Re: [Qemu-devel] target/ppc: gdbstub single-step broken since TranslatorOps conversion

["Emilio G. Cota" <cota@braap.org>]

On Fri, Jun 29, 2018 at 13:17:11 +0100, Mark Cave-Ayland wrote:
> Whilst trying to debug an issue in OpenBIOS, I noticed that the gdbstub
> single-step functionality was broken which I bisected down to this commit:
(snip)
> commit b0c2d5213a14f8b9d44096ee879a5d7f10fbc505
> Author: Emilio G. Cota <cota@braap.org>
> Date:   Thu Feb 15 14:51:49 2018 -0500
> 
>     target/ppc: convert to TranslatorOps
(snip)
> (gdb) c
> Continuing.
> 
> Breakpoint 1, ob_pci_init () at
> /home/build/src/openbios/openbios.git/openbios/drivers/pci.c:2020
> 2020        pci_config_t config = {}; /* host bridge */
> (gdb) n
> 2021        phandle_t phandle_host = 0, intc;
> (gdb) n
> 2025        activate_device("/");
> (gdb) n
> 
> (hangs)
> 
> 
> Hitting CTRL-C sometimes allows the single-step to continue but raises a
> SIGINT at address 0x400 which is in the CPU exception table.

Thanks for reporting this.

I'm afraid I can't reproduce this. I'm using gdb-multiarch as follows:

$ gdb-multiarch ../img/ppc/openbios-qemu.elf.nostrip
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
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For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ../img/ppc/openbios-qemu.elf.nostrip...done.
warning: File "/data/src/qemu/.gdbinit" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
To enable execution of this file add
        add-auto-load-safe-path /data/src/qemu/.gdbinit
line to your configuration file "/home/cota/.gdbinit".
To completely disable this security protection add
        set auto-load safe-path /
line to your configuration file "/home/cota/.gdbinit".
For more information about this security protection see the
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:
        info "(gdb)Auto-loading safe path"
(gdb) set arch powerpc:
403       603       620       7400      EC603e    a35       common64  e500mc    e5500     rs64ii    titan
601       604       630       750       MPC8XX    common    e500      e500mc64  e6500     rs64iii   vle
(gdb) set arch powerpc:403
The target architecture is assumed to be powerpc:403
(gdb) b ob_pci_init
Breakpoint 1 at 0xfff25c58: file /home/build/src/openbios/openbios.git/openbios/drivers/pci.c, line 2020.
(gdb) target remote :1234
Remote debugging using :1234
warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

vector.0x100 () at /home/build/src/openbios/openbios.git/openbios/arch/ppc/qemu/start.S:284
284     /home/build/src/openbios/openbios.git/openbios/arch/ppc/qemu/start.S: No such file or directory.
warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

warning: (Internal error: pc 0xfff00100 in read in psymtab, but not in symtab.)

(gdb) c
Continuing.

Breakpoint 1, ob_pci_init () at /home/build/src/openbios/openbios.git/openbios/drivers/pci.c:2020
2020    /home/build/src/openbios/openbios.git/openbios/drivers/pci.c: No such file or directory.
(gdb) n
2021    in /home/build/src/openbios/openbios.git/openbios/drivers/pci.c
(gdb)
2025    in /home/build/src/openbios/openbios.git/openbios/drivers/pci.c
(gdb)
2029    in /home/build/src/openbios/openbios.git/openbios/drivers/pci.c

Here I can keep executing for a while, with no hangs. Eventually I hit
(gdb) 
arch_of_init () at /home/build/src/openbios/openbios.git/openbios/arch/ppc/qemu/init.c:939
939     in /home/build/src/openbios/openbios.git/openbios/arch/ppc/qemu/init.c
Cannot access memory at address 0xfff0c054

I tried the last known good commit (as per your bisect) and get the same results
as above.

Is there any other way I could try reproducing this? Also, can you
reproduce with another gdb version?

If I don't manage to reproduce, I can send you off-list some patches to try,
basically breaking down the diff in the first bad commit into very small chunks,
to see where the problem was introduced.

Thanks,

		Emilio