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

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

Hi Sumit,

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

quoted

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.

This problem won't apply where the toolchain implements
-fpatchable-function-entry as we'll use that in preference to regular -pg,
i.e. this won't show up with recent versions of clang.

Fixes: 9186ad8e66bab6a1 ("arm64: allow CONFIG_CFI_CLANG to be selected")
Signed-off-by: Sumit Garg <redacted>
Acked-by: Mark Rutland <mark.rutland@arm.com>
---

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

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.

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

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.

Thanks,
Mark.