Re: [PATCH RFC] util/error.c: Print backtrace on error

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Aug 05, 2025 at 07:22:14PM +0300, Manos Pitsidianakis wrote:
> On Tue, Aug 5, 2025 at 7:00 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > On Tue, Aug 05, 2025 at 12:19:26PM +0300, Manos Pitsidianakis wrote:
> > > Add a backtrace_on_error meson feature (enabled with
> > > --enable-backtrace-on-error) that compiles system binaries with
> > > -rdynamic option and prints a function backtrace on error to stderr.
> > >
> > > Example output by adding an unconditional error_setg on error_abort in hw/arm/boot.c:
> > >
> > >   ./qemu-system-aarch64(+0x13b4a2c) [0x55d015406a2c]
> > >   ./qemu-system-aarch64(+0x13b4abd) [0x55d015406abd]
> > >   ./qemu-system-aarch64(+0x13b4d49) [0x55d015406d49]
> > >   ./qemu-system-aarch64(error_setg_internal+0xe7) [0x55d015406f62]
> > >   ./qemu-system-aarch64(arm_load_dtb+0xbf) [0x55d014d7686f]
> > >   ./qemu-system-aarch64(+0xd2f1d8) [0x55d014d811d8]
> > >   ./qemu-system-aarch64(notifier_list_notify+0x44) [0x55d01540a282]
> > >   ./qemu-system-aarch64(qdev_machine_creation_done+0xa0) [0x55d01476ae17]
> > >   ./qemu-system-aarch64(+0xaa691e) [0x55d014af891e]
> > >   ./qemu-system-aarch64(qmp_x_exit_preconfig+0x72) [0x55d014af8a5d]
> > >   ./qemu-system-aarch64(qemu_init+0x2a89) [0x55d014afb657]
> > >   ./qemu-system-aarch64(main+0x2f) [0x55d01521e836]
> > >   /lib/x86_64-linux-gnu/libc.so.6(+0x29ca8) [0x7f3033d67ca8]
> > >   /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85) [0x7f3033d67d65]
> > >   ./qemu-system-aarch64(_start+0x21) [0x55d0146814f1]
> > >
> > >   Unexpected error in arm_load_dtb() at ../hw/arm/boot.c:529:
> >
> > From an end-user POV, IMHO the error messages need to be good enough
> > that such backtraces aren't needed to understand the problem. For
> > developers, GDB can give much better backtraces (file+line numbers,
> > plus parameters plus local variables) in the ideally rare cases that
> > the error message alone has insufficient info. So I'm not really
> > convinced that programs (in general, not just QEMU) should try to
> > create backtraces themselves.
> 
> I don't think there's value in replacing gdb debugging with this, I
> agree. I think it has value for "fire and forget" uses, when errors
> happen unexpectedly and are hard to replicate and you only end up with
> log entries and no easy way to debug it.

If the log entry with the error message is useless for devs, then it
is even worse for end users... who will be copying that message into
bug reports anyway. This patch doesn't feel like something we could
enable in formal builds in the distro, so we still need better error
reporting without it, such that user bug reports are actionable.

Was there a specific place where you found things hard to debug
from the error message alone ?  I'm sure we have plenty of examples
of errors that can be improved, but wondering if there are some
general patterns we're doing badly that would be a good win
to improve ?

With regards,
Daniel
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