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

Tony Ambardar
2024-06-10T22:56:55Z

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.

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>