Re: [PATCH bpf v2 2/2] bpf: Harden __bpf_kfunc tag against linker kfunc removal

Geert Uytterhoeven
2024-06-26T11:40:21Z

Hi Jiri,

On Wed, Jun 26, 2024 at 11:52 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> On Tue, Jun 25, 2024 at 12:46:48PM +0200, Geert Uytterhoeven wrote:
> > On Mon, 3 Jun 2024, Tony Ambardar wrote:
> > > BPF kfuncs are often not directly referenced and may be inadvertently
> > > removed by optimization steps during kernel builds, thus the __bpf_kfunc
> > > tag mitigates against this removal by including the __used macro. However,
> > > this macro alone does not prevent removal during linking, and may still
> > > yield build warnings (e.g. on mips64el):
> > >
> > > LD vmlinux
> > > BTFIDS vmlinux
> > > WARN: resolve_btfids: unresolved symbol bpf_verify_pkcs7_signature
> > > WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
> > > WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
> > > WARN: resolve_btfids: unresolved symbol bpf_key_put
> > > WARN: resolve_btfids: unresolved symbol bpf_iter_task_next
> > > WARN: resolve_btfids: unresolved symbol bpf_iter_css_task_new
> > > WARN: resolve_btfids: unresolved symbol bpf_get_file_xattr
> > > WARN: resolve_btfids: unresolved symbol bpf_ct_insert_entry
> > > WARN: resolve_btfids: unresolved symbol bpf_cgroup_release
> > > WARN: resolve_btfids: unresolved symbol bpf_cgroup_from_id
> > > WARN: resolve_btfids: unresolved symbol bpf_cgroup_acquire
> > > WARN: resolve_btfids: unresolved symbol bpf_arena_free_pages
> > > NM System.map
> > > SORTTAB vmlinux
> > > OBJCOPY vmlinux.32
> > >
> > > Update the __bpf_kfunc tag to better guard against linker optimization by
> > > including the new __retain compiler macro, which fixes the warnings above.
> > >
> > > Verify the __retain macro with readelf by checking object flags for 'R':
> > >
> > > $ readelf -Wa kernel/trace/bpf_trace.o
> > > Section Headers:
> > > [Nr] Name Type Address Off Size ES Flg Lk Inf Al
> > > ...
> > > [178] .text.bpf_key_put PROGBITS 00000000 6420 0050 00 AXR 0 0 8
> > > ...
> > > Key to Flags:
> > > ...
> > > R (retain), D (mbind), p (processor specific)
> > >
> > > Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Closes: https://lore.kernel.org/r/202401211357.OCX9yllM-lkp@intel.com/
> > > Fixes: 57e7c169cd6a ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
> > > Cc: stable@vger.kernel.org # v6.6+
> > > Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
> >
> > Thanks for your patch, which is now commit 7bdcedd5c8fb88e7
> > ("bpf: Harden __bpf_kfunc tag against linker kfunc removal") in
> > v6.10-rc5.
> >
> > This is causing build failures on ARM with
> > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y:
> >
> > net/core/filter.c:11859:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 11859 | {
> > | ^
> > net/core/filter.c:11872:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 11872 | {
> > | ^
> > net/core/filter.c:11885:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 11885 | {
> > | ^
> > net/core/filter.c:11906:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 11906 | {
> > | ^
> > net/core/filter.c:12092:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 12092 | {
> > | ^
> > net/core/xdp.c:713:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 713 | {
> > | ^
> > net/core/xdp.c:736:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 736 | {
> > | ^
> > net/core/xdp.c:769:1: error: ‘retain’ attribute ignored [-Werror=attributes]
> > 769 | {
> > | ^
> > [...]
> >
> > My compiler is arm-linux-gnueabihf-gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04).
>
> hum, so it'd mean __has_attribute(__retain__) returns true while gcc still
> ignores the retain attribute.. like in this bug which seems similar:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587
> but not sure how it got fixed.. any chance you can upgrade gcc and retest?

Indeed, __has_attribute(__retain__) returns true, while the attribute
is not supported.

My test program:

cat > /tmp/a.c <<EOF
#if __has_attribute(__retain__)
#warning __retain__ OK
#else
#warning No __retain__
#endif

int x __attribute__((__retain__));
EOF

$ arm-linux-gnueabihf-gcc-11 -c /tmp/a.c # gcc version 11.4.0 (Ubuntu
11.4.0-1ubuntu1~22.04))

/tmp/a.c:2:2: warning: #warning __retain__ OK [-Wcpp]
2 | #warning __retain__ OK
| ^~~~~~~
/tmp/a.c:7:1: warning: ‘retain’ attribute ignored [-Wattributes]
7 | int x __attribute__((__retain__));
| ^~~

Oops

$ arm-linux-gnueabihf-gcc-12 -c /tmp/a.c # gcc version 12.3.0 (Ubuntu
12.3.0-1ubuntu1~22.04)
/tmp/a.c:2:2: warning: #warning __retain__ OK [-Wcpp]
2 | #warning __retain__ OK
| ^~~~~~~

Fixed

It works fine with the native gcc-11:

$ gcc-11 -c /tmp/a.c # gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)
/tmp/a.c:2:2: warning: #warning __retain__ OK [-Wcpp]
2 | #warning __retain__ OK
| ^~~~~~~

I gave it a try on all installed gcc-11 compilers.

/usr/bin/aarch64-linux-gnu-gcc-11
/usr/bin/alpha-linux-gnu-gcc-11
/usr/bin/arm-linux-gnueabi-gcc-11
/usr/bin/arm-linux-gnueabihf-gcc-11
/usr/bin/hppa64-linux-gnu-gcc-11
/usr/bin/hppa-linux-gnu-gcc-11
/usr/bin/m68k-linux-gnu-gcc-11
/usr/bin/powerpc64le-linux-gnu-gcc-11
/usr/bin/powerpc64-linux-gnu-gcc-11
/usr/bin/powerpc-linux-gnu-gcc-11
/usr/bin/riscv64-linux-gnu-gcc-11
/usr/bin/s390x-linux-gnu-gcc-11
/usr/bin/sh4-linux-gnu-gcc-11
/usr/bin/sparc64-linux-gnu-gcc-11
/usr/bin/x86_64-linux-gnu-gcc-11
/usr/bin/x86_64-linux-gnux32-gcc-11

All of them failed (incl. x32), except for the native x86_64-linux-gnu-gcc-11.

It works fine with all installed gcc-12 compilers
(arm-linux-gnueabihf-gcc-12, m68k-linux-gnu-gcc-12, x86_64-linux-gnu-gcc-12).

With gcc-9, the absence of __retain__ is detected correctly.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds