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@xxxxxxxxxxxxxxx # v6.6+ > > > > Signed-off-by: Tony Ambardar <Tony.Ambardar@xxxxxxxxx> > > > > --- > > > > 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>
