linux-kernel - [PATCH] ARM: ftrace: Implement HAVE_FUNCTION_GRAPH

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Message-ID: <20250818103931.1100084-1-richard@nod.at>
Date: Mon, 18 Aug 2025 12:39:31 +0200
From: Richard Weinberger <richard@....at>
To: linux@...linux.org.uk,
	rostedt@...dmis.org,
	mhiramat@...nel.org,
	mark.rutland@....com
Cc: ardb@...nel.org,
	ebiggers@...nel.org,
	nathan@...nel.org,
	linus.walleij@...aro.org,
	dave@...ilevsky.ca,
	ruanjinjie@...wei.com,
	liuyuntao12@...wei.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Richard Weinberger <richard@....at>,
	Donglin Peng <pengdonglin@...gfor.com.cn>
Subject: [PATCH] ARM: ftrace: Implement HAVE_FUNCTION_GRAPH_FREGS

Enable support for ftrace's funcgraph-retval feature by capturing r0-r3
and fp.
Since ARM does not provide its own __arch_ftrace_regs structure,
we instead populate pt_regs with the registers required by ftrace.

Cc: Donglin Peng <pengdonglin@...gfor.com.cn>
Signed-off-by: Richard Weinberger <richard@....at>
---
I noticed that funcgraph-retval is not available on ARM.
There was a patch [0] to enable the feature, but it never made it
upstream. Since then, the infrastructure around
CONFIG_FUNCTION_GRAPH_RETVAL has changed, so I have
reimplemented the original patch.

Donglin Peng, your change also captured the link register, but it
seems it was never used. Therefore, I skipped storing LR.

Thanks,
//richard

[0]: https://lore.kernel.org/all/c61eb9290c3e817d4d70c429c0e987e3ec51a3c4.1680954589.git.pengdonglin@sangfor.com.cn/
---
 arch/arm/Kconfig               |  1 +
 arch/arm/kernel/entry-ftrace.S | 18 ++++++++++++++----
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index b1f3df39ed406..ba84c6f7f5f7a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -108,6 +108,7 @@ config ARM
 	select HAVE_GUP_FAST if ARM_LPAE
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
+	select HAVE_FUNCTION_GRAPH_FREGS
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
 	select HAVE_GCC_PLUGINS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
index bc598e3d8dd23..e24ee559af81d 100644
--- a/arch/arm/kernel/entry-ftrace.S
+++ b/arch/arm/kernel/entry-ftrace.S
@@ -257,11 +257,21 @@ ENDPROC(ftrace_graph_regs_caller)
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 ENTRY(return_to_handler)
-	stmdb	sp!, {r0-r3}
-	add	r0, sp, #16		@ sp at exit of instrumented routine
+	mov	ip, sp				@ sp at exit of instrumented routine
+	sub	sp, #PT_REGS_SIZE
+	str	r0, [sp, #S_R0]
+	str	r1, [sp, #S_R1]
+	str	r2, [sp, #S_R2]
+	str	r3, [sp, #S_R3]
+	str	ip, [sp, #S_FP]
+	mov	r0, sp
 	bl	ftrace_return_to_handler
-	mov	lr, r0			@ r0 has real ret addr
-	ldmia	sp!, {r0-r3}
+	mov	lr, r0				@ r0 has real ret addr
+	ldr	r3, [sp, #S_R3]
+	ldr	r2, [sp, #S_R2]
+	ldr	r1, [sp, #S_R1]
+	ldr	r0, [sp, #S_R0]
+	add	sp, sp, #PT_REGS_SIZE		@ restore stack pointer
 	ret	lr
 ENDPROC(return_to_handler)
 #endif
-- 
2.50.1