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@xxxxxxxxxxxxxxx # v6.6+
Signed-off-by: Tony Ambardar <Tony.Ambardar@xxxxxxxxx>
---
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?
+# define __retain __attribute__((__retain__))
+#else
+# define __retain
+#endif
+
/* Compiler specific macros. */
#ifdef __clang__
#include <linux/compiler-clang.h>
