From 935244cd54b86ca46e69bc6604d2adfb1aec2d42 Mon Sep 17 00:00:00 2001 From: Nicolai Stange Date: Thu, 5 May 2016 19:46:19 -0400 Subject: [PATCH] ext4: silence UBSAN in ext4_mb_init() 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: [] dump_stack+0xbc/0x117 [] ? _atomic_dec_and_lock+0x169/0x169 [] ubsan_epilogue+0xd/0x4e [] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254 [] ? __ubsan_handle_load_invalid_value+0x158/0x158 [] ? kmem_cache_alloc+0x101/0x390 [] ? ext4_mb_init+0x13b/0xfd0 [] ? create_cache+0x57/0x1f0 [] ? create_cache+0x11a/0x1f0 [] ? mutex_lock+0x38/0x60 [] ? mutex_unlock+0x1b/0x50 [] ? put_online_mems+0x5b/0xc0 [] ? kmem_cache_create+0x117/0x2c0 [] 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 Cc: stable@vger.kernel.org Signed-off-by: Nicolai Stange Signed-off-by: Theodore Ts'o --- fs/ext4/mballoc.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 49e444b51a0c..c1ab3ec30423 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -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);