[RFC][PATCH] x86: ptrace: Add function argument access API

[Steven Rostedt]

[
  A while ago I posted an RFC patchset for dynamic function based
  events. But Masami pointed out that this could be done with kprobes
  with minimal changes. He posted a patch set back in March
  http://lkml.kernel.org/r/152049860385.7289.14079393589900496424.stgit@devbox
  I've pulled this in locally, but haven't had the time until recently
  to look at it seriously. I even plan to talk about these changes in
  my talk at Open Source Summit in Edinburgh less than two weeks away
  (talk about conference driven development!).
  Anyway, the one patch that really needs external approval is the one
  that creates a new architecture dependent API to retrieve function
  arguments from pt_regs if the ip is at the start of the function call
  (via a breakpoint or ftrace fentry). That's this patch.

  Anyone have any issues with it? If not, I'm going to start doing some
  serious testing of this code and try to get it into the next merge
  window.

  Thanks!

  -- Steve
]

From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Add regs_get_argument() which returns N th argument of the
function call.
Note that this chooses most probably assignment, in some case
it can be incorrect (e.g. passing data structure or floating
point etc.)

This is expected to be called from kprobes or ftrace with regs
where the top of stack is the return address.

Link: http://lkml.kernel.org/r/152465885737.26224.2822487520472783854.stgit@devbox

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 arch/Kconfig                  |  7 +++++++
 arch/x86/Kconfig              |  1 +
 arch/x86/include/asm/ptrace.h | 38 +++++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 6801123932a5..facace0c90fc 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -290,6 +290,13 @@ config HAVE_RSEQ
 	  This symbol should be selected by an architecture if it
 	  supports an implementation of restartable sequences.
 
+config HAVE_FUNCTION_ARG_ACCESS_API
+	bool
+	help
+	  This symbol should be selected by an architecure if it supports
+	  the API needed to access function arguments from pt_regs,
+	  declared in asm/ptrace.h
+
 config HAVE_CLK
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1a0be022f91d..972973851779 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -184,6 +184,7 @@ config X86
 	select HAVE_RCU_TABLE_INVALIDATE	if HAVE_RCU_TABLE_FREE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE		if X86_64 && (UNWINDER_FRAME_POINTER || UNWINDER_ORC) && STACK_VALIDATION
+	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_STACKPROTECTOR		if CC_HAS_SANE_STACKPROTECTOR
 	select HAVE_STACK_VALIDATION		if X86_64
 	select HAVE_RSEQ
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 6de1fd3d0097..c2304b25e2fd 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -256,6 +256,44 @@ static inline unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs,
 		return 0;
 }
 
+/**
+ * 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 most probably assignment, in some case
+ * it can be incorrect.
+ * This is expected to be called from kprobes or ftrace with regs
+ * where the top of stack is the return address.
+ */
+static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
+						     unsigned int n)
+{
+	static const unsigned int argument_offs[] = {
+#ifdef __i386__
+		offsetof(struct pt_regs, ax),
+		offsetof(struct pt_regs, cx),
+		offsetof(struct pt_regs, dx),
+#define NR_REG_ARGUMENTS 3
+#else
+		offsetof(struct pt_regs, di),
+		offsetof(struct pt_regs, si),
+		offsetof(struct pt_regs, dx),
+		offsetof(struct pt_regs, cx),
+		offsetof(struct pt_regs, r8),
+		offsetof(struct pt_regs, r9),
+#define NR_REG_ARGUMENTS 6
+#endif
+	};
+
+	if (n >= NR_REG_ARGUMENTS) {
+		n -= NR_REG_ARGUMENTS - 1;
+		return regs_get_kernel_stack_nth(regs, n);
+	} else
+		return regs_get_register(regs, argument_offs[n]);
+}
+
 #define arch_has_single_step()	(1)
 #ifdef CONFIG_X86_DEBUGCTLMSR
 #define arch_has_block_step()	(1)
-- 
2.19.0