[PATCH] arm64: fix missing syscall trace exit

[Josh Stone]

On 06/02/2015 06:01 PM, Josh Stone wrote:
> If a syscall is entered without TIF_SYSCALL_TRACE set, then it goes on
> the fast path.  It's then possible to have TIF_SYSCALL_TRACE added in
> the middle of the syscall, but ret_fast_syscall doesn't check this flag
> again.  This causes a ptrace syscall-exit-stop to be missed.
> 
> For instance, from a PTRACE_EVENT_FORK reported during do_fork, the
> tracer might resume with PTRACE_SYSCALL, setting TIF_SYSCALL_TRACE.
> Now the completion of the fork should have a syscall-exit-stop.
> 
> Russell King fixed this on arm by re-checking _TIF_SYSCALL_WORK in the
> fast exit path.  Do the same on arm64.
> 
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will.deacon at arm.com>
> Cc: Russell King <rmk at arm.linux.org.uk>
> Signed-off-by: Josh Stone <jistone at redhat.com>
> ---
>  arch/arm64/kernel/entry.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 959fe8733560..a547a3e8a198 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -608,7 +608,9 @@ ENDPROC(cpu_switch_to)
>   */
>  ret_fast_syscall:
>  	disable_irq				// disable interrupts
> -	ldr	x1, [tsk, #TI_FLAGS]
> +	ldr	x1, [tsk, #TI_FLAGS]		// re-check for syscall tracing
> +	and	x2, x1, #_TIF_SYSCALL_WORK
> +	cbnz	x2, __sys_trace_return
>  	and	x2, x1, #_TIF_WORK_MASK
>  	cbnz	x2, fast_work_pending
>  	enable_step_tsk x1, x2

I do have one concern about this, also in Russell's ARM patch.  Is it
really ok to branch to __sys_trace_return with interrupts disabled?

I didn't hit any issue from that, but my testcase only exercises this
path once each run.  So that might have just been lucky not to hit any
gross scenario...