| Message ID | 20201028171506.15682-2-ardb@kernel.org (mailing list archive) |
|---|---|
| State | Accepted |
| Delegated to: | BPF |
| Headers | show |
| Series | get rid of GCC __attribute__((optimize)) for BPF | expand |
| Context | Check | Description |
|---|---|---|
| tsipa740/Travis | fail | vmtest |
| bpf/vmtest | fail | Travis CI - Pull Request |
On Wed, 28 Oct 2020 at 22:39, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > > On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel wrote: > > Commit 3193c0836 ("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. > > > > This reverts commit 3193c0836, and instead, it disables the -fgcse > > optimization for the entire source file, but only when building for > > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > > original commit states that CONFIG_RETPOLINE=n triggers the issue, > > whereas CONFIG_RETPOLINE=y performs better without the optimization, > > so it is kept disabled in both cases. > > > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > --- > > include/linux/compiler-gcc.h | 2 -- > > include/linux/compiler_types.h | 4 ---- > > kernel/bpf/Makefile | 6 +++++- > > kernel/bpf/core.c | 2 +- > > 4 files changed, 6 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"))) > > See my reply in the other thread. > I prefer > -#define __no_fgcse __attribute__((optimize("-fno-gcse"))) > +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > Potentially with -fno-asynchronous-unwind-tables. > So how would that work? arm64 has the following: KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables ifeq ($(CONFIG_SHADOW_CALL_STACK), y) KBUILD_CFLAGS += -ffixed-x18 endif and it adds -fpatchable-function-entry=2 for compilers that support it, but only when CONFIG_FTRACE is enabled. Also, as Nick pointed out, -fno-gcse does not work on Clang. Every architecture will have a different set of requirements here. And there is no way of knowing which -f options are disregarded when you use the function attribute. So how on earth are you going to #define __no-fgcse correctly for every configuration imaginable? > __attribute__((optimize("")) is not as broken as you're claiming to be. > It has quirky gcc internal logic, but it's still widely used > in many software projects. So it's fine because it is only a little bit broken? I'm sorry, but that makes no sense whatsoever. If you insist on sticking with this broken construct, can you please make it GCC/x86-only at least?
On Wed, Oct 28, 2020 at 11:15:04PM +0100, Ard Biesheuvel wrote: > On Wed, 28 Oct 2020 at 22:39, Alexei Starovoitov > <alexei.starovoitov@gmail.com> wrote: > > > > On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel wrote: > > > Commit 3193c0836 ("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. > > > > > > This reverts commit 3193c0836, and instead, it disables the -fgcse > > > optimization for the entire source file, but only when building for > > > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > > > original commit states that CONFIG_RETPOLINE=n triggers the issue, > > > whereas CONFIG_RETPOLINE=y performs better without the optimization, > > > so it is kept disabled in both cases. > > > > > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > > > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > --- > > > include/linux/compiler-gcc.h | 2 -- > > > include/linux/compiler_types.h | 4 ---- > > > kernel/bpf/Makefile | 6 +++++- > > > kernel/bpf/core.c | 2 +- > > > 4 files changed, 6 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"))) > > > > See my reply in the other thread. > > I prefer > > -#define __no_fgcse __attribute__((optimize("-fno-gcse"))) > > +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > > > Potentially with -fno-asynchronous-unwind-tables. > > > > So how would that work? arm64 has the following: > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables > > ifeq ($(CONFIG_SHADOW_CALL_STACK), y) > KBUILD_CFLAGS += -ffixed-x18 > endif > > and it adds -fpatchable-function-entry=2 for compilers that support > it, but only when CONFIG_FTRACE is enabled. I think you're assuming that GCC drops all flags when it sees __attribute__((optimize)). That's not the case. > Also, as Nick pointed out, -fno-gcse does not work on Clang. yes and what's the point? #define __no_fgcse is GCC only. clang doesn't need this workaround. > Every architecture will have a different set of requirements here. And > there is no way of knowing which -f options are disregarded when you > use the function attribute. > > So how on earth are you going to #define __no-fgcse correctly for > every configuration imaginable? > > > __attribute__((optimize("")) is not as broken as you're claiming to be. > > It has quirky gcc internal logic, but it's still widely used > > in many software projects. > > So it's fine because it is only a little bit broken? I'm sorry, but > that makes no sense whatsoever. > > If you insist on sticking with this broken construct, can you please > make it GCC/x86-only at least? I'm totally fine with making #define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) to be gcc+x86 only. I'd like to get rid of it, but objtool is not smart enough to understand generated asm without it.
On Wed, 28 Oct 2020 at 23:59, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > > On Wed, Oct 28, 2020 at 11:15:04PM +0100, Ard Biesheuvel wrote: > > On Wed, 28 Oct 2020 at 22:39, Alexei Starovoitov > > <alexei.starovoitov@gmail.com> wrote: > > > > > > On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel wrote: > > > > Commit 3193c0836 ("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. > > > > > > > > This reverts commit 3193c0836, and instead, it disables the -fgcse > > > > optimization for the entire source file, but only when building for > > > > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > > > > original commit states that CONFIG_RETPOLINE=n triggers the issue, > > > > whereas CONFIG_RETPOLINE=y performs better without the optimization, > > > > so it is kept disabled in both cases. > > > > > > > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > > > > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > --- > > > > include/linux/compiler-gcc.h | 2 -- > > > > include/linux/compiler_types.h | 4 ---- > > > > kernel/bpf/Makefile | 6 +++++- > > > > kernel/bpf/core.c | 2 +- > > > > 4 files changed, 6 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"))) > > > > > > See my reply in the other thread. > > > I prefer > > > -#define __no_fgcse __attribute__((optimize("-fno-gcse"))) > > > +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > > > > > Potentially with -fno-asynchronous-unwind-tables. > > > > > > > So how would that work? arm64 has the following: > > > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables > > > > ifeq ($(CONFIG_SHADOW_CALL_STACK), y) > > KBUILD_CFLAGS += -ffixed-x18 > > endif > > > > and it adds -fpatchable-function-entry=2 for compilers that support > > it, but only when CONFIG_FTRACE is enabled. > > I think you're assuming that GCC drops all flags when it sees __attribute__((optimize)). > That's not the case. > So which flags does it drop, and which doesn't it drop? Is that documented somewhere? Is that the same for all versions of GCC? > > Also, as Nick pointed out, -fno-gcse does not work on Clang. > > yes and what's the point? > #define __no_fgcse is GCC only. clang doesn't need this workaround. > Ah ok, that's at least something. > > Every architecture will have a different set of requirements here. And > > there is no way of knowing which -f options are disregarded when you > > use the function attribute. > > > > So how on earth are you going to #define __no-fgcse correctly for > > every configuration imaginable? > > > > > __attribute__((optimize("")) is not as broken as you're claiming to be. > > > It has quirky gcc internal logic, but it's still widely used > > > in many software projects. > > > > So it's fine because it is only a little bit broken? I'm sorry, but > > that makes no sense whatsoever. > > > > If you insist on sticking with this broken construct, can you please > > make it GCC/x86-only at least? > > I'm totally fine with making > #define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > to be gcc+x86 only. > I'd like to get rid of it, but objtool is not smart enough to understand > generated asm without it. I'll defer to the x86 folks to make the final call here, but I would be perfectly happy doing index d1e3c6896b71..68ddb91fbcc6 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -176,4 +176,6 @@ #define __diag_GCC_8(s) #endif +#ifdef CONFIG_X86 #define __no_fgcse __attribute__((optimize("-fno-gcse"))) +#endif and end the conversation here, because I honestly cannot wrap my head around the fact that you are willing to work around an x86 specific objtool shortcoming by arbitrarily disabling some GCC optimization for all architectures, using a construct that may or may not affect other compiler settings in unpredictable ways, where the compiler is being used to compile a BPF language runtime for executing BPF programs inside the kernel. What on earth could go wrong?
On Thu, Oct 29, 2020 at 12:10:52AM +0100, Ard Biesheuvel wrote: > On Wed, 28 Oct 2020 at 23:59, Alexei Starovoitov > <alexei.starovoitov@gmail.com> wrote: > > > > On Wed, Oct 28, 2020 at 11:15:04PM +0100, Ard Biesheuvel wrote: > > > On Wed, 28 Oct 2020 at 22:39, Alexei Starovoitov > > > <alexei.starovoitov@gmail.com> wrote: > > > > > > > > On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel wrote: > > > > > Commit 3193c0836 ("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. > > > > > > > > > > This reverts commit 3193c0836, and instead, it disables the -fgcse > > > > > optimization for the entire source file, but only when building for > > > > > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > > > > > original commit states that CONFIG_RETPOLINE=n triggers the issue, > > > > > whereas CONFIG_RETPOLINE=y performs better without the optimization, > > > > > so it is kept disabled in both cases. > > > > > > > > > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > > > > > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > > --- > > > > > include/linux/compiler-gcc.h | 2 -- > > > > > include/linux/compiler_types.h | 4 ---- > > > > > kernel/bpf/Makefile | 6 +++++- > > > > > kernel/bpf/core.c | 2 +- > > > > > 4 files changed, 6 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"))) > > > > > > > > See my reply in the other thread. > > > > I prefer > > > > -#define __no_fgcse __attribute__((optimize("-fno-gcse"))) > > > > +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > > > > > > > Potentially with -fno-asynchronous-unwind-tables. > > > > > > > > > > So how would that work? arm64 has the following: > > > > > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables > > > > > > ifeq ($(CONFIG_SHADOW_CALL_STACK), y) > > > KBUILD_CFLAGS += -ffixed-x18 > > > endif > > > > > > and it adds -fpatchable-function-entry=2 for compilers that support > > > it, but only when CONFIG_FTRACE is enabled. > > > > I think you're assuming that GCC drops all flags when it sees __attribute__((optimize)). > > That's not the case. > > > > So which flags does it drop, and which doesn't it drop? Is that > documented somewhere? Is that the same for all versions of GCC? > > > > Also, as Nick pointed out, -fno-gcse does not work on Clang. > > > > yes and what's the point? > > #define __no_fgcse is GCC only. clang doesn't need this workaround. > > > > Ah ok, that's at least something. > > > > Every architecture will have a different set of requirements here. And > > > there is no way of knowing which -f options are disregarded when you > > > use the function attribute. > > > > > > So how on earth are you going to #define __no-fgcse correctly for > > > every configuration imaginable? > > > > > > > __attribute__((optimize("")) is not as broken as you're claiming to be. > > > > It has quirky gcc internal logic, but it's still widely used > > > > in many software projects. > > > > > > So it's fine because it is only a little bit broken? I'm sorry, but > > > that makes no sense whatsoever. > > > > > > If you insist on sticking with this broken construct, can you please > > > make it GCC/x86-only at least? > > > > I'm totally fine with making > > #define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > to be gcc+x86 only. > > I'd like to get rid of it, but objtool is not smart enough to understand > > generated asm without it. > > I'll defer to the x86 folks to make the final call here, but I would > be perfectly happy doing > > index d1e3c6896b71..68ddb91fbcc6 100644 > --- a/include/linux/compiler-gcc.h > +++ b/include/linux/compiler-gcc.h > @@ -176,4 +176,6 @@ > #define __diag_GCC_8(s) > #endif > > +#ifdef CONFIG_X86 > #define __no_fgcse __attribute__((optimize("-fno-gcse"))) > +#endif If you're going to submit this patch could you please add ,-fno-omit-frame-pointer to the above as well? > and end the conversation here, because I honestly cannot wrap my head > around the fact that you are willing to work around an x86 specific > objtool shortcoming by arbitrarily disabling some GCC optimization for > all architectures, using a construct that may or may not affect other > compiler settings in unpredictable ways, where the compiler is being > used to compile a BPF language runtime for executing BPF programs > inside the kernel. > > What on earth could go wrong? Frankly I'm move worried that -Os will generate incorrect code. All compilers have bugs. Kernel has bugs. What can go wrong?
On Wed, Oct 28, 2020 at 04:20:01PM -0700, Alexei Starovoitov wrote: > On Thu, Oct 29, 2020 at 12:10:52AM +0100, Ard Biesheuvel wrote: > > On Wed, 28 Oct 2020 at 23:59, Alexei Starovoitov > > <alexei.starovoitov@gmail.com> wrote: > > > > > > On Wed, Oct 28, 2020 at 11:15:04PM +0100, Ard Biesheuvel wrote: > > > > On Wed, 28 Oct 2020 at 22:39, Alexei Starovoitov > > > > <alexei.starovoitov@gmail.com> wrote: > > > > > > > > > > On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel wrote: > > > > > > Commit 3193c0836 ("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. > > > > > > > > > > > > This reverts commit 3193c0836, and instead, it disables the -fgcse > > > > > > optimization for the entire source file, but only when building for > > > > > > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > > > > > > original commit states that CONFIG_RETPOLINE=n triggers the issue, > > > > > > whereas CONFIG_RETPOLINE=y performs better without the optimization, > > > > > > so it is kept disabled in both cases. > > > > > > > > > > > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > > > > > > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > > > --- > > > > > > include/linux/compiler-gcc.h | 2 -- > > > > > > include/linux/compiler_types.h | 4 ---- > > > > > > kernel/bpf/Makefile | 6 +++++- > > > > > > kernel/bpf/core.c | 2 +- > > > > > > 4 files changed, 6 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"))) > > > > > > > > > > See my reply in the other thread. > > > > > I prefer > > > > > -#define __no_fgcse __attribute__((optimize("-fno-gcse"))) > > > > > +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > > > > > > > > > Potentially with -fno-asynchronous-unwind-tables. > > > > > > > > > > > > > So how would that work? arm64 has the following: > > > > > > > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables > > > > > > > > ifeq ($(CONFIG_SHADOW_CALL_STACK), y) > > > > KBUILD_CFLAGS += -ffixed-x18 > > > > endif > > > > > > > > and it adds -fpatchable-function-entry=2 for compilers that support > > > > it, but only when CONFIG_FTRACE is enabled. > > > > > > I think you're assuming that GCC drops all flags when it sees __attribute__((optimize)). > > > That's not the case. > > > > > > > So which flags does it drop, and which doesn't it drop? Is that > > documented somewhere? Is that the same for all versions of GCC? > > > > > > Also, as Nick pointed out, -fno-gcse does not work on Clang. > > > > > > yes and what's the point? > > > #define __no_fgcse is GCC only. clang doesn't need this workaround. > > > > > > > Ah ok, that's at least something. > > > > > > Every architecture will have a different set of requirements here. And > > > > there is no way of knowing which -f options are disregarded when you > > > > use the function attribute. > > > > > > > > So how on earth are you going to #define __no-fgcse correctly for > > > > every configuration imaginable? > > > > > > > > > __attribute__((optimize("")) is not as broken as you're claiming to be. > > > > > It has quirky gcc internal logic, but it's still widely used > > > > > in many software projects. > > > > > > > > So it's fine because it is only a little bit broken? I'm sorry, but > > > > that makes no sense whatsoever. > > > > > > > > If you insist on sticking with this broken construct, can you please > > > > make it GCC/x86-only at least? > > > > > > I'm totally fine with making > > > #define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > > to be gcc+x86 only. > > > I'd like to get rid of it, but objtool is not smart enough to understand > > > generated asm without it. > > > > I'll defer to the x86 folks to make the final call here, but I would > > be perfectly happy doing > > > > index d1e3c6896b71..68ddb91fbcc6 100644 > > --- a/include/linux/compiler-gcc.h > > +++ b/include/linux/compiler-gcc.h > > @@ -176,4 +176,6 @@ > > #define __diag_GCC_8(s) > > #endif > > > > +#ifdef CONFIG_X86 > > #define __no_fgcse __attribute__((optimize("-fno-gcse"))) > > +#endif > > If you're going to submit this patch could you please add > ,-fno-omit-frame-pointer > to the above as well? > > > and end the conversation here, because I honestly cannot wrap my head > > around the fact that you are willing to work around an x86 specific > > objtool shortcoming by arbitrarily disabling some GCC optimization for > > all architectures, using a construct that may or may not affect other > > compiler settings in unpredictable ways, where the compiler is being > > used to compile a BPF language runtime for executing BPF programs > > inside the kernel. > > > > What on earth could go wrong? > > Frankly I'm move worried that -Os will generate incorrect code. > All compilers have bugs. Kernel has bugs. What can go wrong? +linux-toolchains. GCC updated the documentation in 7.x to discourage people from using the optimize attribute. https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=893100c3fa9b3049ce84dcc0c1a839ddc7a21387
On Wed, Oct 28, 2020 at 6:15 PM Ard Biesheuvel <ardb@kernel.org> wrote: > Commit 3193c0836 ("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. > > This reverts commit 3193c0836, and instead, it disables the -fgcse > optimization for the entire source file, but only when building for > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > original commit states that CONFIG_RETPOLINE=n triggers the issue, > whereas CONFIG_RETPOLINE=y performs better without the optimization, > so it is kept disabled in both cases. > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> (probably you missed by tag on v1 due to kernel.org hickups) 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
On Wed, Oct 28, 2020 at 10:57:45PM -0400, Arvind Sankar wrote: > On Wed, Oct 28, 2020 at 04:20:01PM -0700, Alexei Starovoitov wrote: > > All compilers have bugs. Kernel has bugs. What can go wrong? Heh. > +linux-toolchains. GCC updated the documentation in 7.x to discourage > people from using the optimize attribute. > > https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=893100c3fa9b3049ce84dcc0c1a839ddc7a21387 https://patchwork.ozlabs.org/project/gcc/patch/20151213081911.GA320@x4/ has all the discussion around that GCC patch. Segher
On Thu, 29 Oct 2020 at 21:35, Segher Boessenkool <segher@kernel.crashing.org> wrote: > > On Wed, Oct 28, 2020 at 10:57:45PM -0400, Arvind Sankar wrote: > > On Wed, Oct 28, 2020 at 04:20:01PM -0700, Alexei Starovoitov wrote: > > > All compilers have bugs. Kernel has bugs. What can go wrong? > > Heh. > > > +linux-toolchains. GCC updated the documentation in 7.x to discourage > > people from using the optimize attribute. > > > > https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=893100c3fa9b3049ce84dcc0c1a839ddc7a21387 > > https://patchwork.ozlabs.org/project/gcc/patch/20151213081911.GA320@x4/ > has all the discussion around that GCC patch. > For everyone's convenience, let me reproduce here how the GCC developers describe this attribute on their wiki [0]: """ Currently (2015), this attribute is known to have several critical bugs (PR37565, PR63401, PR60580, PR50782). Using it may produce not effect at all or lead to wrong-code. Quoting one GCC maintainer: "I consider the optimize attribute code seriously broken and unmaintained (but sometimes useful for debugging - and only that)." source Unfortunately, the people who added it are either not working on GCC anymore or not interested in fixing it. Do not try to guess how it is supposed to work by trial-and-error. There is not a list of options that are safe to use or known to be broken. Bug reports about the optimize attribute being broken will probably be closed as WONTFIX (PR59262), thus it is not worth to open new ones. If it works for you for a given version of GCC, it doesn't mean it will work on a different machine or a different version. The only realistic choices are to not use it, to use it and accept its brokenness (current or future one, since it is unmaintained), or join GCC and fix it (perhaps motivating other people along the way to join your effort). """ [0] https://gcc.gnu.org/wiki/FAQ#optimize_attribute_broken
On Wed, Oct 28, 2020 at 4:20 PM Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > > On Thu, Oct 29, 2020 at 12:10:52AM +0100, Ard Biesheuvel wrote: > > On Wed, 28 Oct 2020 at 23:59, Alexei Starovoitov > > <alexei.starovoitov@gmail.com> wrote: > > > > > > I'm totally fine with making > > > #define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer"))) > > > to be gcc+x86 only. > > > I'd like to get rid of it, but objtool is not smart enough to understand > > > generated asm without it. > > > > I'll defer to the x86 folks to make the final call here, but I would > > be perfectly happy doing > > > > index d1e3c6896b71..68ddb91fbcc6 100644 > > --- a/include/linux/compiler-gcc.h > > +++ b/include/linux/compiler-gcc.h > > @@ -176,4 +176,6 @@ > > #define __diag_GCC_8(s) > > #endif > > > > +#ifdef CONFIG_X86 > > #define __no_fgcse __attribute__((optimize("-fno-gcse"))) > > +#endif > > If you're going to submit this patch could you please add > ,-fno-omit-frame-pointer > to the above as well? You'll be playing whack-a-mole with other -f flags that should have been used, which changes even based on the config. The -fsanitize= flags come to mind with the sanitizers. defconfig shows: $ make LLVM=1 -j71 kernel/bpf/core.o V=1 2>&1 | grep "\-f" the following -f flags set: -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -fno-asynchronous-unwind-tables -fno-delete-null-pointer-checks -fomit-frame-pointer -fmacro-prefix-map=./= -fstack-protector-strong We already know that -fno-asynchronous-unwind-tables get dropped, hence this patch. And we know -fomit-frame-pointer or -fno-omit-frame-pointer I guess gets dropped, hence your ask. We might not know the full extent which other flags get dropped with the optimize attribute, but I'd argue that my list above can all result in pretty bad bugs when accidentally omitted (ok, maybe not -fshort-wchar or -fmacro-prefix-map, idk what those do) or when mixed with code that has different values those flags control. Searching GCC's bug tracker for `__attribute__((optimize` turns up plenty of reports to make me think this attribute maybe doesn't work the way folks suspect or intend: https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=__attribute__%28%28optimize&list_id=283390. There's plenty of folks arguing against the use of the optimize attribute in favor of the command line flag. I urge you to please reconsider the request. > Frankly I'm more worried that -Os will generate incorrect code. If you have observed bugs as a result of setting CONFIG_CC_OPTIMIZE_FOR_SIZE, we would love to help you get to the bottom of them and help you debug them. But we should also remain vigilant against rejecting progress on the status quo for known issues over hypothetical issues without proper regard for evidence. Correctness is the chief concern of a compiler; that it generates incorrect code unless default-on optimizations are explicitly disabled would be concerning, if that was in fact the case. Such a bug report would be invaluable to this code base, and likely others. I trust you've seen bugs here, but I would like to help verify this claim. > All compilers have bugs. Kernel has bugs. What can go wrong? This is more terrifyingly precise and infinitely wise than you may have initially intended. That my phone and laptop don't catch fire simultaneously now is nothing short of miraculous. I'm still holding my breath. -- Thanks, ~Nick Desaulniers
On Wed, Oct 28, 2020 at 10:15 AM Ard Biesheuvel <ardb@kernel.org> wrote: > > Commit 3193c0836 ("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. > > This reverts commit 3193c0836, and instead, it disables the -fgcse > optimization for the entire source file, but only when building for > X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the > original commit states that CONFIG_RETPOLINE=n triggers the issue, > whereas CONFIG_RETPOLINE=y performs better without the optimization, > so it is kept disabled in both cases. > > Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > --- > include/linux/compiler-gcc.h | 2 -- > include/linux/compiler_types.h | 4 ---- > kernel/bpf/Makefile | 6 +++++- > kernel/bpf/core.c | 2 +- > 4 files changed, 6 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..c1b9f71ee6aa 100644 > --- a/kernel/bpf/Makefile > +++ b/kernel/bpf/Makefile > @@ -1,6 +1,10 @@ > # SPDX-License-Identifier: GPL-2.0 > obj-y := core.o > -CFLAGS_core.o += $(call cc-disable-warning, override-init) > +ifneq ($(CONFIG_BPF_JIT_ALWAYS_ON),y) > +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details > +cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse > +endif > +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy) Writing multiple conditions in a conditional block in GNU make is painful, hence the double `y` trick. I feel like either 3 nested conditionals (one for CONFIG_BPF_JIT_ALWAYS_ON, CONFIG_X86, and CONFIG_CC_IS_GCC) would have been clearer, or using three `y`, rather than mixing and matching `if`s with multiple `y`s, but regardless of what color I think we should paint the bikeshed: Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> This also doesn't resolve all issues here, but is a step in the right direction, IMO. > > obj-$(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 >
On Thu, Oct 29, 2020 at 05:28:11PM -0700, Nick Desaulniers wrote: > > We already know that -fno-asynchronous-unwind-tables get dropped, > hence this patch. On arm64 only. Not on x86 > And we know -fomit-frame-pointer or > -fno-omit-frame-pointer I guess gets dropped, hence your ask. yep. that one is bugged. > We might not know the full extent which other flags get dropped with the > optimize attribute, but I'd argue that my list above can all result in > pretty bad bugs when accidentally omitted (ok, maybe not -fshort-wchar > or -fmacro-prefix-map, idk what those do) or when mixed with code that true. Few month back I've checked that strict-aliasing and no-common flags from your list are not dropped by this attr in gcc [6789]. I've also checked that no-red-zone and model=kernel preserved as well. > has different values those flags control. Searching GCC's bug tracker > for `__attribute__((optimize` turns up plenty of reports to make me > think this attribute maybe doesn't work the way folks suspect or > intend: https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=__attribute__%28%28optimize&list_id=283390. There is a risk. Is it a footgun? Sure. Yet. gcc testsuite is using __attribute__((optimize)). And some of these tests were added _after_ offical gcc doc said that this attribute is broken. imo it's like 'beware of the dog' sign. > There's plenty of folks arguing against the use of the optimize > attribute in favor of the command line flag. I urge you to please > reconsider the request. ok. Applied this first patch to bpf tree and will get it to Linus soon. Second patch that is splitting interpreter out because of this mess is dropped. The effect of gcse on performance is questionable. iirc some interpreters used to do -fno-gcse to gain performance.
On Fri, 30 Oct 2020 at 04:22, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > > On Thu, Oct 29, 2020 at 05:28:11PM -0700, Nick Desaulniers wrote: > > > > We already know that -fno-asynchronous-unwind-tables get dropped, > > hence this patch. > > On arm64 only. Not on x86 > > > And we know -fomit-frame-pointer or > > -fno-omit-frame-pointer I guess gets dropped, hence your ask. > > yep. that one is bugged. > > > We might not know the full extent which other flags get dropped with the > > optimize attribute, but I'd argue that my list above can all result in > > pretty bad bugs when accidentally omitted (ok, maybe not -fshort-wchar > > or -fmacro-prefix-map, idk what those do) or when mixed with code that > > true. > Few month back I've checked that strict-aliasing and no-common flags > from your list are not dropped by this attr in gcc [6789]. > I've also checked that no-red-zone and model=kernel preserved as well. > > > has different values those flags control. Searching GCC's bug tracker > > for `__attribute__((optimize` turns up plenty of reports to make me > > think this attribute maybe doesn't work the way folks suspect or > > intend: https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=__attribute__%28%28optimize&list_id=283390. > > There is a risk. > Is it a footgun? Sure. > Yet. gcc testsuite is using __attribute__((optimize)). > And some of these tests were added _after_ offical gcc doc said that this > attribute is broken. > imo it's like 'beware of the dog' sign. > > > There's plenty of folks arguing against the use of the optimize > > attribute in favor of the command line flag. I urge you to please > > reconsider the request. > > ok. Applied this first patch to bpf tree and will get it to Linus soon. > Second patch that is splitting interpreter out because of this mess > is dropped. The effect of gcse on performance is questionable. > iirc some interpreters used to do -fno-gcse to gain performance. That is absolutely fine. I only included the second patch to address Daniel's concern that -fno-gcse could affect unrelated code living in the same source file as __bpf_prog_run(), but if you don't care about that, nor will I.
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..c1b9f71ee6aa 100644 --- a/kernel/bpf/Makefile +++ b/kernel/bpf/Makefile @@ -1,6 +1,10 @@ # SPDX-License-Identifier: GPL-2.0 obj-y := core.o -CFLAGS_core.o += $(call cc-disable-warning, override-init) +ifneq ($(CONFIG_BPF_JIT_ALWAYS_ON),y) +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details +cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse +endif +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy) obj-$(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
Commit 3193c0836 ("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. This reverts commit 3193c0836, and instead, it disables the -fgcse optimization for the entire source file, but only when building for X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the original commit states that CONFIG_RETPOLINE=n triggers the issue, whereas CONFIG_RETPOLINE=y performs better without the optimization, so it is kept disabled in both cases. Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ Signed-off-by: Ard Biesheuvel <ardb@kernel.org> --- include/linux/compiler-gcc.h | 2 -- include/linux/compiler_types.h | 4 ---- kernel/bpf/Makefile | 6 +++++- kernel/bpf/core.c | 2 +- 4 files changed, 6 insertions(+), 8 deletions(-)