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

Yonghong Song
2024-06-14T18:47:31Z

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>