On Apr 29, 2016 3:41 PM, "Josh Poimboeuf" [off-list ref] wrote:

On Fri, Apr 29, 2016 at 02:37:41PM -0700, Andy Lutomirski wrote:

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On Fri, Apr 29, 2016 at 2:25 PM, Josh Poimboeuf [off-list ref] wrote:

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I think the easiest way to make it work would be to modify the idtentry
macro to put all the idt entries in a dedicated section.  Then the
unwinder could easily detect any calls from that code.

That would work.  Would it make sense to do the same for the irq entries?

Yes, I think so.

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I suppose we could try to rejigger the code so that rbp points to
pt_regs or similar.

I think we should avoid doing something like that because it would break
gdb and all the other unwinders who don't know about it.

How so?

Currently, rbp in the entry code is meaningless.  I'm suggesting that,
when we do, for example, 'call \do_sym' in idtentry, we point rbp to
the pt_regs.  Currently it points to something stale (which the
dump_stack code might be relying on.  Hmm.)  But it's probably also
safe to assume that if you unwind to the 'call \do_sym', then pt_regs
is the next thing on the stack, so just doing the section thing would
work.

Yes, rbp is meaningless on the entry from user space.  But if an
in-kernel interrupt occurs (e.g. page fault, preemption) and you have
nested entry, rbp keeps its old value, right?  So the unwinder can walk
past the nested entry frame and keep going until it gets to the original
entry.

Yes.

It would be nice if we could do better, though, and actually notice
the pt_regs and identify the entry.  For example, I'd love to see
"page fault, RIP=xyz" printed in the middle of a stack dump on a
crash.  Also, I think that just following rbp links will lose the
actual function that took the page fault (or whatever function
pt_regs->ip actually points to).

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We should really re-add DWARF some day.

Working on it :-)

Excellent.

Have you looked at my vdso unwinding test at all?  If we could do
something similar for the kernel, IMO it would make testing much more
pleasant.

--Andy