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

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

On Tue, Aug 05, 2025 at 07:57:38PM +0300, Manos Pitsidianakis wrote:
> On Tue, Aug 5, 2025 at 7:49 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > 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 ?
> 
> Some months ago I was debugging a MemoryRegion use-after-free and used
> this code to figure out that the free was called from RCU context
> instead of the main thread.

We give useful names to many (but not neccessarily all) threads that we
spawn. Perhaps we should call pthread_getname_np() to fetch the current
thread name, and used that as a prefix on the error message we print
out, as a bit of extra context ?

Obviously not as much info as a full stack trace, but that is something
we could likely enable unconditionally without any overheads to worry
about, so a likely incremental wni. 

With regards,
Daniel
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