On Friday 16 November 2012 10:56 AM, Al Viro wrote:

quoted

+	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.

Thx,
Vineet