On Sat, Mar 19, 2016 at 10:12:05PM +0100, Nicolai Stange wrote:
> 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.
>
> Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx>
Applied, thanks.
- Ted
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