From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH v2 2/2] fs/ext4: ext4_mb_init(): silence UBSAN
Date: Thu, 5 May 2016 19:47:50 -0400
Message-ID: <20160505234750.GB30122@thunk.org>
References: <1458421925-5481-1-git-send-email-nicstange@gmail.com>
 <1458421925-5481-3-git-send-email-nicstange@gmail.com>
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Cc: Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
To: Nicolai Stange <nicstange@gmail.com>
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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@gmail.com>

Applied, thanks.

						- Ted