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SubjectRe: [RFC PATCH v1 16/31] ARC: Signal handling
On Friday 16 November 2012 10:56 AM, Al Viro wrote:
>> + if (insyscall) {
>> + /* No handler for syscall: restart it */
>> + if (regs->r0 == -ERESTARTNOHAND ||
>> + regs->r0 == -ERESTARTSYS || regs->r0 == -ERESTARTNOINTR) {
>> + regs->r0 = regs->orig_r0;
>> + regs->ret -= 4;
>> + } else if (regs->r0 == -ERESTART_RESTARTBLOCK) {
>> + regs->r8 = __NR_restart_syscall;
>> + regs->ret -= 4;
>> + }
> What's to prevent double decrement on ->ret if two signals arrive? Note
> that e.g. x86 gets away with similar code only because it uses the same
> register for syscall number and return value; since none of -ERESTART...
> is a valid syscall number, we either won't get into an analog of that code at
> all (-ENOSYS is not restart-worthy) or will revert to a value that is
> a valid syscall number, so all subsequent do_signal() calls will not hit
> that code. This is subtle and unfortunately not spelled out in the
> architectures where it is enough.

Ok that is fixed now, by saving additional state in pt_regs->orig_r8 (which
required redoing how we kept information in it).

- long orig_r8; /*to distinguish bet excp, sys call, int1 or int2
- * syscalls -> 1 to NR_SYSCALLS
- * Exceptions -> NR_SYSCALLS + 1
- * Break-point-> NR_SYSCALLS + 2
- */
+ unsigned long event_type:16, orig_r8:16;

Alternately, I could have set orig_r8 to a special value, that would have been
simpler, but it would have meant potential breakage if one of the intermediate
signals was tracing related - leading to a ptrace(peekxxx) which relies on orig_r8
to have correct state. Also there's tracehook stuff which could enquire for orig
syscall number in orig_r8.

> You need to make sure that after the first restart in_syscall() will be false.
> Same ought to be done in sigreturn(), BTW...

Sure, this becomes very simple given the infrastructure above - patches follow for
you to take a quick peek.

Also on the topic, it seems the altstack handling is done unconditionally by most
of arches - while ARC Port does it based on a magic value. The reason being to
avoid needless calling (only to return) in sigreturn path. What's the
recommendation there.


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