Re: [PATCH] arm64: Fix PTRACE_SYSEMU semantics

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

On Mon, May 18, 2020 at 12:41:20PM +0100, Will Deacon wrote:
> 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@juliacomputing.com>
> > ---
> >  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@kernel.org>
> 
> Also needs:
> 
> Cc: <stable@vger.kernel.org>
> Fixes: f086f67485c5 ("arm64: ptrace: add support for syscall emulation")
> 
> Catalin -- can you pick this up for 5.7 please, with my 'nit' addressed?

I'll queue it with the above addressed. I think flags also needs to be
unsigned long rather than u32.

However, before sending the pull request, I'd like Sudeep to confirm
that it doesn't break his original use-case for this feature.

-- 
Catalin

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