Re: [PATCH bpf v2 1/2] compiler_types.h: Define __retain for __attribute__((__retain__))

Yonghong Song
2024-06-05T05:55:49Z

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>