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Subject: Re: [PATCH] fs: Clear a UBSAN shift-out-of-bounds warning
To:     Eric Biggers <ebiggers@kernel.org>
CC:     Alexander Viro <viro@zeniv.linux.org.uk>,
        <linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>
References: <20221110031024.204-1-thunder.leizhen@huawei.com>
 <Y3kw4IP2BVcbFoGT@sol.localdomain>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <5dbf72b4-e596-8954-4d8d-1e92edf46b90@huawei.com>
Date:   Mon, 21 Nov 2022 09:38:38 +0800
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On 2022/11/20 3:39, Eric Biggers wrote:
> On Thu, Nov 10, 2022 at 11:10:24AM +0800, Zhen Lei wrote:
>> UBSAN: shift-out-of-bounds in fs/locks.c:2572:16
>> left shift of 1 by 63 places cannot be represented in type 'long long int'
>>
>> Switch the calculation method to ((quarter - 1) * 2 + 1) can help us
>> eliminate this false positive.
>>
>> On the other hand, the old implementation has problems with char and
>> short types, although not currently involved.
>> printf("%d: %x\n", sizeof(char),  INT_LIMIT(char));
>> printf("%d: %x\n", sizeof(short), INT_LIMIT(short));
>> 1: ffffff7f
>> 2: ffff7fff
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>> ---
>>  include/linux/fs.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index e654435f16512c1..88d42e2daed9f6c 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -1131,7 +1131,7 @@ struct file_lock_context {
>>  
>>  /* The following constant reflects the upper bound of the file/locking space */
>>  #ifndef OFFSET_MAX
>> -#define INT_LIMIT(x)	(~((x)1 << (sizeof(x)*8 - 1)))
>> +#define INT_LIMIT(x)	((((x)1 << (sizeof(x) * 8 - 2)) - 1) * 2  + 1)
>>  #define OFFSET_MAX	INT_LIMIT(loff_t)
>>  #define OFFT_OFFSET_MAX	INT_LIMIT(off_t)
>>  #endif
> 
> This problem has already been solved by type_max() in include/linux/overflow.h.
> How about removing INT_LIMIT() and using type_max() instead?

It's better to use type_max(). INT_LIMIT() is currently valid only for signed type.
Besides, my method is actually the same as type_max(). Using type_max() does not
result in code duplication.

> 
> - Eric
> .
> 

-- 
Regards,
  Zhen Lei