[PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

[AKASHI Takahiro]

Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
its value either to:
  * any valid syscall number to alter a system call, or
  * -1 to skip a system call

This patch implements this behavior by reloading that value into syscallno
in struct pt_regs after tracehook_report_syscall_entry() or
secure_computing(). In case of '-1', a return value of system call can also
be changed by the tracer setting the value to x0 register, and so
sys_ni_nosyscall() should not be called.

See also:
    42309ab4, ARM: 8087/1: ptrace: reload syscall number after
	      secure_computing() check

Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
---
 arch/arm64/kernel/entry.S  |    2 ++
 arch/arm64/kernel/ptrace.c |   13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5141e79..de8bdbc 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -628,6 +628,8 @@ ENDPROC(el0_svc)
 __sys_trace:
 	mov	x0, sp
 	bl	syscall_trace_enter
+	cmp	w0, #-1				// skip syscall?
+	b.eq	ret_to_user
 	adr	lr, __sys_trace_return		// return address
 	uxtw	scno, w0			// syscall number (possibly new)
 	mov	x1, sp				// pointer to regs
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 70526cf..100d7d1 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -21,6 +21,7 @@
 
 #include <linux/audit.h>
 #include <linux/compat.h>
+#include <linux/errno.h>
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/mm.h>
@@ -1109,9 +1110,21 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 
 asmlinkage int syscall_trace_enter(struct pt_regs *regs)
 {
+	unsigned long saved_x0, saved_x8;
+
+	saved_x0 = regs->regs[0];
+	saved_x8 = regs->regs[8];
+
 	if (test_thread_flag(TIF_SYSCALL_TRACE))
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
 
+	regs->syscallno = regs->regs[8];
+	if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
+		regs->regs[8] = saved_x8;
+		if (regs->regs[0] == saved_x0) /* not changed by user */
+			regs->regs[0] = -ENOSYS;
+	}
+
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, regs->syscallno);
 
-- 
1.7.9.5