Re: [PATCH v2] arm64: ftrace: use function_nocfi for _mcount as well

Hi Sumit,

On Wed, Oct 06, 2021 at 11:05:52AM +0530, Sumit Garg wrote:

On Tue, 5 Oct 2021 at 21:05, Mark Rutland [off-list ref] wrote:

quoted

On Tue, Oct 05, 2021 at 08:20:02AM -0700, Sami Tolvanen wrote:

quoted

On Tue, Oct 5, 2021 at 5:37 AM Sumit Garg [off-list ref] wrote:

quoted

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

Clang >= 10 supports -fpatchable-function-entry and CFI requires Clang
12, so I assume this is only an issue if
CONFIG_DYNAMIC_FTRACE_WITH_REGS is explicitly disabled?

I don't believe it's possible to disable explicitly, since
DYNAMIC_FTRACE_WITH_REGS isn't user selectable, and is def bool y,
depending on HAVE_DYNAMIC_FTRACE_WITH_REGS.

Ah, I see.

quoted

Sumit, have you actually seen a problem, or was this found by
inspection?

Actually I have seen this ftrace problem with the android11-5.4-lts
kernel and AOSP master user-space on db845c. The reason being kernel
v5.4 LTS doesn't support ftrace with -fpatchable-function-entry on
arm64.

With the mainline, I haven't tried to reproduce this issue but it was
rather by inspection that this needs to be fixed as well.

quoted

If this isn't an issue in practice, we could add the funciton_nocfi()
for consistency, but we should make that clear in the commit message,
and drop the fixes tag.

Sure, let me drop the fixes tag and update the commit description in
v3 as mainline only enabled CFI_CLANG for arm64 when
"-fpatchable-function-entry" is supported.

Did you post a v3? Just want to make sure I didn't miss it.

Will

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