[PATCH v3] arm64: ftrace: use function_nocfi for _mcount as well

[PATCH v3] arm64: ftrace: use function_nocfi for _mcount as well

[Sumit Garg]

Commit 800618f955a9 ("arm64: ftrace: use function_nocfi for ftrace_call")
only fixed address of ftrace_call but address of _mcount needs to be
fixed as well. Use function_nocfi() to get the actual address of _mcount
function as with CONFIG_CFI_CLANG, the compiler replaces function pointers
with jump table addresses which breaks dynamic ftrace as the address of
_mcount is replaced with the address of _mcount.cfi_jt.

With mainline, this won't be a problem since by default
CONFIG_DYNAMIC_FTRACE_WITH_REGS=y with Clang >= 10 as it supports
-fpatchable-function-entry and CFI requires Clang 12 but for consistency
we should add function_nocfi() for _mcount as well.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
---

Changes in v3:
- Droped fixes tag as it isn't critical.
- Modified commit description in order to make this change for consistency.
- Picked up Sami's review tag.

Changes in v2:
- Added fixes tag.
- Extended commit description.
- Picked up Mark's ack.

 arch/arm64/include/asm/ftrace.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 91fa4baa1a93..347b0cc68f07 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -15,7 +15,7 @@
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #else
-#define MCOUNT_ADDR		((unsigned long)_mcount)
+#define MCOUNT_ADDR		((unsigned long)function_nocfi(_mcount))
 #endif
 
 /* The BL at the callsite's adjusted rec->ip */
-- 
2.17.1

Re: [PATCH v3] arm64: ftrace: use function_nocfi for _mcount as well

[Will Deacon]

On Mon, 11 Oct 2021 18:20:59 +0530, Sumit Garg wrote:
> Commit 800618f955a9 ("arm64: ftrace: use function_nocfi for ftrace_call")
> only fixed address of ftrace_call but address of _mcount needs to be
> fixed as well. Use function_nocfi() to get the actual address of _mcount
> function as with CONFIG_CFI_CLANG, the compiler replaces function pointers
> with jump table addresses which breaks dynamic ftrace as the address of
> _mcount is replaced with the address of _mcount.cfi_jt.
> 
> [...]

Applied to arm64 (for-next/misc), thanks!

[1/1] arm64: ftrace: use function_nocfi for _mcount as well
      https://git.kernel.org/arm64/c/de56379f21c7

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev