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[Qemu-devel] [Bug 1364501] Re: Gdb hangs when trying to single-step afte
From: |
Martin |
Subject: |
[Qemu-devel] [Bug 1364501] Re: Gdb hangs when trying to single-step after an invalid instruction |
Date: |
Tue, 02 Sep 2014 19:38:35 -0000 |
>> Public bug reported:
>>
>> When using Gdb to remote-debug a program and manually setting its PC to
>> point to an address containing an invalid instruction and then doing a
>> single step, Qemu will never return control to the remote Gdb.
>>
>> For instance, let's say address 0x114 contains an invalid instruction.
>> On the remote Gdb, we'd do:
>>
>> (gdb) set $pc = 0x114
>> (gdb) stepi
>>
>> After doing that we won't get the (gdb) prompt unless we do a Ctrl-C. If
>> we do so we'll be left at 0x114 instead of going towards the exception
>> handler as we should. This happens with stepi, step and next. If instead
>> of single-stepping we used continue, the program will proceed into the
>> exception handler as it should.
>>
>> The reason this is happening is that when Qemu realizes it's about to
>> translate an instruction it doesn't recognize it'll generate a call to
>> helper_exception_with_syndrome(), which will register the exception and
>> then call cpu_loop_exit(). At the same time, because we're doing a
>> single-step, Qemu will also generate a call to
>> helper_exception_internal() passing it an EXCP_DEBUG, which lets the
>> system know it'll give control back to the remote debugger, and it also
>> ends with a call to cpu_loop_exit(). However, because the syndrome
>> exception calls cpu_loop_exit() first, the call to the internal
>> exception won't be reached and Qemu will be stuck in a loop without
>> returning control to the remote debugger.
>
> Just to check, does your system image include a valid handler
> for the undef exception?
Yes, it does. RTEMS sets a default handler for every exception, and just
to be sure I added a bunch of small wrappers so I'll be able to know
what kind of exception are we dealing with.
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https://bugs.launchpad.net/bugs/1364501
Title:
Gdb hangs when trying to single-step after an invalid instruction
Status in QEMU:
New
Bug description:
When using Gdb to remote-debug a program and manually setting its PC
to point to an address containing an invalid instruction and then
doing a single step, Qemu will never return control to the remote Gdb.
For instance, let's say address 0x114 contains an invalid instruction.
On the remote Gdb, we'd do:
(gdb) set $pc = 0x114
(gdb) stepi
After doing that we won't get the (gdb) prompt unless we do a Ctrl-C.
If we do so we'll be left at 0x114 instead of going towards the
exception handler as we should. This happens with stepi, step and
next. If instead of single-stepping we used continue, the program will
proceed into the exception handler as it should.
The reason this is happening is that when Qemu realizes it's about to
translate an instruction it doesn't recognize it'll generate a call to
helper_exception_with_syndrome(), which will register the exception
and then call cpu_loop_exit(). At the same time, because we're doing a
single-step, Qemu will also generate a call to
helper_exception_internal() passing it an EXCP_DEBUG, which lets the
system know it'll give control back to the remote debugger, and it
also ends with a call to cpu_loop_exit(). However, because the
syndrome exception calls cpu_loop_exit() first, the call to the
internal exception won't be reached and Qemu will be stuck in a loop
without returning control to the remote debugger.
What makes this a bit tricky to fix is that we must call
cpu_loop_exit() at the end of helper_exception_with_syndrome(),
otherwise the target exception will go undetected and its handler
won't be excecuted.
Tested on latest head by emulating a Stellaris lm3s6965 board and
running RTEMS 4.11:
$ qemu-system-arm -nographic -s -S -M lm3s6965evb -kernel my_rtems_app
Commit hash in qemu.git: 30eaca3acdf17d7bcbd1213eb149c02037edfb0b
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