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
- */
+#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>