new helper: signal_pt_regs()

Always equal to task_pt_regs(current); defined only when we are in
signal delivery.  It may be different from current_pt_regs() - e.g.
architectures like m68k may have pt_regs location on exception
different from that on a syscall and signals (just as ptrace handling)
may happen on exceptions as well as on syscalls.

When they are equal, it's often better to have signal_pt_regs
defined (in asm/ptrace.h) as current_pt_regs - that tends to be
optimized better than default would be.  However, optimisation is
the only reason why we might want an arch-specific definition;
if current_pt_regs() and task_pt_regs(current) have different
values, the latter one is right.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/alpha/include/asm/ptrace.h

@@ -78,6 +78,7 @@ struct switch_stack {
 
 #define current_pt_regs() \
   ((struct pt_regs *) ((char *)current_thread_info() + 2*PAGE_SIZE) - 1)
+#define signal_pt_regs current_pt_regs
 
 #define force_successful_syscall_return() (current_pt_regs()->r0 = 0)
 

arch/h8300/include/asm/ptrace.h

@@ -62,6 +62,7 @@ struct pt_regs {
 #define profile_pc(regs) instruction_pointer(regs)
 #define current_pt_regs() ((struct pt_regs *) \
 	(THREAD_SIZE + (unsigned long)current_thread_info()) - 1)
+#define signal_pt_regs() ((struct pt_regs *)current->thread.esp0)
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 #endif /* _H8300_PTRACE_H */

include/linux/ptrace.h

@@ -333,6 +333,15 @@ static inline void user_single_step_siginfo(struct task_struct *tsk,
 #define ptrace_signal_deliver(regs, cookie) do { } while (0)
 #endif
 
+/*
+ * unlike current_pt_regs(), this one is equal to task_pt_regs(current)
+ * on *all* architectures; the only reason to have a per-arch definition
+ * is optimisation.
+ */
+#ifndef signal_pt_regs
+#define signal_pt_regs() task_pt_regs(current)
+#endif
+
 extern int task_current_syscall(struct task_struct *target, long *callno,
 				unsigned long args[6], unsigned int maxargs,
 				unsigned long *sp, unsigned long *pc);