Re: [PATCH v6 23/25] tests: add plugin asserting correctness of discon event's to_pc

["Alex Bennée" <alex.bennee@linaro.org>]

"Julian Ganz" <neither@nut.email> writes:

(add Richard to Cc)

> September 23, 2025 at 10:29 PM, "Julian Ganz" wrote:
>> September 22, 2025 at 12:11 PM, "Julian Ganz" wrote:
>> > September 21, 2025 at 6:46 PM, "Alex Bennée" wrote:
>> > This is firing on:
>> >  
>> >  🕙17:35:50 alex@draig:tests/tcg/i386-linux-user on  review/tcg-discon-v6 [$!?] 
>> >  ➜ make run-plugin-catch-syscalls-with-libdiscons.so V=1
>> >  timeout -s KILL --foreground 120 env
>> > QEMU=/home/alex/lsrc/qemu.git/builds/sanitisers/qemu-i386
>> > /home/alex/lsrc/qemu.git/builds/sanitisers/qemu-i386 -plugin
>> > ../plugins/libdiscons.so -d plugin -D
>> > catch-syscalls-with-libdiscons.so.pout catch-syscalls >
>> > run-plugin-catch-syscalls-with-libdiscons.so.out
>> >  Aborted
>> >  make: *** [Makefile:226: run-plugin-catch-syscalls-with-libdiscons.so] Error 134
<snip>
>> >  (gdb) p report->str
>> >  $2 = (gchar *) 0x51100001fbc0 "Discon target PC mismatch on VCPU 0\nExpected: 8057369\nEncountered: 805736b\nExecuted Last: 805736b\nEvent type: exception\n"
>> >  (gdb) 
>> >  
>> >  I think this is where it is going wrong:
>> >  
>> >  IN: _dl_early_allocate
>> >  0x0805736b: 89 c2 movl %eax, %edx
>> >  0x0805736d: 8d 1c 28 leal (%eax, %ebp), %ebx
>> >  0x08057370: 89 c8 movl %ecx, %eax
>> >  0x08057372: cd 80 int $0x80
>> >  
>> >  Thanks! I'll have a closer look.
>> > 
>> I probably didn't configure the target I need for this test on my
>> private machine (which uses musl, so some targets are awkward).
>
> Turned out I just ran `make` in the wrong directory.
>
>> Could it
>> be that this doesn't run in system emulation mode and the exception is
>> somehow handled natively? I did not account for that possibility and I
>> don't think I'll get the testing plugin to do anything meaningful
>> outside system emulation.
>
> As expected the plugin ran in a qemu configuration in which the API does
> not behave as expected. Which I could have figured out by the logs Alex
> provided if I paid attention. Sorry for the noise.
>
> I added a check to the plugin that bails out if qemu does not run in
> system emulation mode (which I thought I did, but that was only for the
> contrib plugin).
>
> Even if we cannot test the API for user mode, it may still provide some
> utility,  so I suggest _just_ letting the testing plugin do nothing in
> that case. In fact, I'm considering removing the check from the contrib
> "traps" plugin now that I saw that the API _is_ triggered in user
> mode.

Is this just because we are missing the hooks into the linux-user
run-loop? I assume int $0x80 is a syscall so we should report an
exception type at that point?

In fact I think you could probably generate the event at
qemu_plugin_vcpu_syscall()?

force_sig_fault() and force_sig() might be the other places that should
be hooked. Or possibly handle_pending_signal() where all the other
signals affect the code flow.

I think that can avoid patching every arches run loop.

Richard, WDYT?

>
> Regards,
> Julian

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro