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

["tomas.vanek at fbl dot cz" <sourceware-bugzilla@sourceware.org>]

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

tomas.vanek at fbl dot cz changed:

           What    |Removed                     |Added
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                 CC|                            |tomas.vanek at fbl dot cz

--- Comment #14 from tomas.vanek at fbl dot cz ---
Fredrik,

I'm afraid that simply adding lockup addresses to the list of magic address is
not correct. The Cortex-M lockup differs from an exception! The lockup does not
stack anything, it just sets PC (and other special regs). So there is no point
in trying to unwind lockup frame. It does not seem possible, the previous PC is
lost.

Unwinding lockup state as it was an exception may result again in peeking not
existing memory (arm_m_exception_cache() reads word @ unwound_sp + 28)

So I would recommend splitting arm_m_addr_is_magic() to
arm_m_addr_is_exc_return()
and arm_m_addr_is_lockup().
Then introduce another Cortex-M specific unwinder "arm m lockup" with a trivial
implementation:
arm_m_lockup_this_id() will always return outer_frame_id
and
arm_m_lockup_unwind_sniffer() will check PC for arm_m_addr_is_lockup()

Tom

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