ARM: 9183/1: unwind: avoid spurious warnings on bogus code addresses

Corentin reports that since commit 538b9265c0 ("ARM: unwind: track
location of LR value in stack frame"), numerous spurious warnings are
emitted into the kernel log:

  [    0.000000] unwind: Index not found c0f0c440
  [    0.000000] unwind: Index not found 00000000
  [    0.000000] unwind: Index not found c0f0c440
  [    0.000000] unwind: Index not found 00000000

This is due to the fact that the commit in question removes a check
whether the PC value in the unwound frame is actually a kernel text
address, on the assumption that such an address will not be associated
with valid unwind data to begin with, which is checked right after.

The reason for removing this check was that unwind_frame() will be
called by the ftrace graph tracer code, which means that it can no
longer be safely instrumented itself, or any code that it calls, as it
could cause infinite recursion.

In order to prevent the spurious diagnostics, let's add back the call to
kernel_text_address(), but this time, only call it if no unwind data
could be found for the address in question. This is more efficient for
the common successful case, and should avoid any unintended recursion,
considering that kernel_text_address() will only be called if no unwind
data was found.

Cc: Corentin Labbe <clabbe.montjoie@gmail.com>
Fixes: 538b9265c0 ("ARM: unwind: track location of LR value in stack frame")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/unwind.c

@@ -400,7 +400,8 @@ int unwind_frame(struct stackframe *frame)
 
 	idx = unwind_find_idx(frame->pc);
 	if (!idx) {
-		pr_warn("unwind: Index not found %08lx\n", frame->pc);
+		if (frame->pc && kernel_text_address(frame->pc))
+			pr_warn("unwind: Index not found %08lx\n", frame->pc);
 		return -URC_FAILURE;
 	}