Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[kernel test robot]

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In-Reply-To: <20201027205723.12514-1-ardb@kernel.org>
References: <20201027205723.12514-1-ardb@kernel.org>
TO: Ard Biesheuvel <ardb@kernel.org>
TO: linux-kernel(a)vger.kernel.org
CC: netdev(a)vger.kernel.org
CC: bpf(a)vger.kernel.org
CC: arnd(a)arndb.de
CC: Ard Biesheuvel <ardb@kernel.org>
CC: Nick Desaulniers <ndesaulniers@google.com>
CC: Arvind Sankar <nivedita@alum.mit.edu>
CC: Randy Dunlap <rdunlap@infradead.org>
CC: Josh Poimboeuf <jpoimboe@redhat.com>
CC: Thomas Gleixner <tglx@linutronix.de>

Hi Ard,

I love your patch! Perhaps something to improve:

[auto build test WARNING on bpf-next/master]
[also build test WARNING on bpf/master linus/master linux/master v5.10-rc1 next-20201027]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Ard-Biesheuvel/bpf-don-t-rely-on-GCC-__attribute__-optimize-to-disable-GCSE/20201028-045850
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: x86_64-randconfig-a001-20201026 (attached as .config)
compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project f2c25c70791de95d2466e09b5b58fc37f6ccd7a4)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install x86_64 cross compiling tool for clang build
        # apt-get install binutils-x86-64-linux-gnu
        # https://github.com/0day-ci/linux/commit/edebf279098c74782e939d735bd82311c0030968
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Ard-Biesheuvel/bpf-don-t-rely-on-GCC-__attribute__-optimize-to-disable-GCSE/20201028-045850
        git checkout edebf279098c74782e939d735bd82311c0030968
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=x86_64 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

>> clang-12: warning: optimization flag '-fno-gcse' is not supported [-Wignored-optimization-argument]
   kernel/bpf/core.c:1358:12: warning: no previous prototype for function 'bpf_probe_read_kernel' [-Wmissing-prototypes]
   u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
              ^
   kernel/bpf/core.c:1358:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
   ^
   static 
   1 warning generated.

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org

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Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Nick Desaulniers]

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+ Ard

(I think we knew about this in the v1, but just cc'ing you since 0day
bot did not)

On Tue, Oct 27, 2020 at 10:39 PM kernel test robot <lkp@intel.com> wrote:
>
> In-Reply-To: <20201027205723.12514-1-ardb@kernel.org>
> References: <20201027205723.12514-1-ardb@kernel.org>
> TO: Ard Biesheuvel <ardb@kernel.org>
> TO: linux-kernel(a)vger.kernel.org
> CC: netdev(a)vger.kernel.org
> CC: bpf(a)vger.kernel.org
> CC: arnd(a)arndb.de
> CC: Ard Biesheuvel <ardb@kernel.org>
> CC: Nick Desaulniers <ndesaulniers@google.com>
> CC: Arvind Sankar <nivedita@alum.mit.edu>
> CC: Randy Dunlap <rdunlap@infradead.org>
> CC: Josh Poimboeuf <jpoimboe@redhat.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
>
> Hi Ard,
>
> I love your patch! Perhaps something to improve:
>
> [auto build test WARNING on bpf-next/master]
> [also build test WARNING on bpf/master linus/master linux/master v5.10-rc1 next-20201027]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch]
>
> url:    https://github.com/0day-ci/linux/commits/Ard-Biesheuvel/bpf-don-t-rely-on-GCC-__attribute__-optimize-to-disable-GCSE/20201028-045850
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
> config: x86_64-randconfig-a001-20201026 (attached as .config)
> compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project f2c25c70791de95d2466e09b5b58fc37f6ccd7a4)
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # install x86_64 cross compiling tool for clang build
>         # apt-get install binutils-x86-64-linux-gnu
>         # https://github.com/0day-ci/linux/commit/edebf279098c74782e939d735bd82311c0030968
>         git remote add linux-review https://github.com/0day-ci/linux
>         git fetch --no-tags linux-review Ard-Biesheuvel/bpf-don-t-rely-on-GCC-__attribute__-optimize-to-disable-GCSE/20201028-045850
>         git checkout edebf279098c74782e939d735bd82311c0030968
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=x86_64
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
>
> All warnings (new ones prefixed by >>):
>
> >> clang-12: warning: optimization flag '-fno-gcse' is not supported [-Wignored-optimization-argument]
>    kernel/bpf/core.c:1358:12: warning: no previous prototype for function 'bpf_probe_read_kernel' [-Wmissing-prototypes]
>    u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>               ^
>    kernel/bpf/core.c:1358:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
>    u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>    ^
>    static
>    1 warning generated.
>
> ---
> 0-DAY CI Kernel Test Service, Intel Corporation
> https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org
>
> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe(a)googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/202010281330.KCaqOzGW-lkp%40intel.com.



-- 
Thanks,
~Nick Desaulniers

RE: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[David Laight]

From: Ard Biesheuvel
> Sent: 27 October 2020 20:57
> 
> Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> function scope __attribute__((optimize("-fno-gcse"))), to disable a
> GCC specific optimization that was causing trouble on x86 builds, and
> was not expected to have any positive effect in the first place.

Surely it is possibly to 'adjust' the bpf code so that gcc doesn't
apply (and can't apply) the gcse optimisation?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Geert Uytterhoeven]

Hi Ard,

On Tue, Oct 27, 2020 at 9:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> function scope __attribute__((optimize("-fno-gcse"))), to disable a
> GCC specific optimization that was causing trouble on x86 builds, and
> was not expected to have any positive effect in the first place.
>
> However, as the GCC manual documents, __attribute__((optimize))
> is not for production use, and results in all other optimization
> options to be forgotten for the function in question. This can
> cause all kinds of trouble, but in one particular reported case,
> it causes -fno-asynchronous-unwind-tables to be disregarded,
> resulting in .eh_frame info to be emitted for the function
> inadvertently.
>
> This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> optimization for the entire source file, but only when building for
> X86.
>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Arvind Sankar <nivedita@alum.mit.edu>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Kees Cook <keescook@chromium.org>
> Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Thanks, this gets rid of the following warning, which you may
want to quote in the patch description:

    aarch64-linux-gnu-ld: warning: orphan section `.eh_frame' from
`kernel/bpf/core.o' being placed in section `.eh_frame'

Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>

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

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Ard Biesheuvel]

On Wed, 28 Oct 2020 at 07:51, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Wed, 28 Oct 2020 at 00:04, Daniel Borkmann <daniel@iogearbox.net> wrote:
> >
> > On 10/27/20 9:57 PM, Ard Biesheuvel wrote:
> > > Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> > > ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> > > function scope __attribute__((optimize("-fno-gcse"))), to disable a
> > > GCC specific optimization that was causing trouble on x86 builds, and
> > > was not expected to have any positive effect in the first place.
> > >
> > > However, as the GCC manual documents, __attribute__((optimize))
> > > is not for production use, and results in all other optimization
> > > options to be forgotten for the function in question. This can
> > > cause all kinds of trouble, but in one particular reported case,
> >
> > Looks like there are couple more as well aside from __no_fgcse, are you
> > also planning to fix them?
> >
> >    arch/powerpc/kernel/setup.h:14:#define __nostackprotector __attribute__((__optimize__("no-stack-protector")))
> >    tools/include/linux/compiler-gcc.h:37:#define __no_tail_call __attribute__((optimize("no-optimize-sibling-calls")))
> >
>
> No, but we can notify the respective maintainers.
>
> > > it causes -fno-asynchronous-unwind-tables to be disregarded,
> > > resulting in .eh_frame info to be emitted for the function
> > > inadvertently.
> >
> > Would have been useful to add a pointer to the original discussion with
> > Link tag.
> >
> > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/
> >
>
> Agreed.
>
> > > This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> > > optimization for the entire source file, but only when building for
> > > X86.
> > >
> > > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > > Cc: Arvind Sankar <nivedita@alum.mit.edu>
> > > Cc: Randy Dunlap <rdunlap@infradead.org>
> > > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Alexei Starovoitov <ast@kernel.org>
> > > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > [...]
> > > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > > index bdc8cd1b6767..02b58f44c479 100644
> > > --- a/kernel/bpf/Makefile
> > > +++ b/kernel/bpf/Makefile
> > > @@ -1,6 +1,8 @@
> > >   # SPDX-License-Identifier: GPL-2.0
> > >   obj-y := core.o
> > > -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> > > +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> > > +cflags-core-$(CONFIG_X86) := -fno-gcse
> > > +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
> >
> > Also, this needs to be guarded behind !CONFIG_RETPOLINE and !CONFIG_BPF_JIT_ALWAYS_ON
> > in particular the latter since only in this case interpreter is compiled in ... most
> > distros have the CONFIG_BPF_JIT_ALWAYS_ON set these days for x86.
> >
>
> Is that a new requirement? Because before this patch, -fno-gcse was
> applied unconditionally.
>

Ah never mind. You are saying ___bpf_prog_run() does not even exist if
CONFIG_BPF_JIT_ALWAYS_ON=y, right?


> > Do you have an analysis for the commit log on what else this penalizes in core.c if
> > it's now for the entire translation unit?
> >
>
> No, I simply observed the regression this caused on non-x86
> architectures, and proposed a way to fix it.
>
> Do you have any concerns in particular regarding other things in
> core.c? Would you prefer ___bpf_prog_run() to be moved into a separate
> .c file?
>
>
> > >   obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
> > >   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
> > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > index 9268d77898b7..55454d2278b1 100644
> > > --- a/kernel/bpf/core.c
> > > +++ b/kernel/bpf/core.c
> > > @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
> > >    *
> > >    * Decode and execute eBPF instructions.
> > >    */
> > > -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > > +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > >   {
> > >   #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
> > >   #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> > >
> >

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Ard Biesheuvel]

On Wed, 28 Oct 2020 at 00:04, Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 10/27/20 9:57 PM, Ard Biesheuvel wrote:
> > Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> > ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> > function scope __attribute__((optimize("-fno-gcse"))), to disable a
> > GCC specific optimization that was causing trouble on x86 builds, and
> > was not expected to have any positive effect in the first place.
> >
> > However, as the GCC manual documents, __attribute__((optimize))
> > is not for production use, and results in all other optimization
> > options to be forgotten for the function in question. This can
> > cause all kinds of trouble, but in one particular reported case,
>
> Looks like there are couple more as well aside from __no_fgcse, are you
> also planning to fix them?
>
>    arch/powerpc/kernel/setup.h:14:#define __nostackprotector __attribute__((__optimize__("no-stack-protector")))
>    tools/include/linux/compiler-gcc.h:37:#define __no_tail_call __attribute__((optimize("no-optimize-sibling-calls")))
>

No, but we can notify the respective maintainers.

> > it causes -fno-asynchronous-unwind-tables to be disregarded,
> > resulting in .eh_frame info to be emitted for the function
> > inadvertently.
>
> Would have been useful to add a pointer to the original discussion with
> Link tag.
>
> Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/
>

Agreed.

> > This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> > optimization for the entire source file, but only when building for
> > X86.
> >
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Arvind Sankar <nivedita@alum.mit.edu>
> > Cc: Randy Dunlap <rdunlap@infradead.org>
> > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > Cc: Kees Cook <keescook@chromium.org>
> > Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> [...]
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index bdc8cd1b6767..02b58f44c479 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -1,6 +1,8 @@
> >   # SPDX-License-Identifier: GPL-2.0
> >   obj-y := core.o
> > -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> > +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> > +cflags-core-$(CONFIG_X86) := -fno-gcse
> > +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
>
> Also, this needs to be guarded behind !CONFIG_RETPOLINE and !CONFIG_BPF_JIT_ALWAYS_ON
> in particular the latter since only in this case interpreter is compiled in ... most
> distros have the CONFIG_BPF_JIT_ALWAYS_ON set these days for x86.
>

Is that a new requirement? Because before this patch, -fno-gcse was
applied unconditionally.

> Do you have an analysis for the commit log on what else this penalizes in core.c if
> it's now for the entire translation unit?
>

No, I simply observed the regression this caused on non-x86
architectures, and proposed a way to fix it.

Do you have any concerns in particular regarding other things in
core.c? Would you prefer ___bpf_prog_run() to be moved into a separate
.c file?


> >   obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
> >   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 9268d77898b7..55454d2278b1 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
> >    *
> >    * Decode and execute eBPF instructions.
> >    */
> > -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> >   {
> >   #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
> >   #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> >
>

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Nick Desaulniers]

On Tue, Oct 27, 2020 at 4:04 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 10/27/20 9:57 PM, Ard Biesheuvel wrote:
> > Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> > ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> > function scope __attribute__((optimize("-fno-gcse"))), to disable a
> > GCC specific optimization that was causing trouble on x86 builds, and
> > was not expected to have any positive effect in the first place.
> >
> > However, as the GCC manual documents, __attribute__((optimize))
> > is not for production use, and results in all other optimization
> > options to be forgotten for the function in question. This can
> > cause all kinds of trouble, but in one particular reported case,
>
> Looks like there are couple more as well aside from __no_fgcse, are you
> also planning to fix them?
>
>    arch/powerpc/kernel/setup.h:14:#define __nostackprotector __attribute__((__optimize__("no-stack-protector")))

GCC literally just landed support for
__attribute__((no_stack_protector)) a few days ago.  I was planning on
sending a patch adding it to compiler_attributes.h, but we won't be
able to rely on it for a while.  Now I see I'll have to clean up ppc a
bit. Surely they've had bugs related to optimize attribute
unexpectedly dropping flags.

>    tools/include/linux/compiler-gcc.h:37:#define __no_tail_call __attribute__((optimize("no-optimize-sibling-calls")))

Only used in perf?
tools/perf/tests/dwarf-unwind.c

>
> > it causes -fno-asynchronous-unwind-tables to be disregarded,
> > resulting in .eh_frame info to be emitted for the function
> > inadvertently.
>
> Would have been useful to add a pointer to the original discussion with
> Link tag.
>
> Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/
>
> > This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> > optimization for the entire source file, but only when building for
> > X86.
> >
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Arvind Sankar <nivedita@alum.mit.edu>
> > Cc: Randy Dunlap <rdunlap@infradead.org>
> > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > Cc: Kees Cook <keescook@chromium.org>
> > Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> [...]
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index bdc8cd1b6767..02b58f44c479 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -1,6 +1,8 @@
> >   # SPDX-License-Identifier: GPL-2.0
> >   obj-y := core.o
> > -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> > +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> > +cflags-core-$(CONFIG_X86) := -fno-gcse
> > +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
>
> Also, this needs to be guarded behind !CONFIG_RETPOLINE and !CONFIG_BPF_JIT_ALWAYS_ON
> in particular the latter since only in this case interpreter is compiled in ... most
> distros have the CONFIG_BPF_JIT_ALWAYS_ON set these days for x86.
>
> Do you have an analysis for the commit log on what else this penalizes in core.c if
> it's now for the entire translation unit?
>
> >   obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
> >   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 9268d77898b7..55454d2278b1 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
> >    *
> >    * Decode and execute eBPF instructions.
> >    */
> > -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> >   {
> >   #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
> >   #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> >
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Daniel Borkmann]

On 10/27/20 9:57 PM, Ard Biesheuvel wrote:
> Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> function scope __attribute__((optimize("-fno-gcse"))), to disable a
> GCC specific optimization that was causing trouble on x86 builds, and
> was not expected to have any positive effect in the first place.
> 
> However, as the GCC manual documents, __attribute__((optimize))
> is not for production use, and results in all other optimization
> options to be forgotten for the function in question. This can
> cause all kinds of trouble, but in one particular reported case,

Looks like there are couple more as well aside from __no_fgcse, are you
also planning to fix them?

   arch/powerpc/kernel/setup.h:14:#define __nostackprotector __attribute__((__optimize__("no-stack-protector")))
   tools/include/linux/compiler-gcc.h:37:#define __no_tail_call	__attribute__((optimize("no-optimize-sibling-calls")))

> it causes -fno-asynchronous-unwind-tables to be disregarded,
> resulting in .eh_frame info to be emitted for the function
> inadvertently.

Would have been useful to add a pointer to the original discussion with
Link tag.

Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/

> This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> optimization for the entire source file, but only when building for
> X86.
> 
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Arvind Sankar <nivedita@alum.mit.edu>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Kees Cook <keescook@chromium.org>
> Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
[...]
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index bdc8cd1b6767..02b58f44c479 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -1,6 +1,8 @@
>   # SPDX-License-Identifier: GPL-2.0
>   obj-y := core.o
> -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> +cflags-core-$(CONFIG_X86) := -fno-gcse
> +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)

Also, this needs to be guarded behind !CONFIG_RETPOLINE and !CONFIG_BPF_JIT_ALWAYS_ON
in particular the latter since only in this case interpreter is compiled in ... most
distros have the CONFIG_BPF_JIT_ALWAYS_ON set these days for x86.

Do you have an analysis for the commit log on what else this penalizes in core.c if
it's now for the entire translation unit?

>   obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
>   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 9268d77898b7..55454d2278b1 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>    *
>    * Decode and execute eBPF instructions.
>    */
> -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
>   {
>   #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
>   #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> 

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Nick Desaulniers]

On Tue, Oct 27, 2020 at 3:23 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 27 Oct 2020 at 23:03, Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Tue, Oct 27, 2020 at 2:50 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > >
> > > On Tue, 27 Oct 2020 at 22:20, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > > >
> > > > On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > >
> > > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > > > > index 6e390d58a9f8..ac3fa37a84f9 100644
> > > > > --- a/include/linux/compiler_types.h
> > > > > +++ b/include/linux/compiler_types.h
> > > > > @@ -247,10 +247,6 @@ struct ftrace_likely_data {
> > > > >  #define asm_inline asm
> > > > >  #endif
> > > > >
> > > > > -#ifndef __no_fgcse
> > > > > -# define __no_fgcse
> > > > > -#endif
> > > > > -
> > > > Finally, this is going to disable GCSE for the whole translation unit,
> > > > which may be overkill.   Previously it was isolated to one function
> > > > definition.  You could lower the definition of the preprocessor define
> > > > into kernel/bpf/core.c to keep its use isolated as far as possible.
> > > >
> > >
> > > Which preprocessor define?
> >
> > __no_fgcse
> >
>
> But we can't use that, that is the whole point of this patch.

Ah, right because the attribute drops other command line flags...ok,
then -fno-gcse the whole translation unit it is then.

Still need to avoid new warnings with clang though.
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Ard Biesheuvel]

On Tue, 27 Oct 2020 at 23:03, Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Tue, Oct 27, 2020 at 2:50 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > On Tue, 27 Oct 2020 at 22:20, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > >
> > > On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > >
> > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > > > index 6e390d58a9f8..ac3fa37a84f9 100644
> > > > --- a/include/linux/compiler_types.h
> > > > +++ b/include/linux/compiler_types.h
> > > > @@ -247,10 +247,6 @@ struct ftrace_likely_data {
> > > >  #define asm_inline asm
> > > >  #endif
> > > >
> > > > -#ifndef __no_fgcse
> > > > -# define __no_fgcse
> > > > -#endif
> > > > -
> > > Finally, this is going to disable GCSE for the whole translation unit,
> > > which may be overkill.   Previously it was isolated to one function
> > > definition.  You could lower the definition of the preprocessor define
> > > into kernel/bpf/core.c to keep its use isolated as far as possible.
> > >
> >
> > Which preprocessor define?
>
> __no_fgcse
>

But we can't use that, that is the whole point of this patch.

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Nick Desaulniers]

On Tue, Oct 27, 2020 at 2:50 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 27 Oct 2020 at 22:20, Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > >
> > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > > index 6e390d58a9f8..ac3fa37a84f9 100644
> > > --- a/include/linux/compiler_types.h
> > > +++ b/include/linux/compiler_types.h
> > > @@ -247,10 +247,6 @@ struct ftrace_likely_data {
> > >  #define asm_inline asm
> > >  #endif
> > >
> > > -#ifndef __no_fgcse
> > > -# define __no_fgcse
> > > -#endif
> > > -
> > Finally, this is going to disable GCSE for the whole translation unit,
> > which may be overkill.   Previously it was isolated to one function
> > definition.  You could lower the definition of the preprocessor define
> > into kernel/bpf/core.c to keep its use isolated as far as possible.
> >
>
> Which preprocessor define?

__no_fgcse

>
> > I'm fine with either approach, but we should avoid new warnings for
> > clang.  Thanks for the patch!

-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Ard Biesheuvel]

On Tue, 27 Oct 2020 at 22:20, Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> > ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> > function scope __attribute__((optimize("-fno-gcse"))), to disable a
> > GCC specific optimization that was causing trouble on x86 builds, and
> > was not expected to have any positive effect in the first place.
> >
> > However, as the GCC manual documents, __attribute__((optimize))
> > is not for production use, and results in all other optimization
> > options to be forgotten for the function in question. This can
> > cause all kinds of trouble, but in one particular reported case,
> > it causes -fno-asynchronous-unwind-tables to be disregarded,
> > resulting in .eh_frame info to be emitted for the function
> > inadvertently.
> >
> > This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> > optimization for the entire source file, but only when building for
> > X86.
> >
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Arvind Sankar <nivedita@alum.mit.edu>
> > Cc: Randy Dunlap <rdunlap@infradead.org>
> > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > Cc: Kees Cook <keescook@chromium.org>
> > Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >  include/linux/compiler-gcc.h   | 2 --
> >  include/linux/compiler_types.h | 4 ----
> >  kernel/bpf/Makefile            | 4 +++-
> >  kernel/bpf/core.c              | 2 +-
> >  4 files changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index d1e3c6896b71..5deb37024574 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -175,5 +175,3 @@
> >  #else
> >  #define __diag_GCC_8(s)
> >  #endif
> > -
> > -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > index 6e390d58a9f8..ac3fa37a84f9 100644
> > --- a/include/linux/compiler_types.h
> > +++ b/include/linux/compiler_types.h
> > @@ -247,10 +247,6 @@ struct ftrace_likely_data {
> >  #define asm_inline asm
> >  #endif
> >
> > -#ifndef __no_fgcse
> > -# define __no_fgcse
> > -#endif
> > -
> >  /* Are two types/vars the same type (ignoring qualifiers)? */
> >  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
> >
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index bdc8cd1b6767..02b58f44c479 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -1,6 +1,8 @@
> >  # SPDX-License-Identifier: GPL-2.0
> >  obj-y := core.o
> > -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> > +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> > +cflags-core-$(CONFIG_X86) := -fno-gcse
>
> -fno-gcse is not recognized by clang and will produce
> -Wignored-optimization-argument.  It should at least be wrapped in
> cc-option, though since it's unlikely to ever not be compiler
> specific, I think it might be ok to guard with `ifdef
> CONFIG_CC_IS_GCC`.  Also, might we want to only do this for `ifndef
> CONFIG_RETPOLINE`, based on 3193c0836f203?
>
> Finally, this is going to disable GCSE for the whole translation unit,
> which may be overkill.   Previously it was isolated to one function
> definition.  You could lower the definition of the preprocessor define
> into kernel/bpf/core.c to keep its use isolated as far as possible.
>

Which preprocessor define?

> I'm fine with either approach, but we should avoid new warnings for
> clang.  Thanks for the patch!
>
> > +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
> >
> >  obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
> >  obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 9268d77898b7..55454d2278b1 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
> >   *
> >   * Decode and execute eBPF instructions.
> >   */
> > -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> >  {
> >  #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
> >  #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> > --
> > 2.17.1
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Nick Desaulniers]

On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
> ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> function scope __attribute__((optimize("-fno-gcse"))), to disable a
> GCC specific optimization that was causing trouble on x86 builds, and
> was not expected to have any positive effect in the first place.
>
> However, as the GCC manual documents, __attribute__((optimize))
> is not for production use, and results in all other optimization
> options to be forgotten for the function in question. This can
> cause all kinds of trouble, but in one particular reported case,
> it causes -fno-asynchronous-unwind-tables to be disregarded,
> resulting in .eh_frame info to be emitted for the function
> inadvertently.
>
> This reverts commit 3193c0836f203, and instead, it disables the -fgcse
> optimization for the entire source file, but only when building for
> X86.
>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Arvind Sankar <nivedita@alum.mit.edu>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Kees Cook <keescook@chromium.org>
> Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  include/linux/compiler-gcc.h   | 2 --
>  include/linux/compiler_types.h | 4 ----
>  kernel/bpf/Makefile            | 4 +++-
>  kernel/bpf/core.c              | 2 +-
>  4 files changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d1e3c6896b71..5deb37024574 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -175,5 +175,3 @@
>  #else
>  #define __diag_GCC_8(s)
>  #endif
> -
> -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 6e390d58a9f8..ac3fa37a84f9 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -247,10 +247,6 @@ struct ftrace_likely_data {
>  #define asm_inline asm
>  #endif
>
> -#ifndef __no_fgcse
> -# define __no_fgcse
> -#endif
> -
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
>
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index bdc8cd1b6767..02b58f44c479 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -1,6 +1,8 @@
>  # SPDX-License-Identifier: GPL-2.0
>  obj-y := core.o
> -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> +cflags-core-$(CONFIG_X86) := -fno-gcse

-fno-gcse is not recognized by clang and will produce
-Wignored-optimization-argument.  It should at least be wrapped in
cc-option, though since it's unlikely to ever not be compiler
specific, I think it might be ok to guard with `ifdef
CONFIG_CC_IS_GCC`.  Also, might we want to only do this for `ifndef
CONFIG_RETPOLINE`, based on 3193c0836f203?

Finally, this is going to disable GCSE for the whole translation unit,
which may be overkill.   Previously it was isolated to one function
definition.  You could lower the definition of the preprocessor define
into kernel/bpf/core.c to keep its use isolated as far as possible.

I'm fine with either approach, but we should avoid new warnings for
clang.  Thanks for the patch!

> +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
>
>  obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
>  obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 9268d77898b7..55454d2278b1 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>   *
>   * Decode and execute eBPF instructions.
>   */
> -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
>  {
>  #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
>  #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> --
> 2.17.1
>


-- 
Thanks,
~Nick Desaulniers

[PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Ard Biesheuvel]

Commit 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for
___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
function scope __attribute__((optimize("-fno-gcse"))), to disable a
GCC specific optimization that was causing trouble on x86 builds, and
was not expected to have any positive effect in the first place.

However, as the GCC manual documents, __attribute__((optimize))
is not for production use, and results in all other optimization
options to be forgotten for the function in question. This can
cause all kinds of trouble, but in one particular reported case,
it causes -fno-asynchronous-unwind-tables to be disregarded,
resulting in .eh_frame info to be emitted for the function
inadvertently.

This reverts commit 3193c0836f203, and instead, it disables the -fgcse
optimization for the entire source file, but only when building for
X86.

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Kees Cook <keescook@chromium.org>
Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 include/linux/compiler-gcc.h   | 2 --
 include/linux/compiler_types.h | 4 ----
 kernel/bpf/Makefile            | 4 +++-
 kernel/bpf/core.c              | 2 +-
 4 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d1e3c6896b71..5deb37024574 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -175,5 +175,3 @@
 #else
 #define __diag_GCC_8(s)
 #endif
-
-#define __no_fgcse __attribute__((optimize("-fno-gcse")))
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 6e390d58a9f8..ac3fa37a84f9 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -247,10 +247,6 @@ struct ftrace_likely_data {
 #define asm_inline asm
 #endif
 
-#ifndef __no_fgcse
-# define __no_fgcse
-#endif
-
 /* Are two types/vars the same type (ignoring qualifiers)? */
 #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
 
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index bdc8cd1b6767..02b58f44c479 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-y := core.o
-CFLAGS_core.o += $(call cc-disable-warning, override-init)
+# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
+cflags-core-$(CONFIG_X86) := -fno-gcse
+CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
 
 obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 9268d77898b7..55454d2278b1 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
  *
  * Decode and execute eBPF instructions.
  */
-static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
+static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
 {
 #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
 #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
-- 
2.17.1