x86: ptrace: Add function argument access API

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@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

3 files changed, 46 insertions, 0 deletions

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)