[PATCH v2] ARM: kprobes: move __kretprobe_trampoline to out of line assembler

[Nick Desaulniers <ndesaulniers@google.com>]

commit 1069c1dd20a3 ("ARM: 9231/1: Recover kretprobes return address for
EABI stack unwinder")
tickled a bug in clang's integrated assembler where the .save and .pad
directives must have corresponding .fnstart directives. The integrated
assembler is unaware that the compiler will be generating the .fnstart
directive.

  arch/arm/probes/kprobes/core.c:409:30: error: .fnstart must precede
  .save or .vsave directives
  <inline asm>:3:2: note: instantiated into assembly here
  .save   {sp, lr, pc}
  ^
  arch/arm/probes/kprobes/core.c:412:29: error: .fnstart must precede
  .pad directive
  <inline asm>:6:2: note: instantiated into assembly here
  .pad    #52
  ^

__kretprobe_trampoline's definition is already entirely inline asm. Move
it to out-of-line asm to avoid breaking the build.

Link: https://github.com/llvm/llvm-project/issues/57993
Link: https://github.com/ClangBuiltLinux/linux/issues/1718
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Suggested-by: Logan Chien <loganchien@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes v1 -> v2:
* rebase on linux-next again.
* drop commented out declaration of __kretprobe_trampoline from v1.

 arch/arm/probes/kprobes/Makefile              |  1 +
 arch/arm/probes/kprobes/core.c                | 50 +----------------
 .../arm/probes/kprobes/kretprobe-trampoline.S | 55 +++++++++++++++++++
 include/asm-generic/kprobes.h                 | 13 +++--
 4 files changed, 68 insertions(+), 51 deletions(-)
 create mode 100644 arch/arm/probes/kprobes/kretprobe-trampoline.S

diff --git a/arch/arm/probes/kprobes/Makefile b/arch/arm/probes/kprobes/Makefile
index 6159010dac4a..cdbe9dd99e28 100644
--- a/arch/arm/probes/kprobes/Makefile
+++ b/arch/arm/probes/kprobes/Makefile
@@ -3,6 +3,7 @@ KASAN_SANITIZE_actions-common.o := n
 KASAN_SANITIZE_actions-arm.o := n
 KASAN_SANITIZE_actions-thumb.o := n
 obj-$(CONFIG_KPROBES)		+= core.o actions-common.o checkers-common.o
+obj-$(CONFIG_KPROBES)		+= kretprobe-trampoline.o
 obj-$(CONFIG_ARM_KPROBES_TEST)	+= test-kprobes.o
 test-kprobes-objs		:= test-core.o
 
diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
index 9090c3a74dcc..53f17529d2cb 100644
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c
@@ -365,54 +365,8 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 	return NOTIFY_DONE;
 }
 
-/*
- * When a retprobed function returns, trampoline_handler() is called,
- * calling the kretprobe's handler. We construct a struct pt_regs to
- * give a view of registers r0-r11, sp, lr, and pc to the user
- * return-handler. This is not a complete pt_regs structure, but that
- * should be enough for stacktrace from the return handler with or
- * without pt_regs.
- */
-void __naked __kprobes __kretprobe_trampoline(void)
-{
-	__asm__ __volatile__ (
-#ifdef CONFIG_FRAME_POINTER
-		"ldr	lr, =__kretprobe_trampoline	\n\t"
-	/* __kretprobe_trampoline makes a framepointer on pt_regs. */
-#ifdef CONFIG_CC_IS_CLANG
-		"stmdb	sp, {sp, lr, pc}	\n\t"
-		"sub	sp, sp, #12		\n\t"
-		/* In clang case, pt_regs->ip = lr. */
-		"stmdb	sp!, {r0 - r11, lr}	\n\t"
-		/* fp points regs->r11 (fp) */
-		"add	fp, sp,	#44		\n\t"
-#else /* !CONFIG_CC_IS_CLANG */
-		/* In gcc case, pt_regs->ip = fp. */
-		"stmdb	sp, {fp, sp, lr, pc}	\n\t"
-		"sub	sp, sp, #16		\n\t"
-		"stmdb	sp!, {r0 - r11}		\n\t"
-		/* fp points regs->r15 (pc) */
-		"add	fp, sp, #60		\n\t"
-#endif /* CONFIG_CC_IS_CLANG */
-#else /* !CONFIG_FRAME_POINTER */
-		"sub	sp, sp, #16		\n\t"
-		"stmdb	sp!, {r0 - r11}		\n\t"
-#endif /* CONFIG_FRAME_POINTER */
-		"mov	r0, sp			\n\t"
-		"bl	trampoline_handler	\n\t"
-		"mov	lr, r0			\n\t"
-		"ldmia	sp!, {r0 - r11}		\n\t"
-		"add	sp, sp, #16		\n\t"
-#ifdef CONFIG_THUMB2_KERNEL
-		"bx	lr			\n\t"
-#else
-		"mov	pc, lr			\n\t"
-#endif
-		: : : "memory");
-}
-
 /* Called from __kretprobe_trampoline */
-static __used __kprobes void *trampoline_handler(struct pt_regs *regs)
+__kprobes void *trampoline_handler(struct pt_regs *regs)
 {
 	return (void *)kretprobe_trampoline_handler(regs, (void *)regs->ARM_fp);
 }
@@ -420,6 +374,8 @@ static __used __kprobes void *trampoline_handler(struct pt_regs *regs)
 void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
 				      struct pt_regs *regs)
 {
+	extern void __kretprobe_trampoline(void);
+
 	ri->ret_addr = (kprobe_opcode_t *)regs->ARM_lr;
 	ri->fp = (void *)regs->ARM_fp;
 
diff --git a/arch/arm/probes/kprobes/kretprobe-trampoline.S b/arch/arm/probes/kprobes/kretprobe-trampoline.S
new file mode 100644
index 000000000000..261c99b8c17f
--- /dev/null
+++ b/arch/arm/probes/kprobes/kretprobe-trampoline.S
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/linkage.h>
+#include <asm/unwind.h>
+#include <asm-generic/kprobes.h>
+
+/*
+ * When a retprobed function returns, trampoline_handler() is called,
+ * calling the kretprobe's handler. We construct a struct pt_regs to
+ * give a view of registers r0-r11, sp, lr, and pc to the user
+ * return-handler. This is not a complete pt_regs structure, but that
+ * should be enough for stacktrace from the return handler with or
+ * without pt_regs.
+ */
+__KPROBE
+SYM_FUNC_START(__kretprobe_trampoline)
+UNWIND(.fnstart)
+	ldr	lr, =__kretprobe_trampoline
+#ifdef CONFIG_FRAME_POINTER
+	/* __kretprobe_trampoline makes a framepointer on pt_regs. */
+#ifdef CONFIG_CC_IS_CLANG
+	stmdb	sp, {sp, lr, pc}
+	sub	sp, sp, #12
+	/* In clang case, pt_regs->ip = lr. */
+	stmdb	sp!, {r0 - r11, lr}
+	/* fp points regs->r11 (fp) */
+	add	fp, sp, #44
+#else /* !CONFIG_CC_IS_CLANG */
+	/* In gcc case, pt_regs->ip = fp. */
+	stmdb	sp, {fp, sp, lr, pc}
+	sub	sp, sp, #16
+	stmdb	sp!, {r0 - r11}
+	/* fp points regs->r15 (pc) */
+	add	fp, sp, #60
+#endif /* CONFIG_CC_IS_CLANG */
+#else /* !CONFIG_FRAME_POINTER */
+	/* store SP, LR on stack and add EABI unwind hint */
+	stmdb	sp, {sp, lr, pc}
+UNWIND(.save	{sp, lr, pc})
+	sub	sp, sp, #16
+	stmdb	sp!, {r0 - r11}
+UNWIND(.pad	#52)
+#endif /* CONFIG_FRAME_POINTER */
+	mov	r0, sp
+	bl	trampoline_handler
+	mov	lr, r0
+	ldmia	sp!, {r0 - r11}
+	add	sp, sp, #16
+#ifdef CONFIG_THUMB2_KERNEL
+	bx	lr
+#else
+	mov	pc, lr
+#endif
+UNWIND(.fnend)
+SYM_FUNC_END(__kretprobe_trampoline)
diff --git a/include/asm-generic/kprobes.h b/include/asm-generic/kprobes.h
index 060eab094e5a..1509daa281b8 100644
--- a/include/asm-generic/kprobes.h
+++ b/include/asm-generic/kprobes.h
@@ -2,7 +2,11 @@
 #ifndef _ASM_GENERIC_KPROBES_H
 #define _ASM_GENERIC_KPROBES_H
 
-#if defined(__KERNEL__) && !defined(__ASSEMBLY__)
+#ifdef __KERNEL__
+
+#ifdef __ASSEMBLY__
+# define __KPROBE .section ".kprobes.text", "ax"
+#else
 #ifdef CONFIG_KPROBES
 /*
  * Blacklist ganerating macro. Specify functions which is not probed
@@ -16,11 +20,12 @@ static unsigned long __used					\
 /* Use this to forbid a kprobes attach on very low level functions */
 # define __kprobes	__section(".kprobes.text")
 # define nokprobe_inline	__always_inline
-#else
+#else /* !defined(CONFIG_KPROBES) */
 # define NOKPROBE_SYMBOL(fname)
 # define __kprobes
 # define nokprobe_inline	inline
-#endif
-#endif /* defined(__KERNEL__) && !defined(__ASSEMBLY__) */
+#endif /* defined(CONFIG_KPROBES) */
+#endif /* defined(__ASSEMBLY__) */
+#endif /* defined(__KERNEL__) */
 
 #endif /* _ASM_GENERIC_KPROBES_H */
-- 
2.37.3.998.g577e59143f-goog