Re: [RFC PATCH v2 03/24] 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 <[email protected]>
> Signed-off-by: Jens Remus <[email protected]>

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

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