Re: [RFC PATCH v1 03/25] unwind_user: Enable archs that pass RA in a register

Not all architectures/ABIs pass the return address (RA) on the stack on
function entry, like x86-64 does due to its CALL instruction pushing
the RA onto the stack.  Architectures/ABIs, such as s390, also do not
require the RA to be saved on the stack in the function prologue.  In
particular, the RA may never be saved to the stack at all, such as in
leaf functions.  Unwinding must therefore not assume the presence of a
RA saved on stack for the topmost frame.

Treat a RA offset from CFA of zero as indication that the RA is not
saved (on the stack).  For the topmost frame treat it as indication that
the RA is in the link/RA register, such as on arm64 and s390, and obtain
it from there.  For non-topmost frames treat it as error, as the RA must
be saved.

Additionally allow the SP to be unchanged in the topmost frame, for
architectures where SP at function entry == SP at call site, such as
arm64 and s390.

Note that treating a RA offset from CFA of zero as indication that
the RA is not saved on the stack additionally allows for architectures,
such as s390, where the frame pointer (FP) may be saved without the RA
being saved as well.  Provided that such architectures represent this
in SFrame by encoding the "missing" RA offset using a padding RA offset
with a value of zero.

Reviewed-by: Indu Bhagat <redacted>
Signed-off-by: Jens Remus <redacted>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260818144954.2320378-1-jremus@linux.ibm.com?part=3