| Message ID | b31bca5a5e6765a0f32cc8c19b1d9cdbfaa822b5.1717477560.git.Tony.Ambardar@gmail.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 0a5d3258d7c97295a89d22e54733b54aacb62562 |
| Delegated to: | BPF |
| Headers | show |
| Series | bpf: Fix linker optimization removing kfuncs | expand |
On 6/3/24 10:23 PM, Tony Ambardar wrote: > Some code includes the __used macro to prevent functions and data from > being optimized out. This macro implements __attribute__((__used__)), which > operates at the compiler and IR-level, and so still allows a linker to > remove objects intended to be kept. > > Compilers supporting __attribute__((__retain__)) can address this gap by > setting the flag SHF_GNU_RETAIN on the section of a function/variable, > indicating to the linker the object should be retained. This attribute is > available since gcc 11, clang 13, and binutils 2.36. > > Provide a __retain macro implementing __attribute__((__retain__)), whose > first user will be the '__bpf_kfunc' tag. > > Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ > Cc: stable@vger.kernel.org # v6.6+ > Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com> > --- > include/linux/compiler_types.h | 23 +++++++++++++++++++++++ > 1 file changed, 23 insertions(+) > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > index 93600de3800b..f14c275950b5 100644 > --- a/include/linux/compiler_types.h > +++ b/include/linux/compiler_types.h > @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } > # define __preserve_most > #endif > > +/* > + * Annotating a function/variable with __retain tells the compiler to place > + * the object in its own section and set the flag SHF_GNU_RETAIN. This flag > + * instructs the linker to retain the object during garbage-cleanup or LTO > + * phases. > + * > + * Note that the __used macro is also used to prevent functions or data > + * being optimized out, but operates at the compiler/IR-level and may still > + * allow unintended removal of objects during linking. > + * > + * Optional: only supported since gcc >= 11, clang >= 13 > + * > + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute > + * clang: https://clang.llvm.org/docs/AttributeReference.html#retain > + */ > +#if __has_attribute(__retain__) && \ > + (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \ > + defined(CONFIG_LTO_CLANG)) Could you explain why CONFIG_LTO_CLANG is added here? IIUC, the __used macro permits garbage collection at section level, so CLANG_LTO_CLANG without CONFIG_LD_DEAD_CODE_DATA_ELIMINATION shuold not change final section dynamics, right? > +# define __retain __attribute__((__retain__)) > +#else > +# define __retain > +#endif > + > /* Compiler specific macros. */ > #ifdef __clang__ > #include <linux/compiler-clang.h>
On Tue, Jun 04, 2024 at 10:55:39PM -0700, Yonghong Song wrote: > > On 6/3/24 10:23 PM, Tony Ambardar wrote: > > Some code includes the __used macro to prevent functions and data from > > being optimized out. This macro implements __attribute__((__used__)), which > > operates at the compiler and IR-level, and so still allows a linker to > > remove objects intended to be kept. > > > > Compilers supporting __attribute__((__retain__)) can address this gap by > > setting the flag SHF_GNU_RETAIN on the section of a function/variable, > > indicating to the linker the object should be retained. This attribute is > > available since gcc 11, clang 13, and binutils 2.36. > > > > Provide a __retain macro implementing __attribute__((__retain__)), whose > > first user will be the '__bpf_kfunc' tag. > > > > Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ > > Cc: stable@vger.kernel.org # v6.6+ > > Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com> > > --- > > include/linux/compiler_types.h | 23 +++++++++++++++++++++++ > > 1 file changed, 23 insertions(+) > > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > > index 93600de3800b..f14c275950b5 100644 > > --- a/include/linux/compiler_types.h > > +++ b/include/linux/compiler_types.h > > @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } > > # define __preserve_most > > #endif > > +/* > > + * Annotating a function/variable with __retain tells the compiler to place > > + * the object in its own section and set the flag SHF_GNU_RETAIN. This flag > > + * instructs the linker to retain the object during garbage-cleanup or LTO > > + * phases. > > + * > > + * Note that the __used macro is also used to prevent functions or data > > + * being optimized out, but operates at the compiler/IR-level and may still > > + * allow unintended removal of objects during linking. > > + * > > + * Optional: only supported since gcc >= 11, clang >= 13 > > + * > > + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute > > + * clang: https://clang.llvm.org/docs/AttributeReference.html#retain > > + */ > > +#if __has_attribute(__retain__) && \ > > + (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \ > > + defined(CONFIG_LTO_CLANG)) > > Could you explain why CONFIG_LTO_CLANG is added here? > IIUC, the __used macro permits garbage collection at section > level, so CLANG_LTO_CLANG without > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION > shuold not change final section dynamics, right? Hi Yonghong, I included the conditional guard to ensure consistent behaviour between __retain and other features forcing split sections. In particular, the same guard is used in vmlinux.lds.h to merge split sections where needed. For example, using __retain in llvm builds without CONFIG_LTO was failing CI tests on kernel-patches/bpf because the kernel didn't boot properly. And in further testing, the kernel had no issues loading BPF kfunc modules with such split sections, so I left the module (partial) linking scripts alone. Maybe I misunderstand you question re: __used? Thanks, Tony > > > +# define __retain __attribute__((__retain__)) > > +#else > > +# define __retain > > +#endif > > + > > /* Compiler specific macros. */ > > #ifdef __clang__ > > #include <linux/compiler-clang.h>
On 6/10/24 3:56 PM, Tony Ambardar wrote: > On Tue, Jun 04, 2024 at 10:55:39PM -0700, Yonghong Song wrote: >> On 6/3/24 10:23 PM, Tony Ambardar wrote: >>> Some code includes the __used macro to prevent functions and data from >>> being optimized out. This macro implements __attribute__((__used__)), which >>> operates at the compiler and IR-level, and so still allows a linker to >>> remove objects intended to be kept. >>> >>> Compilers supporting __attribute__((__retain__)) can address this gap by >>> setting the flag SHF_GNU_RETAIN on the section of a function/variable, >>> indicating to the linker the object should be retained. This attribute is >>> available since gcc 11, clang 13, and binutils 2.36. >>> >>> Provide a __retain macro implementing __attribute__((__retain__)), whose >>> first user will be the '__bpf_kfunc' tag. >>> >>> Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ >>> Cc: stable@vger.kernel.org # v6.6+ >>> Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com> >>> --- >>> include/linux/compiler_types.h | 23 +++++++++++++++++++++++ >>> 1 file changed, 23 insertions(+) >>> >>> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h >>> index 93600de3800b..f14c275950b5 100644 >>> --- a/include/linux/compiler_types.h >>> +++ b/include/linux/compiler_types.h >>> @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } >>> # define __preserve_most >>> #endif >>> +/* >>> + * Annotating a function/variable with __retain tells the compiler to place >>> + * the object in its own section and set the flag SHF_GNU_RETAIN. This flag >>> + * instructs the linker to retain the object during garbage-cleanup or LTO >>> + * phases. >>> + * >>> + * Note that the __used macro is also used to prevent functions or data >>> + * being optimized out, but operates at the compiler/IR-level and may still >>> + * allow unintended removal of objects during linking. >>> + * >>> + * Optional: only supported since gcc >= 11, clang >= 13 >>> + * >>> + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute >>> + * clang: https://clang.llvm.org/docs/AttributeReference.html#retain >>> + */ >>> +#if __has_attribute(__retain__) && \ >>> + (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \ >>> + defined(CONFIG_LTO_CLANG)) >> Could you explain why CONFIG_LTO_CLANG is added here? >> IIUC, the __used macro permits garbage collection at section >> level, so CLANG_LTO_CLANG without >> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION >> shuold not change final section dynamics, right? > Hi Yonghong, > > I included the conditional guard to ensure consistent behaviour between > __retain and other features forcing split sections. In particular, the same > guard is used in vmlinux.lds.h to merge split sections where needed. For > example, using __retain in llvm builds without CONFIG_LTO was failing CI > tests on kernel-patches/bpf because the kernel didn't boot properly. And in > further testing, the kernel had no issues loading BPF kfunc modules with > such split sections, so I left the module (partial) linking scripts alone. I tried with both bpf and bpf-next tree and I cannot make CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y in .config file. The following are all occurances in Kconfig: $ egrep -r HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/mips/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/powerpc/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT && (!ARCH_USING_PATCHABLE_FUNCTION_ENTRY || (!CC_IS_GCC || GCC_VERSION >= 110100)) arch/riscv/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD init/Kconfig:config HAVE_LD_DEAD_CODE_DATA_ELIMINATION init/Kconfig: depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION Are there some pending patches to enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION for x86? I could foce CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y with the following hack: diff --git a/init/Kconfig b/init/Kconfig index 72404c1f2157..adf8718e2f5b 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1402,7 +1402,7 @@ config CC_OPTIMIZE_FOR_SIZE endchoice config HAVE_LD_DEAD_CODE_DATA_ELIMINATION - bool + def_bool y help This requires that the arch annotates or otherwise protects its external entry points from being discarded. Linker scripts But with the above, I cannot boot the kernel. Did I miss anything? > > Maybe I misunderstand you question re: __used? > > Thanks, > Tony >>> +# define __retain __attribute__((__retain__)) >>> +#else >>> +# define __retain >>> +#endif >>> + >>> /* Compiler specific macros. */ >>> #ifdef __clang__ >>> #include <linux/compiler-clang.h>
On Fri, Jun 14, 2024 at 11:47:19AM -0700, Yonghong Song wrote: > > On 6/10/24 3:56 PM, Tony Ambardar wrote: > > On Tue, Jun 04, 2024 at 10:55:39PM -0700, Yonghong Song wrote: > > > On 6/3/24 10:23 PM, Tony Ambardar wrote: > > > > Some code includes the __used macro to prevent functions and data from > > > > being optimized out. This macro implements __attribute__((__used__)), which > > > > operates at the compiler and IR-level, and so still allows a linker to > > > > remove objects intended to be kept. > > > > > > > > Compilers supporting __attribute__((__retain__)) can address this gap by > > > > setting the flag SHF_GNU_RETAIN on the section of a function/variable, > > > > indicating to the linker the object should be retained. This attribute is > > > > available since gcc 11, clang 13, and binutils 2.36. > > > > > > > > Provide a __retain macro implementing __attribute__((__retain__)), whose > > > > first user will be the '__bpf_kfunc' tag. > > > > > > > > Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ > > > > Cc: stable@vger.kernel.org # v6.6+ > > > > Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com> > > > > --- > > > > include/linux/compiler_types.h | 23 +++++++++++++++++++++++ > > > > 1 file changed, 23 insertions(+) > > > > > > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > > > > index 93600de3800b..f14c275950b5 100644 > > > > --- a/include/linux/compiler_types.h > > > > +++ b/include/linux/compiler_types.h > > > > @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } > > > > # define __preserve_most > > > > #endif > > > > +/* > > > > + * Annotating a function/variable with __retain tells the compiler to place > > > > + * the object in its own section and set the flag SHF_GNU_RETAIN. This flag > > > > + * instructs the linker to retain the object during garbage-cleanup or LTO > > > > + * phases. > > > > + * > > > > + * Note that the __used macro is also used to prevent functions or data > > > > + * being optimized out, but operates at the compiler/IR-level and may still > > > > + * allow unintended removal of objects during linking. > > > > + * > > > > + * Optional: only supported since gcc >= 11, clang >= 13 > > > > + * > > > > + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute > > > > + * clang: https://clang.llvm.org/docs/AttributeReference.html#retain > > > > + */ > > > > +#if __has_attribute(__retain__) && \ > > > > + (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \ > > > > + defined(CONFIG_LTO_CLANG)) > > > Could you explain why CONFIG_LTO_CLANG is added here? > > > IIUC, the __used macro permits garbage collection at section > > > level, so CLANG_LTO_CLANG without > > > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION > > > shuold not change final section dynamics, right? > > Hi Yonghong, > > > > I included the conditional guard to ensure consistent behaviour between > > __retain and other features forcing split sections. In particular, the same > > guard is used in vmlinux.lds.h to merge split sections where needed. For > > example, using __retain in llvm builds without CONFIG_LTO was failing CI > > tests on kernel-patches/bpf because the kernel didn't boot properly. And in > > further testing, the kernel had no issues loading BPF kfunc modules with > > such split sections, so I left the module (partial) linking scripts alone. > > I tried with both bpf and bpf-next tree and I cannot make CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y > in .config file. The following are all occurances in Kconfig: My understanding is one doesn't directly set HAVE_LD_DEAD_CODE_...; it's a per-arch capability flag which guards setting LD_DEAD_CODE_DATA_ELIMINATION but only targets "small systems" (i.e. embedded), so no surprise x86 isn't in the arch list below. > > $ egrep -r HAVE_LD_DEAD_CODE_DATA_ELIMINATION > arch/mips/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION > arch/powerpc/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT && (!ARCH_USING_PATCHABLE_FUNCTION_ENTRY || (!CC_IS_GCC || GCC_VERSION >= 110100)) > arch/riscv/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD > init/Kconfig:config HAVE_LD_DEAD_CODE_DATA_ELIMINATION > init/Kconfig: depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION > > Are there some pending patches to enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION > for x86? I doubt it given the target arches above, but curious what's the need for x86 support? Only x86_32? My patches were motivated seeing resolve_btfids and pahole errors for a couple years on MIPS routers. I don't recall seeing the same for x86 builds, so my testing focussed more on preserving x86 builds rather than adding/testing the arch flag for x86. > > I could foce CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y with the following hack: > diff --git a/init/Kconfig b/init/Kconfig > index 72404c1f2157..adf8718e2f5b 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -1402,7 +1402,7 @@ config CC_OPTIMIZE_FOR_SIZE > endchoice > config HAVE_LD_DEAD_CODE_DATA_ELIMINATION > - bool > + def_bool y > help > This requires that the arch annotates or otherwise protects > its external entry points from being discarded. Linker scripts > > But with the above, I cannot boot the kernel. OK, interesting exercise. Setting HAVE_LD_DEAD_CODE_DATA_ELIMINATION shouldn't change anything itself so I suppose you are also setting LD_DEAD_CODE_DATA_ELIMINATION? From previous testing on kernel-patches/CI, first guess would be vmlinux linker script doing section merges unaware of some x86 quirk. Or x86-specific linker script unhappy with split sections. > > > Did I miss anything? > > > > > Maybe I misunderstand you question re: __used? > > > > Thanks, > > Tony > > > > +# define __retain __attribute__((__retain__)) > > > > +#else > > > > +# define __retain > > > > +#endif > > > > + > > > > /* Compiler specific macros. */ > > > > #ifdef __clang__ > > > > #include <linux/compiler-clang.h>
On 6/14/24 11:57 PM, Tony Ambardar wrote: > On Fri, Jun 14, 2024 at 11:47:19AM -0700, Yonghong Song wrote: >> On 6/10/24 3:56 PM, Tony Ambardar wrote: >>> On Tue, Jun 04, 2024 at 10:55:39PM -0700, Yonghong Song wrote: >>>> On 6/3/24 10:23 PM, Tony Ambardar wrote: >>>>> Some code includes the __used macro to prevent functions and data from >>>>> being optimized out. This macro implements __attribute__((__used__)), which >>>>> operates at the compiler and IR-level, and so still allows a linker to >>>>> remove objects intended to be kept. >>>>> >>>>> Compilers supporting __attribute__((__retain__)) can address this gap by >>>>> setting the flag SHF_GNU_RETAIN on the section of a function/variable, >>>>> indicating to the linker the object should be retained. This attribute is >>>>> available since gcc 11, clang 13, and binutils 2.36. >>>>> >>>>> Provide a __retain macro implementing __attribute__((__retain__)), whose >>>>> first user will be the '__bpf_kfunc' tag. >>>>> >>>>> Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ >>>>> Cc: stable@vger.kernel.org # v6.6+ >>>>> Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com> >>>>> --- >>>>> include/linux/compiler_types.h | 23 +++++++++++++++++++++++ >>>>> 1 file changed, 23 insertions(+) >>>>> >>>>> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h >>>>> index 93600de3800b..f14c275950b5 100644 >>>>> --- a/include/linux/compiler_types.h >>>>> +++ b/include/linux/compiler_types.h >>>>> @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } >>>>> # define __preserve_most >>>>> #endif >>>>> +/* >>>>> + * Annotating a function/variable with __retain tells the compiler to place >>>>> + * the object in its own section and set the flag SHF_GNU_RETAIN. This flag >>>>> + * instructs the linker to retain the object during garbage-cleanup or LTO >>>>> + * phases. >>>>> + * >>>>> + * Note that the __used macro is also used to prevent functions or data >>>>> + * being optimized out, but operates at the compiler/IR-level and may still >>>>> + * allow unintended removal of objects during linking. >>>>> + * >>>>> + * Optional: only supported since gcc >= 11, clang >= 13 >>>>> + * >>>>> + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute >>>>> + * clang: https://clang.llvm.org/docs/AttributeReference.html#retain >>>>> + */ >>>>> +#if __has_attribute(__retain__) && \ >>>>> + (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \ >>>>> + defined(CONFIG_LTO_CLANG)) >>>> Could you explain why CONFIG_LTO_CLANG is added here? >>>> IIUC, the __used macro permits garbage collection at section >>>> level, so CLANG_LTO_CLANG without >>>> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION >>>> shuold not change final section dynamics, right? >>> Hi Yonghong, >>> >>> I included the conditional guard to ensure consistent behaviour between >>> __retain and other features forcing split sections. In particular, the same >>> guard is used in vmlinux.lds.h to merge split sections where needed. For >>> example, using __retain in llvm builds without CONFIG_LTO was failing CI >>> tests on kernel-patches/bpf because the kernel didn't boot properly. And in >>> further testing, the kernel had no issues loading BPF kfunc modules with >>> such split sections, so I left the module (partial) linking scripts alone. >> I tried with both bpf and bpf-next tree and I cannot make CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y >> in .config file. The following are all occurances in Kconfig: > My understanding is one doesn't directly set HAVE_LD_DEAD_CODE_...; it's a > per-arch capability flag which guards setting LD_DEAD_CODE_DATA_ELIMINATION > but only targets "small systems" (i.e. embedded), so no surprise x86 isn't > in the arch list below. I see. Yes, mips should support it but not x86. No wonder why I cannot reproduce. > >> $ egrep -r HAVE_LD_DEAD_CODE_DATA_ELIMINATION >> arch/mips/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION >> arch/powerpc/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT && (!ARCH_USING_PATCHABLE_FUNCTION_ENTRY || (!CC_IS_GCC || GCC_VERSION >= 110100)) >> arch/riscv/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD >> init/Kconfig:config HAVE_LD_DEAD_CODE_DATA_ELIMINATION >> init/Kconfig: depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION >> >> Are there some pending patches to enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION >> for x86? > I doubt it given the target arches above, but curious what's the need for > x86 support? Only x86_32? My patches were motivated seeing resolve_btfids > and pahole errors for a couple years on MIPS routers. I don't recall seeing > the same for x86 builds, so my testing focussed more on preserving x86 > builds rather than adding/testing the arch flag for x86. >> I could foce CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y with the following hack: >> diff --git a/init/Kconfig b/init/Kconfig >> index 72404c1f2157..adf8718e2f5b 100644 >> --- a/init/Kconfig >> +++ b/init/Kconfig >> @@ -1402,7 +1402,7 @@ config CC_OPTIMIZE_FOR_SIZE >> endchoice >> config HAVE_LD_DEAD_CODE_DATA_ELIMINATION >> - bool >> + def_bool y >> help >> This requires that the arch annotates or otherwise protects >> its external entry points from being discarded. Linker scripts >> >> But with the above, I cannot boot the kernel. > OK, interesting exercise. Setting HAVE_LD_DEAD_CODE_DATA_ELIMINATION > shouldn't change anything itself so I suppose you are also setting > LD_DEAD_CODE_DATA_ELIMINATION? From previous testing on kernel-patches/CI, > first guess would be vmlinux linker script doing section merges unaware of > some x86 quirk. Or x86-specific linker script unhappy with split sections. I guess x86 needs additional change to make HAVE_LD_DEAD_CODE_DATA_ELIMINATION work. I still curious about why CONFIG_LTO_CLANG is necessary. In asm-generic/vmlinux.lds.h, /* * LD_DEAD_CODE_DATA_ELIMINATION option enables -fdata-sections, which * generates .data.identifier sections, which need to be pulled in with * .data. We don't want to pull in .data..other sections, which Linux * has defined. Same for text and bss. * * With LTO_CLANG, the linker also splits sections by default, so we need * these macros to combine the sections during the final link. * * RODATA_MAIN is not used because existing code already defines .rodata.x * sections to be brought in with rodata. */ #if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) #define TEXT_MAIN .text .text.[0-9a-zA-Z_]* #define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L* #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]* #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L* #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral* #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]* #else #define TEXT_MAIN .text #define DATA_MAIN .data #define SDATA_MAIN .sdata #define RODATA_MAIN .rodata #define BSS_MAIN .bss #define SBSS_MAIN .sbss #endif If CONFIG_LTO_CLANG is defined and CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not defined, it is not clear whether __used functions will get eliminated or not. I tried with thinlto with a simple example on x86 with some unused function marked with __used, and that function survived in the final binary. But your patch won't hurt, so I am okay with it. > >> >> Did I miss anything? >> >>> Maybe I misunderstand you question re: __used? >>> >>> Thanks, >>> Tony >>>>> +# define __retain __attribute__((__retain__)) >>>>> +#else >>>>> +# define __retain >>>>> +#endif >>>>> + >>>>> /* Compiler specific macros. */ >>>>> #ifdef __clang__ >>>>> #include <linux/compiler-clang.h>
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 93600de3800b..f14c275950b5 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } # define __preserve_most #endif +/* + * Annotating a function/variable with __retain tells the compiler to place + * the object in its own section and set the flag SHF_GNU_RETAIN. This flag + * instructs the linker to retain the object during garbage-cleanup or LTO + * phases. + * + * Note that the __used macro is also used to prevent functions or data + * being optimized out, but operates at the compiler/IR-level and may still + * allow unintended removal of objects during linking. + * + * Optional: only supported since gcc >= 11, clang >= 13 + * + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-retain-function-attribute + * clang: https://clang.llvm.org/docs/AttributeReference.html#retain + */ +#if __has_attribute(__retain__) && \ + (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \ + defined(CONFIG_LTO_CLANG)) +# define __retain __attribute__((__retain__)) +#else +# define __retain +#endif + /* Compiler specific macros. */ #ifdef __clang__ #include <linux/compiler-clang.h>
Some code includes the __used macro to prevent functions and data from being optimized out. This macro implements __attribute__((__used__)), which operates at the compiler and IR-level, and so still allows a linker to remove objects intended to be kept. Compilers supporting __attribute__((__retain__)) can address this gap by setting the flag SHF_GNU_RETAIN on the section of a function/variable, indicating to the linker the object should be retained. This attribute is available since gcc 11, clang 13, and binutils 2.36. Provide a __retain macro implementing __attribute__((__retain__)), whose first user will be the '__bpf_kfunc' tag. Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ Cc: stable@vger.kernel.org # v6.6+ Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com> --- include/linux/compiler_types.h | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)