From b5cb316cdf3a3f5f6125412b0f6065185240cfdc Mon Sep 17 00:00:00 2001 From: Nicolai Stange Date: Thu, 5 May 2016 17:38:03 -0400 Subject: [PATCH] ext4: address UBSAN warning in mb_find_order_for_block() 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: [] 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 [] ? ext4_mb_generate_from_pa+0x590/0x590 [] ? ext4_read_block_bitmap_nowait+0x598/0xe80 [] 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 Cc: stable@vger.kernel.org Signed-off-by: Nicolai Stange Signed-off-by: Theodore Ts'o --- 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 efa111a7606d..49e444b51a0c 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -1266,6 +1266,7 @@ 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); @@ -1278,7 +1279,8 @@ 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; order++; } return 0;