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

[Markus Armbruster <armbru@redhat.com>]

Manos Pitsidianakis <manos.pitsidianakis@linaro.org> writes:

> 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.
>
> Hi Daniel,
>
> 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.

Enable core dumps.  I doubt that's harder than recompiling and
redeploying QEMU with backtraces enabled.