linux-kernel - Re: [PATCH v1 1/1] libceph: Amend checking to fix `make W=1` build breakage

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Message-ID: <8d1983c9d4c84a6c78b72ba23aa196e849b465a1.camel@ibm.com>
Date: Mon, 10 Nov 2025 19:28:36 +0000
From: Viacheslav Dubeyko <Slava.Dubeyko@....com>
To: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>,
        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "nathan@...nel.org" <nathan@...nel.org>,
        "justinstitt@...gle.com"
	<justinstitt@...gle.com>,
        "idryomov@...il.com" <idryomov@...il.com>,
        Xiubo Li
	<xiubli@...hat.com>,
        "nick.desaulniers+lkml@...il.com"
	<nick.desaulniers+lkml@...il.com>,
        "morbo@...gle.com" <morbo@...gle.com>
Subject: Re:  [PATCH v1 1/1] libceph: Amend checking to fix `make W=1` build
 breakage

On Mon, 2025-11-10 at 15:46 +0100, Andy Shevchenko wrote:
> In a few cases the code compares 32-bit value to a SIZE_MAX derived
> constant which is much higher than that value on 64-bit platforms,
> Clang, in particular, is not happy about this
> 
> net/ceph/osdmap.c:1441:10: error: result of comparison of constant 4611686018427387891 with expression of type 'u32' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
>  1441 |         if (len > (SIZE_MAX - sizeof(*pg)) / sizeof(u32))
>       |             ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> net/ceph/osdmap.c:1624:10: error: result of comparison of constant 2305843009213693945 with expression of type 'u32' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
>  1624 |         if (len > (SIZE_MAX - sizeof(*pg)) / (2 * sizeof(u32)))
>       |             ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> fs/ceph/snap.c:377:10: 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]
>   377 |         if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
>       |             ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Fix this by casting to size_t. Note, that possible replacement of SIZE_MAX
> by U32_MAX may lead to the behaviour changes on the corner cases.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  net/ceph/osdmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ceph/osdmap.c b/net/ceph/osdmap.c
> index 295098873861..8e7cb2fde6f1 100644
> --- a/net/ceph/osdmap.c
> +++ b/net/ceph/osdmap.c
> @@ -1438,7 +1438,7 @@ static struct ceph_pg_mapping *__decode_pg_temp(void **p, void *end,
>  	ceph_decode_32_safe(p, end, len, e_inval);
>  	if (len == 0 && incremental)
>  		return NULL;	/* new_pg_temp: [] to remove */
> -	if (len > (SIZE_MAX - sizeof(*pg)) / sizeof(u32))
> +	if ((size_t)len > (SIZE_MAX - sizeof(*pg)) / sizeof(u32))
>  		return ERR_PTR(-EINVAL);
>  
>  	ceph_decode_need(p, end, len * sizeof(u32), e_inval);
> @@ -1621,7 +1621,7 @@ static struct ceph_pg_mapping *__decode_pg_upmap_items(void **p, void *end,
>  	u32 len, i;

I am guessing... What if we change the declaration of len on size_t, then could
it be more clear solution here? For example, let's consider this for both cases:

size_t len, i;

Could it eliminate the issue and to make the Clang happy? Or could it introduce
another warnings/issues?

Thanks,
Slava. 

>  
>  	ceph_decode_32_safe(p, end, len, e_inval);
> -	if (len > (SIZE_MAX - sizeof(*pg)) / (2 * sizeof(u32)))
> +	if ((size_t)len > (SIZE_MAX - sizeof(*pg)) / (2 * sizeof(u32)))
>  		return ERR_PTR(-EINVAL);
>  
>  	ceph_decode_need(p, end, 2 * len * sizeof(u32), e_inval);