linux-kernel - Re: [PATCH] btrfs: Annotate structs with __counted

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Message-ID: <20240814180219.GA2542470@thelio-3990X>
Date: Wed, 14 Aug 2024 11:02:19 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thorsten Blum <thorsten.blum@...lux.com>
Cc: Johannes Thumshirn <Johannes.Thumshirn@....com>,
	"clm@...com" <clm@...com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"dsterba@...e.com" <dsterba@...e.com>,
	"kees@...nel.org" <kees@...nel.org>,
	"gustavoars@...nel.org" <gustavoars@...nel.org>,
	"linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] btrfs: Annotate structs with __counted_by()

On Mon, Aug 12, 2024 at 02:03:44PM +0200, Thorsten Blum wrote:
> On 12. Aug 2024, at 12:54, Johannes Thumshirn <Johannes.Thumshirn@....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.blum@...lux.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.thumshirn@....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