Re: [Qemu-devel] [PATCH] scripts/qemugdb: support coroutine backtrace in coredumps

[Stefan Hajnoczi <stefanha at gmail dot com>]

On Mon, Apr 09, 2018 at 04:01:24PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 04.04.2018 13:34, Stefan Hajnoczi wrote:
> > Use the 'select-frame' GDB command to switch stacks instead of manually
> > setting the debugged thread's registers (this only works when debugging
> > a live process, not in a coredump).
> > 
> > Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > Vladimir: Does this work for you?
> > 
> >   scripts/qemugdb/coroutine.py | 6 ++----
> >   1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/scripts/qemugdb/coroutine.py b/scripts/qemugdb/coroutine.py
> > index ab699794ab..ed96434aee 100644
> > --- a/scripts/qemugdb/coroutine.py
> > +++ b/scripts/qemugdb/coroutine.py
> > @@ -77,13 +77,11 @@ def bt_jmpbuf(jmpbuf):
> >       for i in regs:
> >           old[i] = gdb.parse_and_eval('(uint64_t)$%s' % i)
> > -    for i in regs:
> > -        gdb.execute('set $%s = %s' % (i, regs[i]))
> > +    gdb.execute('select-frame %s %s' % (regs['rsp'], regs['rip']))
> >       gdb.execute('bt')
> > -    for i in regs:
> > -        gdb.execute('set $%s = %s' % (i, old[i]))
> > +    gdb.execute('select-frame %s %s' % (old['rsp'], old['rip']))
> >   def coroutine_to_jmpbuf(co):
> >       coroutine_pointer = co.cast(gdb.lookup_type('CoroutineUContext').pointer())
> 
> strange, but it doesn't work. it prints the same backtrace, as if I just
> call bt.
> (I applied it onto "[PATCH 2/4] scripts/qemugdb: improve "qemu coroutine"
> command")
> 
> also, I can just call select-frame with zeros or any garbage in gdb, with
> same effect:
> (gdb) select-frame 0 0
> (gdb) bt
> 
> and get same backtrace.
> 
> so, bt command not related to selected frame. also, up and down commands
> don't help too, they go to frames in current bt, instead of moving
> relatively to selected frame.

I wonder what the point of select-frame is then...

I have CCed the GDB mailing list.  Maybe someone can help us.  Context:

QEMU implements coroutines using jmpbuf.  We'd like to print coroutine
call stacks in GDB and have a script that works when a process is being
debugged (it sets the registers).

Now we'd like to extend the script to work on core dumps where it's not
possible to set registers (since there is no process being debugged).

Is there a way to backtrace an arbitrary call stack in a core dump?

Stefan

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