Thorsten Blum <thorsten.b...@toblux.com>
> >> ---
> >> fs/btrfs/volumes.h | 4 ++--
> >> 1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> >> index 37a09ebb34dd..f28fa318036b 100644
> >> --- a/fs/btrfs/volumes.h
> >> +++ b/fs/btrfs/volumes.h
> >> @@ -551,7 +551,7 @@ struct btrfs_io_context {
> >> * stripes[data_stripes + 1]: The Q stripe (only for RAID6).
> >> */
> >> u64 full_stripe_logical;
> >> - struct btrfs_io_stripe stripes[];
> >> + struct btrfs_io_stripe stripes[] __counted_by(num_stripes);
> >> };
> >>
> >> struct btrfs_device_info {
> >> @@ -591,7 +591,7 @@ struct btrfs_chunk_map {
> >> int io_width;
> >> int num_stripes;
> >> int sub_stripes;
> >> - struct btrfs_io_stripe stripes[];
> >> + struct btrfs_io_stripe stripes[] __counted_by(num_stripes);
> >> };
> >>
> >> #define btrfs_chunk_map_size(n) (sizeof(struct btrfs_chunk_map) + \
> >
> > Looks good to me,
> > Reviewed-by: Johannes Thumshirn <johannes.thumsh...@wdc.com>
> >
> > Out of curiosity, have you encountered any issues with this patch applied?
>
> I only compile-tested it.
This change is now in next-20240814 and I see a UBSAN warning at runtime
as a result because the assignment of ->num_stripes happens after
accessing ->stripes[] (which breaks one of the requirements for using
__counted_by [1]), meaning that UBSAN thinks this is a zero sized array
due to bioc being allocated with kzalloc().
[ 24.992264] ------------[ cut here ]------------
[ 25.009196] UBSAN: array-index-out-of-bounds in fs/btrfs/volumes.c:6602:11
[ 25.021963] index 1 is out of range for type 'struct btrfs_io_stripe[]
__counted_by(num_stripes)' (aka 'struct btrfs_io_stripe[]')
[ 25.036463] CPU: 28 UID: 0 PID: 1171 Comm: systemd-random- Not tainted
6.11.0-rc3-next-20240814 #1
[ 25.048172] Hardware name: ADLINK Ampere Altra Developer Platform/Ampere
Altra Developer Platform, BIOS TianoCore 2.04.100.11 (SYS: 2.06.20220308)
11/06/2
[ 25.064754] Call trace:
[ 25.069965] dump_backtrace+0x114/0x19c
[ 25.076564] show_stack+0x28/0x3c
[ 25.082642] dump_stack_lvl+0x48/0x94
[ 25.089068] __ubsan_handle_out_of_bounds+0x10c/0x184
[ 25.096883] btrfs_map_block+0x540/0xb3c
[ 25.103570] btrfs_submit_bio+0xf8/0x654
[ 25.110256] write_one_eb+0x290/0x444
[ 25.116682] btree_write_cache_pages+0x44c/0x5a8
[ 25.124063] btree_writepages+0x2c/0x8c
[ 25.130662] do_writepages+0x10c/0x34c
[ 25.137175] filemap_fdatawrite_wbc+0x84/0xb0
[ 25.144295] filemap_fdatawrite_range+0x74/0xac
[ 25.151589] btrfs_write_marked_extents+0xa0/0x140
[ 25.159143] btrfs_sync_log+0x298/0xa98
[ 25.165743] btrfs_sync_file+0x440/0x608
[ 25.172429] __arm64_sys_fsync+0x90/0xd4
[ 25.179115] invoke_syscall+0x8c/0x11c
[ 25.185628] el0_svc_common
[ 25.191185] do_el0_svc+0x2c/0x3c
[ 25.197264] el0_svc+0x48/0xf0
[ 25.203083] el0t_64_sync_handler+0x98/0x108
[ 25.210118] el0t_64_sync+0x19c/0x1a0
[ 25.216552] ---[ end trace ]---
The fix might be as simple as something like
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 4a259bdaa21c..0cabc2ebde71 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6561,6 +6561,7 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum
btrfs_map_op op,
}
bioc->map_type = map->type;
+ bioc->num_stripes = io_geom.num_stripes;
/*
* For RAID56 full map, we need to make sure the stripes[] follows the
* rule that data stripes are all ordered, then followed with P and Q
@@ -6621,7 +6622,6 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum
btrfs_map_op op,
}
*bioc_ret = bioc;
- bioc->num_stripes = io_geom.num_stripes;
bioc->max_errors = io_geom.max_errors;
bioc->mirror_num = io_geom.mirror_num;
but I am not sure of the implications of this change on quick glance
with regards to error handling and such.
[1]:
https://people.kernel.org/gustavoars/how-to-use-the-new-counted_by-attribute-in-c-and-linux
Cheers,
Nathan
