linux-kernel - Re: [PATCH 3/9] rbd: avoid out-of-range warning

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Date: Fri, 29 Mar 2024 08:05:20 +0800
From: Xiubo Li <xiubli@...hat.com>
To: Alex Elder <elder@...e.org>, Arnd Bergmann <arnd@...nel.org>,
 linux-kernel@...r.kernel.org, Ilya Dryomov <idryomov@...il.com>,
 Jens Axboe <axboe@...nel.dk>, Nathan Chancellor <nathan@...nel.org>,
 Alex Elder <elder@...tank.com>, Josh Durgin <josh.durgin@...tank.com>
Cc: Arnd Bergmann <arnd@...db.de>,
 Dongsheng Yang <dongsheng.yang@...ystack.cn>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Hannes Reinecke <hare@...e.de>, Christian Brauner <brauner@...nel.org>,
 Christophe JAILLET <christophe.jaillet@...adoo.fr>,
 "Ricardo B. Marliere" <ricardo@...liere.net>,
 Jinjie Ruan <ruanjinjie@...wei.com>, Alex Elder <elder@...aro.org>,
 ceph-devel@...r.kernel.org, linux-block@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 3/9] rbd: avoid out-of-range warning

On 3/28/24 22:53, Alex Elder wrote:
> On 3/28/24 9:30 AM, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> clang-14 points out that the range check is always true on 64-bit
>> architectures since a u32 is not greater than the allowed size:
>>
>> drivers/block/rbd.c:6079:17: error: result of comparison of constant 
>> 2305843009213693948 with expression of type 'u32' (aka 'unsigned 
>> int') is always false 
>> [-Werror,-Wtautological-constant-out-of-range-compare]
> w
>>              ~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> This is harmless, so just change the type of the temporary to size_t
>> to shut up that warning.
>
> This fixes the warning, but then the now size_t value is passed
> to ceph_decode_32_safe(), which implies a different type conversion.
> That too is not harmful, but...
>
> Could we just cast the value in the comparison instead?
>
>   if ((size_t)snap_count > (SIZE_MAX - sizeof (struct ceph_snap_context))
>
> You could drop the space between sizeof and ( while
> you're at it (I always used the space back then).
>
Agree.

- Xiubo

> -Alex
>
>>
>> Fixes: bb23e37acb2a ("rbd: refactor rbd_header_from_disk()")
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> ---
>>   drivers/block/rbd.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>> index 26ff5cd2bf0a..cb25ee513ada 100644
>> --- a/drivers/block/rbd.c
>> +++ b/drivers/block/rbd.c
>> @@ -6062,7 +6062,7 @@ static int rbd_dev_v2_snap_context(struct 
>> rbd_device *rbd_dev,
>>       void *p;
>>       void *end;
>>       u64 seq;
>> -    u32 snap_count;
>> +    size_t snap_count;
>>       struct ceph_snap_context *snapc;
>>       u32 i;
>
>