Re: [PATCH] arm64: Fix PTRACE_SYSEMU semantics

[Will Deacon]

On Fri, May 15, 2020 at 06:22:53PM -0400, Keno Fischer wrote:
> Quoth the man page:
> ```
>        If the tracee was restarted by PTRACE_SYSCALL or PTRACE_SYSEMU, the
>        tracee enters syscall-enter-stop just prior to entering any system
>        call (which will not be executed if the restart was using
>        PTRACE_SYSEMU, regardless of any change made to registers at this
>        point or how the tracee is restarted after this stop).
> ```
> 
> The parenthetical comment is currently true on x86 and powerpc,
> but not currently true on arm64. arm64 re-checks the _TIF_SYSCALL_EMU
> flag after the syscall entry ptrace stop. However, at this point,
> it reflects which method was used to re-start the syscall
> at the entry stop, rather than the method that was used to reach it.
> Fix that by recording the original flag before performing the ptrace
> stop, bringing the behavior in line with documentation and x86/powerpc.
> 
> Signed-off-by: Keno Fischer <keno@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/ptrace.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index b3d3005d9515..b67b4d14aa17 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1829,10 +1829,12 @@ static void tracehook_report_syscall(struct pt_regs *regs,
>  
>  int syscall_trace_enter(struct pt_regs *regs)
>  {
> -	if (test_thread_flag(TIF_SYSCALL_TRACE) ||
> -		test_thread_flag(TIF_SYSCALL_EMU)) {
> +	u32 flags = READ_ONCE(current_thread_info()->flags) &
> +		(_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE);
> +
> +	if (flags) {

nit: but I'd rather the '&' operation was in the conditional so that the
'flags' variable holds all of the flags.

With that:

Acked-by: Will Deacon <will@xxxxxxxxxx>

Also needs:

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: f086f67485c5 ("arm64: ptrace: add support for syscall emulation")

Catalin -- can you pick this up for 5.7 please, with my 'nit' addressed?

Will