[PATCH 2/2] ARM: entry-common, ptrace: do not pass scno to syscall_trace_enter

[Roman Pen]

In previous patch current_thread_info()->syscall is set with
corresponding syscall number prior to further calls, thus there
is no any need to pass 'scno'.

Also, add explicit comment why do we have to reread 'scno' local
variable.

Signed-off-by: Roman Pen <r.peniaev at gmail.com>
Cc: Russell King <linux at arm.linux.org.uk>
Cc: Christoffer Dall <christoffer.dall at linaro.org>
Cc: Stefano Stabellini <stefano.stabellini at eu.citrix.com>
Cc: Sekhar Nori <nsekhar at ti.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Andy Lutomirski <luto at amacapital.net>
Cc: Eric Paris <eparis at redhat.com>
Cc: Will Deacon <will.deacon at arm.com>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
---
 arch/arm/kernel/entry-common.S | 1 -
 arch/arm/kernel/ptrace.c       | 6 ++++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 89452ff..3d12eb5 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -228,7 +228,6 @@ ENDPROC(vector_swi)
 	 * context switches, and waiting for our parent to respond.
 	 */
 __sys_trace:
-	mov	r1, scno
 	add	r0, sp, #S_OFF
 	bl	syscall_trace_enter
 
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index ef9119f..1238787 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -928,9 +928,9 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 	regs->ARM_ip = ip;
 }
 
-asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
+asmlinkage int syscall_trace_enter(struct pt_regs *regs)
 {
-	current_thread_info()->syscall = scno;
+	int scno = current_thread_info()->syscall;
 
 	/* Do the secure computing check first; failures should be fast. */
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
@@ -944,6 +944,8 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
 	if (test_thread_flag(TIF_SYSCALL_TRACE))
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
 
+	/* Syscall can be aborted (-1 can be set) or even changed
+	 * by the tracer and subsequent PTRACE_SET_SYSCALL request */
 	scno = current_thread_info()->syscall;
 
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
-- 
2.1.3