Hi Jens,

On Fri, 23 Jan 2026 11:06:43 -0700 Jens Axboe <axboe@kernel.dk> wrote:
>
> On 1/23/26 11:00 AM, Jens Axboe wrote:
> > On 1/23/26 10:42 AM, Mark Brown wrote:  
> >> Hi all,
> >>
> >> After merging the block tree, today's linux-next build (x86 allmodconfig)
> >> failed like this:
> >>
> >> In file included from /tmp/next/build/include/linux/string.h:386,
> >>                  from /tmp/next/build/include/linux/bitmap.h:13,
> >>                  from /tmp/next/build/include/linux/cpumask.h:11,
> >>                  from /tmp/next/build/arch/x86/include/asm/paravirt.h:21,
> >>                  from /tmp/next/build/arch/x86/include/asm/cpuid/api.h:57,
> >>                  from /tmp/next/build/arch/x86/include/asm/processor.h:19,
> >>                  from /tmp/next/build/include/linux/sched.h:13,
> >>                  from /tmp/next/build/include/linux/io_uring.h:5,
> >>                  from /tmp/next/build/io_uring/bpf_filter.c:7:
> >> In function 'fortify_memset_chk',
> >>     inlined from 'io_uring_populate_bpf_ctx' at /tmp/next/build/io_uring/bpf_filter.c:33:2:
> >> /tmp/next/build/include/linux/fortify-string.h:480:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
> >>   480 |                         __write_overflow_field(p_size_field, size);
> >>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >> cc1: all warnings being treated as errors
> >>
> >> Caused by commit
> >>
> >>   f1e3672e49e2c (io_uring: add support for BPF filtering for opcode restrictions)  
> > 
> > Huh, that am I missing here? The struct looks as follows:
> > 
> > struct io_uring_bpf_ctx {
> > 	__u64	user_data;
> > 	__u8	opcode;
> > 	__u8	sqe_flags;
> > 	__u8	pad[6];
> > 	union {
> > 		__u64	resv[6];
> > 		struct {
> > 			__u32	family;
> > 			__u32	type;
> > 			__u32	protocol;
> > 		} socket;
> > 		struct {
> > 			__u64	flags;
> > 			__u64	mode;
> > 			__u64	resolve;
> > 		} open;
> > 	};
> > };
> > 
> > and the offending line is:
> > 
> > memset(bctx->pad, 0, sizeof(bctx->pad) + sizeof(bctx->resv));
> > 
> > which should clear from offset 10 (start of pad) for a total of 6 + 48
> > bytes, which is 54 bytes. The size of the struct is 64b.
> > 
> > I guess the part it doesn't like is that it thinks we're clearing the
> > pad field, which would of course be way overwriting it. Guess we can do
> > something ala:
> > 
> > memset((void *) bctx + offsetof(struct io_uring_bpf_ctx, pad), 0,
> > 	sizeof(bctx->pad) + sizeof(bctx->resv));
> > 
> > to make it happier.  
> 
> Folded that in and pushed it out, should be happy for you now. I wonder
> if we have a helper for that...

The origin warning did suggest using struct_group().

-- 
Cheers,
Stephen Rothwell