[PATCH] ARM: Add HAVE_FUNCTION_ARG_ACCESS_API support

[Jinjie Ruan]

Currently, kprobe on ARM32 can not use the '$argx' syntax available on
other architecture. So implement regs_get_kernel_argument() and add
HAVE_FUNCTION_ARG_ACCESS_API support.

Before there is a following error:
	echo 'p:kprobe_submit_bio submit_bio bio=$arg1' > kprobe_events
	sh: write error: Invalid argument

After:
	# echo 'p:kprobe_submit_bio submit_bio bio=$arg1' > kprobe_events
	# echo  1 > events/kprobes/enable
	# echo  1 > events/kprobes/kprobe_submit_bio/enable
	# echo 0 > tracing_on
	# echo > trace
	# echo 1 > tracing_on
	# cat trace
		kworker/u19:0-36      [002] d....    54.175322: kprobe_submit_bio: (submit_bio+0x0/0xf8) bio=0xc24e6000

Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 arch/arm/Kconfig              |  1 +
 arch/arm/include/asm/ptrace.h | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ee5115252aac..4ed504139763 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -102,6 +102,7 @@ config ARM
 	select HAVE_EXIT_THREAD
 	select HAVE_GUP_FAST if ARM_LPAE
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
+	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
diff --git a/arch/arm/include/asm/ptrace.h b/arch/arm/include/asm/ptrace.h
index 6eb311fb2da0..da5dd4cd0324 100644
--- a/arch/arm/include/asm/ptrace.h
+++ b/arch/arm/include/asm/ptrace.h
@@ -52,6 +52,28 @@ struct svc_pt_regs {
 #define fast_interrupts_enabled(regs) \
 	(!((regs)->ARM_cpsr & PSR_F_BIT))
 
+/**
+ * regs_get_kernel_argument() - get Nth function argument in kernel
+ * @regs:	pt_regs of that context
+ * @n:		function argument number (start from 0)
+ *
+ * regs_get_argument() returns @n th argument of the function call.
+ *
+ * Note that this chooses the most likely register mapping. In very rare
+ * cases this may not return correct data, for example, if one of the
+ * function parameters is 16 bytes or bigger. In such cases, we cannot
+ * get access the parameter correctly and the register assignment of
+ * subsequent parameters will be shifted.
+ */
+static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
+						     unsigned int n)
+{
+#define NR_REG_ARGUMENTS 4
+	if (n < NR_REG_ARGUMENTS)
+		return regs->uregs[n];
+	return 0;
+}
+
 /* Are the current registers suitable for user mode?
  * (used to maintain security in signal handlers)
  */
-- 
2.34.1