How to backtrace an separate stack?
Stefan Hajnoczi
stefanha@redhat.com
Tue Mar 8 09:47:31 GMT 2022
On Mon, Mar 07, 2022 at 10:49:47AM +0000, Pedro Alves wrote:
> On 2022-03-03 11:22, Stefan Hajnoczi wrote:
> > Hi,
> > The QEMU emulator uses coroutines with separate stacks. It can be
> > challenging to debug coroutines that have yielded because GDB is not
> > aware of them (no thread is currently executing them).
> >
> > QEMU has a GDB Python script that helps. It "creates" a stack frame for
> > a given coroutine by temporarily setting register values and then using
> > the "bt" command. This works on a live process under ptrace control but
> > not for coredumps where registers can't be set.
> >
> > Here is the script (or see the bottom of this email for an inline copy
> > of the relevant code):
> > https://gitlab.com/qemu-project/qemu/-/blob/master/scripts/qemugdb/coroutine.py
> >
> > I hoped that "select-frame address ADDRESS" could be used instead so
> > this would work on coredumps too. Unfortunately "select-frame" only
> > searches stack frames that GDB is already aware of, so it cannot be used
> > to backtrace coroutine stacks.
> >
> > Is there a way to backtrace a stack at an arbitrary address in GDB?
>
> I don't think there's an easy/great answer. Maybe it could
> be done with a Python unwinder [1]? See gdb.python/py-unwind-user-regs.py
> in the GDB testsuite for an example you could probably start with.
I tried writing an unwinder that returns the topmost coroutine stack
frame. "info threads" + "bt" shows the main stack though:
(gdb) qemu coroutine 0x55be3c592120
Id Target Id Frame
* 1 Thread 0x7f7abbdd4f00 (LWP 58989) Returning a frame with rip 0x55be3ae19ff4
0x00007f7abcd2489e in __ppoll (fds=0x21, nfds=6717500806073509987, timeout=<optimized out>, sigmask=0x1f000) at ../sysdeps/unix/sysv/linux/ppoll.c:43
...
#0 0x00007f7abcd2489e in __ppoll (fds=0x55be3c78a9f0, nfds=43, timeout=<optimized out>, timeout@entry=0x7ffef27cc040, sigmask=sigmask@entry=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:43
#1 0x000055be3ae26435 in ppoll (__ss=0x0, __timeout=0x7ffef27cc040, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/bits/poll2.h:81
I was hoping that frame #1 would be the coroutine stack since the debug
message "Returning a frame with rip 0x55be3ae19ff4" shows the unwinder
was invoked.
I've included the code below in case anyone has suggestions for making
the unwinder work. See bt_jmpbuf() and the Unwinder class.
The idea is that bt_jmpbuf() passes the registers of the coroutine to
the unwinder and invokes "info thread" + "bt". The unwinder only returns
a stack frame the first time it's invoked. It cannot unwind successive
stack frames so it disables itself after returning the topmost one (I
was hoping GDB's built-in unwinder would take over from there).
Thanks,
Stefan
---
#
# GDB debugging support
#
# Copyright 2012 Red Hat, Inc. and/or its affiliates
#
# Authors:
# Avi Kivity <avi@redhat.com>
#
# This work is licensed under the terms of the GNU GPL, version 2
# or later. See the COPYING file in the top-level directory.
import gdb
import gdb.unwinder
VOID_PTR = gdb.lookup_type('void').pointer()
class FrameId(object):
def __init__(self, sp, pc):
self.sp = sp
self.pc = pc
class Unwinder(gdb.unwinder.Unwinder):
def __init__(self):
super(Unwinder, self).__init__('QEMU coroutine unwinder')
self._regs = None
def arm(self, regs):
self._regs = regs
def __call__(self, pending_frame):
print('A')
if not self._regs:
return None
regs = self._regs
self._regs = None
frame_id = FrameId(regs['rbp'], regs['rip'])
unwind_info = pending_frame.create_unwind_info(frame_id)
for reg_name in regs:
unwind_info.add_saved_register(reg_name, regs[reg_name])
print('Returning a frame with rip 0x%x' % regs['rip'])
return unwind_info
unwinder = Unwinder()
gdb.unwinder.register_unwinder(None, unwinder)
def pthread_self():
'''Fetch pthread_self() from the glibc start_thread function.'''
f = gdb.newest_frame()
while f.name() != 'start_thread':
f = f.older()
if f is None:
return gdb.parse_and_eval('$fs_base')
try:
return f.read_var("arg")
except ValueError:
return gdb.parse_and_eval('$fs_base')
def get_glibc_pointer_guard():
'''Fetch glibc pointer guard value'''
fs_base = pthread_self()
return gdb.parse_and_eval('*(uint64_t*)((uint64_t)%s + 0x30)' % fs_base)
def glibc_ptr_demangle(val, pointer_guard):
'''Undo effect of glibc's PTR_MANGLE()'''
return gdb.parse_and_eval('(((uint64_t)%s >> 0x11) | ((uint64_t)%s << (64 - 0x11))) ^ (uint64_t)%s' % (val, val, pointer_guard))
def get_jmpbuf_regs(jmpbuf):
JB_RBX = 0
JB_RBP = 1
JB_R12 = 2
JB_R13 = 3
JB_R14 = 4
JB_R15 = 5
JB_RSP = 6
JB_PC = 7
pointer_guard = get_glibc_pointer_guard()
return {'rbx': jmpbuf[JB_RBX],
'rbp': glibc_ptr_demangle(jmpbuf[JB_RBP], pointer_guard),
'rsp': glibc_ptr_demangle(jmpbuf[JB_RSP], pointer_guard),
'r12': jmpbuf[JB_R12],
'r13': jmpbuf[JB_R13],
'r14': jmpbuf[JB_R14],
'r15': jmpbuf[JB_R15],
'rip': glibc_ptr_demangle(jmpbuf[JB_PC], pointer_guard) }
def bt_jmpbuf(jmpbuf):
'''Backtrace a jmpbuf'''
regs = get_jmpbuf_regs(jmpbuf)
unwinder.arm(regs)
gdb.execute('info threads')
gdb.execute('bt')
def co_cast(co):
return co.cast(gdb.lookup_type('CoroutineUContext').pointer())
def coroutine_to_jmpbuf(co):
coroutine_pointer = co_cast(co)
return coroutine_pointer['env']['__jmpbuf']
class CoroutineCommand(gdb.Command):
'''Display coroutine backtrace'''
def __init__(self):
gdb.Command.__init__(self, 'qemu coroutine', gdb.COMMAND_DATA,
gdb.COMPLETE_NONE)
def invoke(self, arg, from_tty):
argv = gdb.string_to_argv(arg)
if len(argv) != 1:
gdb.write('usage: qemu coroutine <coroutine-pointer>\n')
return
bt_jmpbuf(coroutine_to_jmpbuf(gdb.parse_and_eval(argv[0])))
class CoroutineBt(gdb.Command):
'''Display backtrace including coroutine switches'''
def __init__(self):
gdb.Command.__init__(self, 'qemu bt', gdb.COMMAND_STACK,
gdb.COMPLETE_NONE)
def invoke(self, arg, from_tty):
gdb.execute("bt")
if gdb.parse_and_eval("qemu_in_coroutine()") == False:
return
co_ptr = gdb.parse_and_eval("qemu_coroutine_self()")
while True:
co = co_cast(co_ptr)
co_ptr = co["base"]["caller"]
if co_ptr == 0:
break
gdb.write("Coroutine at " + str(co_ptr) + ":\n")
bt_jmpbuf(coroutine_to_jmpbuf(co_ptr))
class CoroutineSPFunction(gdb.Function):
def __init__(self):
gdb.Function.__init__(self, 'qemu_coroutine_sp')
def invoke(self, addr):
return get_jmpbuf_regs(coroutine_to_jmpbuf(addr))['rsp'].cast(VOID_PTR)
class CoroutinePCFunction(gdb.Function):
def __init__(self):
gdb.Function.__init__(self, 'qemu_coroutine_pc')
def invoke(self, addr):
return get_jmpbuf_regs(coroutine_to_jmpbuf(addr))['rip'].cast(VOID_PTR)
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