Re: [PATCH v2] arm64: ptrace: use live x0 for seccomp and audit after ptrace

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jun 30, 2026 at 06:29:29PM +0100, Catalin Marinas wrote:
> I think we need to keep orig_x0 as our original arg0 throughout the
> kernel and just fix the tracer path to sync it on the syscall entry. It
> doesn't unclutter the code but it shouldn't break the ABI either (unless
> someone relied on the ptrace change x0 and not being noticed by
> seccomp). Something like below:
> 
> ----------------8<-----------------------------
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 4d08598e2891..cd21b301e154 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -2417,6 +2417,18 @@ int syscall_trace_enter(struct pt_regs *regs)
>  		ret = report_syscall_entry(regs);
>  		if (ret || (flags & _TIF_SYSCALL_EMU))
>  			return NO_SYSCALL;
> +		/*
> +		 * Keep orig_x0 authoritative so that seccomp (via
> +		 * syscall_get_arguments()), audit and the restart path all
> +		 * see the same first argument the syscall is dispatched with,
> +		 * even if it has been updated by a tracer. Skip this for
> +		 * NO_SYSCALL (set either by the user or the tracer) as
> +		 * regs[0] holds the return value (see the comment in
> +		 * el0_svc_common()). For compat, orig_r0 is provided directly
> +		 * through GPR index 17.
> +		 */
> +		if (!is_compat_task() && regs->syscallno != NO_SYSCALL)
> +			regs->orig_x0 = regs->regs[0];
>  	}
>  
>  	/* Do the secure computing after ptrace; failures should be fast. */
> ----------------8<-----------------------------
> 
> If we want to change the ABI, we could do like riscv and only set the
> arguments via PTRACE_SET_SYSCALL_INFO while the GPR ptrace accesses
> whatever is in regs[0] - either the original arg or the return value. I
> think they changed this inadvertently in 2023 when they moved to the
> generic syscall.

Looking at some of the history, the ABI break on riscv was noticed, so
definitely not an option for us. I think the change would have looked
something like below. We could keep regs[0] match orig_x0 for entry but
it gets out of sync later, so still confusing for gdb/lldb/strace.

---------------8<----------------------
diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 5e4c7fc44f73..c58ac8d25692 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -93,19 +93,12 @@ static inline void syscall_set_arguments(struct task_struct *task,
 					 struct pt_regs *regs,
 					 const unsigned long *args)
 {
-	regs->regs[0] = args[0];
+	regs->orig_x0 = args[0];
 	regs->regs[1] = args[1];
 	regs->regs[2] = args[2];
 	regs->regs[3] = args[3];
 	regs->regs[4] = args[4];
 	regs->regs[5] = args[5];
-
-	/*
-	 * Also copy the first argument into orig_x0
-	 * so that syscall_get_arguments() would return it
-	 * instead of the previous value.
-	 */
-	regs->orig_x0 = regs->regs[0];
 }
 
 /*
diff --git a/arch/arm64/include/asm/syscall_wrapper.h b/arch/arm64/include/asm/syscall_wrapper.h
index abb57bc54305..6b13d7c8ad95 100644
--- a/arch/arm64/include/asm/syscall_wrapper.h
+++ b/arch/arm64/include/asm/syscall_wrapper.h
@@ -12,7 +12,7 @@
 
 #define SC_ARM64_REGS_TO_ARGS(x, ...)				\
 	__MAP(x,__SC_ARGS					\
-	      ,,regs->regs[0],,regs->regs[1],,regs->regs[2]	\
+	      ,,regs->orig_x0,,regs->regs[1],,regs->regs[2]	\
 	      ,,regs->regs[3],,regs->regs[4],,regs->regs[5])
 
 #ifdef CONFIG_COMPAT
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 358ddfbf1401..a80596531a5c 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -66,6 +66,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 
 	regs->orig_x0 = regs->regs[0];
 	regs->syscallno = scno;
+	syscall_set_return_value(current, regs, -ENOSYS, 0);
 
 	/*
 	 * BTI note:
@@ -111,8 +112,6 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 		 * setting the return value is unlikely to do anything sensible
 		 * anyway.
 		 */
-		if (scno == NO_SYSCALL)
-			syscall_set_return_value(current, regs, -ENOSYS, 0);
 		scno = syscall_trace_enter(regs);
 		if (scno == NO_SYSCALL)
 			goto trace_exit;

-- 
Catalin