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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: [<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 Cc: stable@vger.kernel.org Signed-off-by: Nicolai Stange <nicstange@gmail.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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1 changed files with 3 additions and 1 deletions
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@ -1266,6 +1266,7 @@ static void ext4_mb_unload_buddy(struct ext4_buddy *e4b)
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static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
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{
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int order = 1;
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int bb_incr = 1 << (e4b->bd_blkbits - 1);
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void *bb;
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BUG_ON(e4b->bd_bitmap == e4b->bd_buddy);
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@ -1278,7 +1279,8 @@ static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
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/* this block is part of buddy of order 'order' */
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return order;
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}
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bb += 1 << (e4b->bd_blkbits - order);
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bb += bb_incr;
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bb_incr >>= 1;
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order++;
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}
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return 0;
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