linux-kernel - Re: include/linux/build_bug.h:78:41: error: static assertion failed: "struct bpf_fib

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Date: Wed, 3 Apr 2024 12:09:29 +0200
From: Anton Protopopov <aspsk@...valent.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Arnd Bergmann <arnd@...db.de>, Naresh Kamboju <naresh.kamboju@...aro.org>, 
	open list <linux-kernel@...r.kernel.org>, lkft-triage@...ts.linaro.org, 
	Linux Regressions <regressions@...ts.linux.dev>, Anders Roxell <anders.roxell@...aro.org>, 
	Dan Carpenter <dan.carpenter@...aro.org>, Daniel Borkmann <daniel@...earbox.net>, 
	David Ahern <dsahern@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Russell King <rmk+kernel@...linux.org.uk>, Ard Biesheuvel <ardb@...nel.org>, 
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: include/linux/build_bug.h:78:41: error: static assertion failed:
 "struct bpf_fib_lookup size check"

On Wed, Apr 3, 2024 at 11:57 AM Anton Protopopov <aspsk@...valent.com> wrote:
>
> On Wed, Apr 3, 2024 at 11:39 AM Alexander Lobakin
> <aleksander.lobakin@...el.com> wrote:
> >
> > From: Arnd Bergmann <arnd@...db.de>
> > Date: Wed, 03 Apr 2024 10:45:36 +0200
> >
> > > On Wed, Apr 3, 2024, at 10:10, Anton Protopopov wrote:
> > >> On Wed, Apr 3, 2024 at 10:03 AM Naresh Kamboju
> > >> <naresh.kamboju@...aro.org> wrote:
> > >>>
> > >>> The arm footbridge_defconfig failed with gcc-13 and gcc-8 on Linux next
> > >>> starting from next-20240328..next-20240402.
> > >>>
> > >>> arm:
> > >>>   build:
> > >>>     * gcc-8-footbridge_defconfig - Failed
> > >>>     * gcc-13-footbridge_defconfig - Failed
> > >>>
> > >>> Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
> > >>>
> > >>> In file included from include/linux/bitfield.h:10,
> > >>>                  from arch/arm/include/asm/ptrace.h:13,
> > >>>                  from arch/arm/include/asm/processor.h:14,
> > >>>                  from include/linux/prefetch.h:15,
> > >>>                  from arch/arm/include/asm/atomic.h:12,
> > >>>                  from include/linux/atomic.h:7,
> > >>>                  from net/core/filter.c:20:
> > >>> include/linux/build_bug.h:78:41: error: static assertion failed:
> > >>> "struct bpf_fib_lookup size check"
> > >>>    78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
> > >>>       |                                         ^~~~~~~~~~~~~~
> > >>
> > >> Thanks, I will take a look today
> >
> > Naresh,
> >
> > Could you please remove that static_assert() and dump bpf_bif_lookup
> > layout from pahole?
> >
> > Anton unionized { smac, dmac } with __u32 mark. On x86_64, the offset of
> > smac was 52 (aligned to 4) already, so I don't really get what AEABI
> > does here. IIRC it aligns every structure to 8 bytes?
> >
> > Maybe we could just add __attribute__((__packed__))
> > __attribute__((__aligned__(4))) to that anonymous union at the end.
>
> Yeah, I am sending a patch for this right now. Better not to depend on
> compiler options

One __packed__ was not enough though. The problem was also with the
union of two __u16's which is padded to be 32 bits when AEABI=n and
the whole structure is packed (so total size is 66 in this case).