Re: [PATCH 3/4] arch/arm: allow a scno of -1 to not cause a SIGILL

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Nov 01, 2012 at 12:46:38PM -0700, Kees Cook wrote:
> From: Will Drewry <wad@chromium.org>
> 
> On tracehook-friendly platforms, a system call number of -1 falls
> through without running much code or taking much action.
> 
> ARM is different.  This adds a lightweight check to arm_syscall()
> to make sure that ARM behaves the same way.
> 
> Signed-off-by: Will Drewry <wad@chromium.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm/kernel/traps.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index b0179b8..f303ea6 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -540,6 +540,10 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
>  	struct thread_info *thread = current_thread_info();
>  	siginfo_t info;
>  
> +	/* Emulate/fallthrough. */
> +	if (no == -1)
> +		return regs->ARM_r0;
> +

This won't work properly with OABI.  The problem is that OABI has an
offset on its syscall numbers which is removed/added at appropriate
times, and this is one of the places where it's put back.  So you end
up with -1 XOR 0x900000 here, not -1.

It'd probably be better to do this check in the asm code here, which
prevents that yuckyness from affecting this.

__sys_trace:
        mov     r1, scno
        add     r0, sp, #S_OFF
        bl      syscall_trace_enter

        adr     lr, BSYM(__sys_trace_return)    @ return address
        mov     scno, r0                        @ syscall number (possibly new)
        add     r1, sp, #S_R0 + S_OFF           @ pointer to regs
        cmp     scno, #NR_syscalls              @ check upper syscall limit
        ldmccia r1, {r0 - r6}                   @ have to reload r0 - r6
        stmccia sp, {r4, r5}                    @ and update the stack args
        ldrcc   pc, [tbl, scno, lsl #2]         @ call sys_* routine
+	cmp	scno, #-1
        bne     2b
+	b	ret_slow_syscall