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From:   Arnd Bergmann <arnd@kernel.org>
To:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Anand Jain <anand.jain@oracle.com>,
        Naohiro Aota <naohiro.aota@wdc.com>
Cc:     Arnd Bergmann <arnd@arndb.de>, David Sterba <dsterba@suse.cz>,
        Nikolay Borisov <nborisov@suse.com>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] [v2] btrfs: zoned: bail out in btrfs_alloc_chunk for bad input
Date:   Tue, 23 Mar 2021 15:31:19 +0100
Message-Id: <20210323143128.1476527-1-arnd@kernel.org>
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From: Arnd Bergmann <arnd@arndb.de>

gcc complains that 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;
      |                                ^~~

If we ever get into this condition, something is seriously
wrong, so the same logic as in init_alloc_chunk_ctl_policy_regular()
and a few other places should be applied. This avoids both further
data corruption, and the compile-time warning.

Fixes: 1cd6121f2a38 ("btrfs: zoned: implement zoned chunk allocator")
Link: https://lore.kernel.org/lkml/20210323132343.GF7604@twin.jikos.cz/
Suggested-by: David Sterba <dsterba@suse.cz>
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.
---
 fs/btrfs/volumes.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index bc3b33efddc5..d2994305ed77 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4989,6 +4989,8 @@ static void init_alloc_chunk_ctl_policy_zoned(
 		ctl->max_chunk_size = 2 * ctl->max_stripe_size;
 		ctl->devs_max = min_t(int, ctl->devs_max,
 				      BTRFS_MAX_DEVS_SYS_CHUNK);
+	} else {
+		BUG();
 	}
 
 	/* We don't want a chunk larger than 10% of writable space */
-- 
2.29.2