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

[Emilio G. Cota]

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 <address@hidden>
> 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>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
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/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
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