Patch "ext4: silence UBSAN in ext4_mb_init()" has been added to the 4.6-stable tree

This is a note to let you know that I've just added the patch titled

ext4: silence UBSAN in ext4_mb_init()

to the 4.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ext4-silence-ubsan-in-ext4_mb_init.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Nicolai Stange <[email protected]>
Date: Thu, 5 May 2016 19:46:19 -0400
Subject: ext4: silence UBSAN in ext4_mb_init()

From: Nicolai Stange <[email protected]>

commit 935244cd54b86ca46e69bc6604d2adfb1aec2d42 upstream.

Currently, in ext4_mb_init(), there's a loop like the following:

  do {
    ...
    offset += 1 << (sb->s_blocksize_bits - i);
    i++;
  } while (i <= sb->s_blocksize_bits + 1);

Note that the updated offset is used in the loop's next iteration only.

However, at the last iteration, that is at i == sb->s_blocksize_bits + 1, the shift count becomes equal to (unsigned)-1 > 31 (c.f. C99 6.5.7(3)) and UBSAN reports

  UBSAN: Undefined behaviour in fs/ext4/mballoc.c:2621:15
  shift exponent 4294967295 is too large for 32-bit type 'int'
  [...]
  Call Trace:
   [<ffffffff818c4d25>] dump_stack+0xbc/0x117
   [<ffffffff818c4c69>] ? _atomic_dec_and_lock+0x169/0x169
   [<ffffffff819411ab>] ubsan_epilogue+0xd/0x4e
   [<ffffffff81941cac>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
   [<ffffffff81941ab1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
   [<ffffffff814b6dc1>] ? kmem_cache_alloc+0x101/0x390
   [<ffffffff816fc13b>] ? ext4_mb_init+0x13b/0xfd0
   [<ffffffff814293c7>] ? create_cache+0x57/0x1f0
   [<ffffffff8142948a>] ? create_cache+0x11a/0x1f0
   [<ffffffff821c2168>] ? mutex_lock+0x38/0x60
   [<ffffffff821c23ab>] ? mutex_unlock+0x1b/0x50
   [<ffffffff814c26ab>] ? put_online_mems+0x5b/0xc0
   [<ffffffff81429677>] ? kmem_cache_create+0x117/0x2c0
   [<ffffffff816fcc49>] ext4_mb_init+0xc49/0xfd0
   [...]

Observe that the mentioned shift exponent, 4294967295, equals (unsigned)-1.

Unless compilers start to do some fancy transformations (which at least GCC 6.0.0 doesn't currently do), the issue is of cosmetic nature only: the such calculated value of offset is never used again.

Silence UBSAN by introducing another variable, offset_incr, holding the next increment to apply to offset and adjust that one by right shifting it by one position per loop iteration.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=114701
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=112161

---
 fs/ext4/mballoc.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

@@ -2585,7 +2585,7 @@ int ext4_mb_init(struct super_block *sb)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	unsigned i, j;
-	unsigned offset;
+	unsigned offset, offset_incr;
 	unsigned max;
 	int ret;
 

@@ -2614,11 +2614,13 @@ int ext4_mb_init(struct super_block *sb)
 
 	i = 1;
 	offset = 0;
+	offset_incr = 1 << (sb->s_blocksize_bits - 1);
 	max = sb->s_blocksize << 2;
 	do {
 		sbi->s_mb_offsets[i] = offset;
 		sbi->s_mb_maxs[i] = max;
-		offset += 1 << (sb->s_blocksize_bits - i);
+		offset += offset_incr;
+		offset_incr = offset_incr >> 1;
 		max = max >> 1;
 		i++;
 	} while (i <= sb->s_blocksize_bits + 1);

Patches currently in stable-queue which might be from [email protected] are

queue-4.6/ext4-address-ubsan-warning-in-mb_find_order_for_block.patch queue-4.6/ext4-silence-ubsan-in-ext4_mb_init.patch