[POWERPC] Ensure FULL_REGS on exec

When PTRACE_O_TRACEEXEC is used, a ptrace call to fetch the registers at
the PTRACE_EVENT_EXEC stop (PTRACE_PEEKUSR) will oops in CHECK_FULL_REGS.
With recent versions, "gdb --args /bin/sh -c 'exec /bin/true'" and "run" at
the (gdb) prompt is sufficient to produce this.  I also have written an
isolated test case, see https://bugzilla.redhat.com/show_bug.cgi?id=301791#c15.

This change fixes the problem by clearing the low bit of pt_regs.trap in
start_thread so that FULL_REGS is true again.  This is correct since all of
the GPRs that "full" refers to are cleared in start_thread.
Signed-off-by: NRoland McGrath <roland@redhat.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/process.c

@@ -605,6 +605,13 @@ void start_thread(struct pt_regs *regs, unsigned long start, unsigned long sp)
 	regs->ccr = 0;
 	regs->gpr[1] = sp;
 
+	/*
+	 * We have just cleared all the nonvolatile GPRs, so make
+	 * FULL_REGS(regs) return true.  This is necessary to allow
+	 * ptrace to examine the thread immediately after exec.
+	 */
+	regs->trap &= ~1UL;
+
 #ifdef CONFIG_PPC32
 	regs->mq = 0;
 	regs->nip = start;