Re: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build breakage

[david laight <david.laight@runbox.com>]

On Mon, 10 Nov 2025 15:44:04 +0100
Andy Shevchenko <andriy.shevchenko@linux.intel.com> 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
> 
> 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.

Did you really read the code?
The test itself needs moving into ceph_create_snap_context().
Possibly by using kmalloc_array() to do the multiply.

But in any case are large values sane at all?
Allocating very large kernel memory blocks isn't a good idea at all.

In fact this does a kmalloc(... GFP_NOFS) which is pretty likely to
fail for even moderate sized requests. I bet it fails 64k (order 4?)
on a regular basis.

Perhaps all three value that get added to make 'num' need 'sanity limits'
that mean a large allocation just can't happen.

	David

> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>  fs/ceph/snap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
> index c65f2b202b2b..521507ea8260 100644
> --- a/fs/ceph/snap.c
> +++ b/fs/ceph/snap.c
> @@ -374,7 +374,7 @@ static int build_snap_context(struct ceph_mds_client *mdsc,
>  
>  	/* alloc new snap context */
>  	err = -ENOMEM;
> -	if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> +	if ((size_t)num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
>  		goto fail;
>  	snapc = ceph_create_snap_context(num, GFP_NOFS);
>  	if (!snapc)