[PATCH] powerpc/ftrace: Ignore weak functions

Extend commit b39181f7c6907d ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to
avoid adding weak function") to ppc32 and ppc64 -mprofile-kernel by
defining FTRACE_MCOUNT_MAX_OFFSET.

For ppc64 -mprofile-kernel ABI, we can have two instructions at function
entry for TOC setup followed by 'mflr r0' and 'bl _mcount'. So, the
mcount location is at most the 4th instruction in a function. For ppc32,
mcount location is always the 3rd instruction in a function, preceded by
'mflr r0' and 'stw r0,4(r1)'.

With this patch, and with ppc64le_guest_defconfig and some ftrace/bpf
config items enabled:
  # grep __ftrace_invalid_address available_filter_functions | wc -l
  79

Signed-off-by: Naveen N. Rao <redacted>
---
 arch/powerpc/include/asm/ftrace.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index 3cee7115441b41..ade406dc6504e3 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h

@@ -10,6 +10,13 @@
 
 #define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 
+/* Ignore unused weak functions which will have larger offsets */
+#ifdef CONFIG_MPROFILE_KERNEL
+#define FTRACE_MCOUNT_MAX_OFFSET	12
+#elif defined(CONFIG_PPC32)
+#define FTRACE_MCOUNT_MAX_OFFSET	8
+#endif
+
 #ifndef __ASSEMBLY__
 extern void _mcount(void);
 

base-commit: ff1ed171e05c971652a0ede3d716997de8ee41c9
-- 
2.37.1

On Tue,  9 Aug 2022 16:24:25 +0530
"Naveen N. Rao" [off-list ref] wrote:

Extend commit b39181f7c6907d ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to
avoid adding weak function") to ppc32 and ppc64 -mprofile-kernel by
defining FTRACE_MCOUNT_MAX_OFFSET.

For ppc64 -mprofile-kernel ABI, we can have two instructions at function
entry for TOC setup followed by 'mflr r0' and 'bl _mcount'. So, the
mcount location is at most the 4th instruction in a function. For ppc32,
mcount location is always the 3rd instruction in a function, preceded by
'mflr r0' and 'stw r0,4(r1)'.

With this patch, and with ppc64le_guest_defconfig and some ftrace/bpf
config items enabled:
  # grep __ftrace_invalid_address available_filter_functions | wc -l
  79

I wonder if this patch answers the question to my last email. ;-)

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

On Tue, 9 Aug 2022 16:24:25 +0530, Naveen N. Rao wrote:

Extend commit b39181f7c6907d ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to
avoid adding weak function") to ppc32 and ppc64 -mprofile-kernel by
defining FTRACE_MCOUNT_MAX_OFFSET.

For ppc64 -mprofile-kernel ABI, we can have two instructions at function
entry for TOC setup followed by 'mflr r0' and 'bl _mcount'. So, the
mcount location is at most the 4th instruction in a function. For ppc32,
mcount location is always the 3rd instruction in a function, preceded by
'mflr r0' and 'stw r0,4(r1)'.

[...]

Applied to powerpc/next.

[1/1] powerpc/ftrace: Ignore weak functions
      https://git.kernel.org/powerpc/c/7af82ff90a2b0690c2c45818fcce4c4ac3b187f3

cheers