sys_exit: NR -1

[Paul Clarke]

On 6/12/19 1:32 AM, Naveen N. Rao wrote:
> Paul Clarke wrote:
>> What are the circumstances in which raw_syscalls:sys_exit reports "-1" for the syscall ID?
>>
>>     perf  5375 [007] 59632.478528:   raw_syscalls:sys_enter: NR 1 (3, 9fb888, 8, 2d83740, 1, 7ffff)
>>     perf  5375 [007] 59632.478532:    raw_syscalls:sys_exit: NR 1 = 8
>>     perf  5375 [007] 59632.478538:   raw_syscalls:sys_enter: NR 15 (11, 7ffffca734b0, 7ffffca73380, 2d83740, 1, 7ffff)
>>     perf  5375 [007] 59632.478539:    raw_syscalls:sys_exit: NR -1 = 8
>>     perf  5375 [007] 59632.478543:   raw_syscalls:sys_enter: NR 16 (4, 2401, 0, 2d83740, 1, 0)
>>     perf  5375 [007] 59632.478551:    raw_syscalls:sys_exit: NR 16 = 0
> 
> Which architecture?
> For powerpc, see:
> 
> static inline int syscall_get_nr(struct task_struct *task, struct pt_regs *regs)
> {
>     /*
>      * Note that we are returning an int here. That means 0xffffffff, ie.
>      * 32-bit negative 1, will be interpreted as -1 on a 64-bit kernel.
>      * This is important for seccomp so that compat tasks can set r0 = -1
>      * to reject the syscall.
>      */
>     return TRAP(regs) == 0xc00 ? regs->gpr[0] : -1;
> }

So, that's intentional?  And has some special meaning?  (I confess I don't understand what the comment is saying exactly.)

Is this documented?  Does something depend on this ABI?

To me, it just makes parsing more difficult, both by humans and machines.

PC