[Bug gdb/28549] ARM/Cortex-M: improper stack unwinding when the target is in lockup state

["tarek.bouchkati at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28549

--- Comment #5 from Tarek BOCHKATI <tarek.bouchkati at gmail dot com> ---
0xfffffffe is just one missing magic address.
according to arm: 
> During LOCKUP, the PC will be forced to 0xFFFFFFFx

so all 0xFFFFFFFx should be considered magic as well, and this hides several
EXC_RETURN

I'm wondering why we are linking this to EXC_RETURN, since this function is
used to check PC values and EXC_RETURN values are loaded into LR.

more literature:
according to ARM v6M Arch ref man (ARM DDI 0419E page B1-198),
section B1.5.8: Exception return behavior:
> An exception return occurs when the processor is in Handler mode
> and one of the following instructions loads a value of 0xFXXXXXXX into the PC

according to this all 0xfxxxxxxx values are magic
the same text is present in ARM v7M arch ref man

in ARM v8M ref man, section B3.22:
> ... loads an EXC_RETURN value, 0xFFXXXXXX, into the PC


============================================

regarding: 
is it right to display something like <signal handler called>?
it's fine for me, unless there is a way to display something similar to
"exception" 

but I agree, if we want to have a minor fix/change:
we can limit to adding the 3 magic addresses:
+      case 0xeffffffe:
+      case 0xfffffffe:
+      case 0xffffffff:

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