[PATCH v3 6/7] arm64: ptrace: Use NO_SYSCALL instead of -1 in syscall_trace_enter()

[Kees Cook]

On Fri, Jul 10, 2020 at 02:07:01PM +0100, Will Deacon wrote:
> Setting a system call number of -1 is special, as it indicates that the
> current system call should be skipped.
> 
> Use NO_SYSCALL instead of -1 when checking for this scenario, which is
> different from the -1 returned due to a seccomp failure.

I can't understand this paragraph. NO_SYSCALL is -1, so how is this
"different"?

arch/arm64/include/asm/ptrace.h:#define NO_SYSCALL (-1)

Do you just mean "stop using a literal '-1'"?

-Kees

> 
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Keno Fischer <keno at juliacomputing.com>
> Cc: Luis Machado <luis.machado at linaro.org>
> Signed-off-by: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/kernel/ptrace.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 89fbee3991a2..1e02e98e68dd 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1856,12 +1856,12 @@ int syscall_trace_enter(struct pt_regs *regs)
>  	if (flags & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE)) {
>  		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>  		if (!in_syscall(regs) || (flags & _TIF_SYSCALL_EMU))
> -			return -1;
> +			return NO_SYSCALL;
>  	}
>  
>  	/* Do the secure computing after ptrace; failures should be fast. */
>  	if (secure_computing() == -1)
> -		return -1;
> +		return NO_SYSCALL;
>  
>  	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
>  		trace_sys_enter(regs, regs->syscallno);
> -- 
> 2.27.0.383.g050319c2ae-goog
> 

-- 
Kees Cook