[3.19.y-ckt stable] Patch "ext4: address UBSAN warning in mb_find_order_for_block()" has been added to the 3.19.y-ckt tree

Kamal Mostafa kamal at canonical.com
Wed Jul 6 21:00:30 UTC 2016

This is a note to let you know that I have just added a patch titled

    ext4: address UBSAN warning in mb_find_order_for_block()

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:


This patch is scheduled to be released in version 3.19.8-ckt23.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see



>From 27eea48416f948e7adba4f1dcbd1778f18ba5c75 Mon Sep 17 00:00:00 2001
From: Nicolai Stange <nicstange at gmail.com>
Date: Thu, 5 May 2016 17:38:03 -0400
Subject: ext4: address UBSAN warning in mb_find_order_for_block()

commit b5cb316cdf3a3f5f6125412b0f6065185240cfdc upstream.

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

  while (order <= e4b->bd_blkbits + 1) {
    bb += 1 << (e4b->bd_blkbits - order);

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

However, at the last iteration, that is at order == e4b->bd_blkbits + 1,
the shift count becomes negative (c.f. C99 6.5.7(3)) and UBSAN reports

  UBSAN: Undefined behaviour in fs/ext4/mballoc.c:1281:11
  shift exponent -1 is negative
  Call Trace:
   [<ffffffff818c4d35>] dump_stack+0xbc/0x117
   [<ffffffff818c4c79>] ? _atomic_dec_and_lock+0x169/0x169
   [<ffffffff819411bb>] ubsan_epilogue+0xd/0x4e
   [<ffffffff81941cbc>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
   [<ffffffff81941ac1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
   [<ffffffff816e93a0>] ? ext4_mb_generate_from_pa+0x590/0x590
   [<ffffffff816502c8>] ? ext4_read_block_bitmap_nowait+0x598/0xe80
   [<ffffffff816e7b7e>] mb_find_order_for_block+0x1ce/0x240

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 bb is never used again.

Silence UBSAN by introducing another variable, bb_incr, holding the next
increment to apply to bb 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

Signed-off-by: Nicolai Stange <nicstange at gmail.com>
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
 fs/ext4/mballoc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4126048..900500c 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1248,6 +1248,7 @@ static void ext4_mb_unload_buddy(struct ext4_buddy *e4b)
 static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
 	int order = 1;
+	int bb_incr = 1 << (e4b->bd_blkbits - 1);
 	void *bb;

 	BUG_ON(e4b->bd_bitmap == e4b->bd_buddy);
@@ -1260,7 +1261,8 @@ static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
 			/* this block is part of buddy of order 'order' */
 			return order;
-		bb += 1 << (e4b->bd_blkbits - order);
+		bb += bb_incr;
+		bb_incr >>= 1;
 	return 0;

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