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

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Date: Thu, 28 Mar 2024 09:53:55 -0500
From: Alex Elder <elder@...e.org>
To: 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 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).

					-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;
>