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

Yonghong Song
2024-06-17T03:27:00Z

On 6/14/24 11:57 PM, Tony Ambardar wrote:
> 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@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:
> 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.

I see. Yes, mips should support it but not x86. No wonder why I cannot reproduce.

>
>> $ 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.

I guess x86 needs additional change to make HAVE_LD_DEAD_CODE_DATA_ELIMINATION
work. I still curious about why CONFIG_LTO_CLANG is necessary.

In asm-generic/vmlinux.lds.h,

/*
* LD_DEAD_CODE_DATA_ELIMINATION option enables -fdata-sections, which
* generates .data.identifier sections, which need to be pulled in with
* .data. We don't want to pull in .data..other sections, which Linux
* has defined. Same for text and bss.
*
* With LTO_CLANG, the linker also splits sections by default, so we need
* these macros to combine the sections during the final link.
*
* RODATA_MAIN is not used because existing code already defines .rodata.x
* sections to be brought in with rodata.
*/
#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L*
#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral*
#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
#else
#define TEXT_MAIN .text
#define DATA_MAIN .data
#define SDATA_MAIN .sdata
#define RODATA_MAIN .rodata
#define BSS_MAIN .bss
#define SBSS_MAIN .sbss
#endif

If CONFIG_LTO_CLANG is defined and CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is
not defined, it is not clear whether __used functions will get eliminated
or not. I tried with thinlto with a simple example on x86 with some unused
function marked with __used, and that function survived in the final binary.

But your patch won't hurt, so I am okay with it.

>
>>
>> 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>