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. 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>
