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

[David Sterba <dsterba@suse.cz>]

On Tue, Mar 23, 2021 at 01:46:19PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> 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 <arnd@arndb.de>
> ---
> 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