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[bpf,v1,1/2] Compiler Attributes: Add __retain macro

Message ID 5416a38a194bb97930f5a2e672165e573b82857b.1717413886.git.Tony.Ambardar@gmail.com (mailing list archive)
State Superseded
Delegated to: BPF
Headers show
Series bpf: Fix linker optimization removing kfuncs | expand

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Commit Message

Tony Ambardar June 3, 2024, 12:16 p.m. UTC 
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_attributes.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

Comments

Miguel Ojeda June 3, 2024, 1:57 p.m. UTC | #1 
On Mon, Jun 3, 2024 at 2:16 PM Tony Ambardar <tony.ambardar@gmail.com> wrote:
>
> +#if __has_attribute(__retain__) && \
> +       (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \
> +        defined(CONFIG_LTO_CLANG))

Since this attribute depends on the configuration, please move it to
`compiler_types.h` instead.

Cheers,
Miguel
Tony Ambardar June 4, 2024, 2:37 a.m. UTC | #2 
On Mon, Jun 03, 2024 at 03:57:38PM +0200, Miguel Ojeda wrote:
> On Mon, Jun 3, 2024 at 2:16 PM Tony Ambardar <tony.ambardar@gmail.com> wrote:
> >
> > +#if __has_attribute(__retain__) && \
> > +       (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \
> > +        defined(CONFIG_LTO_CLANG))
> 
> Since this attribute depends on the configuration, please move it to
> `compiler_types.h` instead.

Noted and thanks for clarifying; I'll change in v2.

Cheers,
Tony

> 
> Cheers,
> Miguel
diff mbox series

Patch

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 32284cd26d52..1c22e1a734dc 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -326,6 +326,20 @@ 
  */
 #define __pure                          __attribute__((__pure__))
 
+/*
+ * 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))
+# define __retain			__attribute__((__retain__))
+#else
+# define __retain
+#endif
+
 /*
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-section-function-attribute
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-section-variable-attribute