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

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Message-ID: <20251125120543.49107d8d@pumpkin>
Date: Tue, 25 Nov 2025 12:05:43 +0000
From: david laight <david.laight@...box.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, Xiubo Li <xiubli@...hat.com>, Ilya Dryomov
 <idryomov@...il.com>, Nathan Chancellor <nathan@...nel.org>, Nick
 Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build
 breakage

On Tue, 25 Nov 2025 12:17:10 +0200
Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:

> On Tue, Nov 25, 2025 at 09:55:16AM +0000, david laight wrote:
> > On Mon, 10 Nov 2025 15:44:04 +0100
> > Andy Shevchenko <andriy.shevchenko@...ux.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?  
> 
> I read the piece that prevents builds. The exercise on how to fix this properly
> is delegated to the authors and maintainers.
> 
> > 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.  
> 
> Nice, can you send a followup to fix all that in a better way?
> (I don't care about the fix as long as it doesn't break my builds)
> 

Perhaps -Wtautological-constant-out-of-range-compare should just be delegated
to W=2 like (IIRC) -Wtype-bounds has been which is pretty much the same test.

	David