Re: [REGRESSION] x86/entry: Tracer no longer has opportunity to change the syscall number at entry via orig_ax

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Wed, Aug 19 2020 at 10:14, Kyle Huey wrote:
> tl;dr: after 27d6b4d14f5c3ab21c4aef87dd04055a2d7adf14 ptracer
> modifications to orig_ax in a syscall entry trace stop are not honored
> and this breaks our code.

My fault and I have no idead why none of the silly test cases
noticed. Fix below.

Thanks,

        tglx
---
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 9852e0d62d95..fcae019158ca 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -65,7 +65,8 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
 
 	syscall_enter_audit(regs, syscall);
 
-	return ret ? : syscall;
+	/* The above might have changed the syscall number */
+	return ret ? : syscall_get_nr(current, regs);
 }
 
 noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)