[PATCH v2 2/6] ptrace: introduce ptrace_syscall_enter to consolidate PTRACE_SYSEMU handling

[Sudeep Holla <sudeep.holla@arm.com>]

Currently each architecture handles PTRACE_SYSEMU in very similar way.
It's completely arch independent and can be handled in the code helping
to consolidate PTRACE_SYSEMU handling.

Let's introduce a hook 'ptrace_syscall_enter' that arch specific syscall
entry code can call.

Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
---
 include/linux/ptrace.h |  1 +
 kernel/ptrace.c        | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index edb9b040c94c..e30f51e3363e 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -407,6 +407,7 @@ static inline void user_single_step_report(struct pt_regs *regs)
 #define current_user_stack_pointer() user_stack_pointer(current_pt_regs())
 #endif
 
+extern long ptrace_syscall_enter(struct pt_regs *regs);
 extern int task_current_syscall(struct task_struct *target, long *callno,
 				unsigned long args[6], unsigned int maxargs,
 				unsigned long *sp, unsigned long *pc);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 4fa3b7f4c3c7..c9c505c483df 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -29,6 +29,7 @@
 #include <linux/hw_breakpoint.h>
 #include <linux/cn_proc.h>
 #include <linux/compat.h>
+#include <linux/tracehook.h>
 
 /*
  * Access another process' address space via ptrace.
@@ -557,6 +558,27 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 	return 0;
 }
 
+/*
+ * Hook to check and report for PTRACE_SYSEMU, can be called from arch
+ * arch syscall entry code
+ */
+long ptrace_syscall_enter(struct pt_regs *regs)
+{
+#ifdef TIF_SYSCALL_EMU
+	if (test_thread_flag(TIF_SYSCALL_EMU)) {
+		if (tracehook_report_syscall_entry(regs))
+			/*
+			 * We can ignore the return code here as we need
+			 * return -1 always for syscall emulation irrespective
+			 * of whether the tracehook report fails or succeed.
+			 */
+			;
+		return -1L;
+	}
+#endif
+	return 0;
+}
+
 /*
  * Detach all tasks we were using ptrace on. Called with tasklist held
  * for writing.
-- 
2.17.1