On Friday 16 November 2012 10:28 AM, Al Viro wrote:

quoted

+	; --------- check for signals/restore-sigmask ------------
+	bbit0  r9, TIF_SIGPENDING, chk_next_work
+
+	; save CALLEE Regs.
+	; (i)  If this signal causes coredump - full regfile needed
+	; (ii) If signal is SIGTRAP/SIGSTOP, task is being traced thus
+	;      tracer might call PEEKUSR for a CALLEE reg
+	;
+	; NOTE: SP will grow up by size of CALLEE Reg-File
+	SAVE_CALLEE_SAVED_USER		; clobbers r12
+
+	; save location of saved Callee Regs @ thread_struct->callee
+	GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r10
+	st  sp, [r10, THREAD_CALLEE_REG]
+
+	bl  @do_signal
+
+	; unwind SP for cheap discard of Callee saved Regs
+	DISCARD_CALLEE_SAVED_USER

Uh-oh...  And what if tracer wanted to modify callee-saved regs?

So the solution would be to either unconditionally restore all the 13 callee regs
- or add additional state (struct thread) where ptrace makes a note that it wrote
to a callee reg which is used here to conditional-ize the restore. Former is
simpler to do - although it might ill-affect micro-benchmarks such as LMBench
lat_sig. Anyhow correctness comes before optimization.

quoted

+	b      resume_user_mode_begin	; loop back to start of U mode ret
+
+	; --- notify_resume ---
+chk_next_work:
+	btst   r9, TIF_NOTIFY_RESUME
+	blnz   @do_notify_resume
+
+	;--------- All things done, go back to Userland ------
+
+	b restore_regs

No.  After NOTIFY_RESUME stuff you need to recheck SIGPENDING.  This should
go to resume_user_mode_begin, not restore regs.  

Thanks for pointing this - fixed in v2 series.

Another problem here is
IRQ handling - you hit do_signal()/do_notify_resume() with IRQs disabled,
which is broken - you need to re-enable it before going into either.

Thanks - fixed that as well. I presume it is broken for "interrupt latency
reasons" and not because IRQs could remain disabled all the way into usermode -
since given the way RTIE insn works - that won't happen. Although independently
this was indeed cause of a WARN_ON_ONCE when bh were enabled with IRQs disabled.