2021-03-23 12:49:43

by Arnd Bergmann

Subject: [PATCH] btrfs: zoned: fix uninitialized max_chunk_size

From: Arnd Bergmann <[email protected]>

The ctl->max_chunk_size member might be used uninitialized
when none of the three conditions for initializing it in
init_alloc_chunk_ctl_policy_zoned() are true:

In function ‘init_alloc_chunk_ctl_policy_zoned’,
inlined from ‘init_alloc_chunk_ctl’ at fs/btrfs/volumes.c:5023:3,
inlined from ‘btrfs_alloc_chunk’ at fs/btrfs/volumes.c:5340:2:
include/linux/compiler-gcc.h:48:45: error: ‘ctl.max_chunk_size’ may be used uninitialized [-Werror=maybe-uninitialized]
4998 | ctl->max_chunk_size = min(limit, ctl->max_chunk_size);
| ^~~
fs/btrfs/volumes.c: In function ‘btrfs_alloc_chunk’:
fs/btrfs/volumes.c:5316:32: note: ‘ctl’ declared here
5316 | struct alloc_chunk_ctl ctl;
| ^~~

Initialize it to UINT_MAX and rely on the min() expression to limit
it.

Fixes: 1cd6121f2a38 ("btrfs: zoned: implement zoned chunk allocator")
Signed-off-by: Arnd Bergmann <[email protected]>
---
Note that the -Wmaybe-unintialized warning is globally disabled
by default. For some reason I got this warning anyway when building
this specific file with gcc-11.
---
fs/btrfs/volumes.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index bc3b33efddc5..b42b423b6a10 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4980,6 +4980,7 @@ static void init_alloc_chunk_ctl_policy_zoned(
u64 type = ctl->type;

ctl->max_stripe_size = zone_size;
+ ctl->max_chunk_size = UINT_MAX;
if (type & BTRFS_BLOCK_GROUP_DATA) {
ctl->max_chunk_size = round_down(BTRFS_MAX_DATA_CHUNK_SIZE,
zone_size);
--
2.29.2

2021-03-24 00:56:42

by David Sterba

Subject: Re: [PATCH] btrfs: zoned: fix uninitialized max_chunk_size

On Tue, Mar 23, 2021 at 01:46:19PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <[email protected]>
>
> The ctl->max_chunk_size member might be used uninitialized
> when none of the three conditions for initializing it in
> init_alloc_chunk_ctl_policy_zoned() are true:
>
> In function ‘init_alloc_chunk_ctl_policy_zoned’,
> inlined from ‘init_alloc_chunk_ctl’ at fs/btrfs/volumes.c:5023:3,
> inlined from ‘btrfs_alloc_chunk’ at fs/btrfs/volumes.c:5340:2:
> include/linux/compiler-gcc.h:48:45: error: ‘ctl.max_chunk_size’ may be used uninitialized [-Werror=maybe-uninitialized]
> 4998 | ctl->max_chunk_size = min(limit, ctl->max_chunk_size);
> | ^~~
> fs/btrfs/volumes.c: In function ‘btrfs_alloc_chunk’:
> fs/btrfs/volumes.c:5316:32: note: ‘ctl’ declared here
> 5316 | struct alloc_chunk_ctl ctl;
> | ^~~
>
> Initialize it to UINT_MAX and rely on the min() expression to limit
> it.
>
> Fixes: 1cd6121f2a38 ("btrfs: zoned: implement zoned chunk allocator")
> Signed-off-by: Arnd Bergmann <[email protected]>
> ---
> Note that the -Wmaybe-unintialized warning is globally disabled
> by default. For some reason I got this warning anyway when building
> this specific file with gcc-11.

The warning catches a theoretical case but this would not happen in
pracitce. There are three bits to check and that covers all valid
options, but there should be a final else {} like is in
init_alloc_chunk_ctl_policy_regular that does not let the function
continue as that would mean there are worse problems.

btrfs_alloc_chunk
init_alloc_chunk_ctl
init_alloc_chunk_ctl_policy_zoned

and btrfs_alloc_chunk validates the ctl->flags against
BTRFS_BLOCK_GROUP_TYPE_MASK, which is exactly the tree branches.

> ---
> fs/btrfs/volumes.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index bc3b33efddc5..b42b423b6a10 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4980,6 +4980,7 @@ static void init_alloc_chunk_ctl_policy_zoned(
> u64 type = ctl->type;
>
> ctl->max_stripe_size = zone_size;
> + ctl->max_chunk_size = UINT_MAX;

This would allow the min() work but otherwise is not an expected to
happen at all.

> if (type & BTRFS_BLOCK_GROUP_DATA) {
> ctl->max_chunk_size = round_down(BTRFS_MAX_DATA_CHUNK_SIZE,
> zone_size);
> --
> 2.29.2