How to backtrace an separate stack?

Stefan Hajnoczi via Gdb <[email protected]> Thu, 3 Mar 2022 11:22:32 +0000

Newsgroups	gmane.comp.gdb.devel,gmane.comp.emulators.qemu
Message-ID	<[email protected]>

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?

Thanks,
Stefan
---
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)
    old = dict()

    # remember current stack frame and select the topmost
    # so that register modifications don't wreck it
    selected_frame = gdb.selected_frame()
    gdb.newest_frame().select()

    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('bt')

    for i in regs:
        gdb.execute('set $%s = %s' % (i, old[i]))

    selected_frame.select()

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