Re: [PATCH v5] Add negative repeat count to 'x' command

[Thomas Preudhomme <thomas.preudhomme@foss.arm.com>]

Hi Pedro,

Sorry for the delay in answering.

On 19/08/16 19:59, Pedro Alves wrote:
>>
>
> First I think we should assess whether the behavior is correct or
> whether there's a qemu or gdb bug.  If it's a gdb bug, we should fix
> it, of course.  If it's a qemu bug, we should report it and maybe XFAIL
> it.  If it's not a bug but a qemu personality quirk, then we can make
> the tests cope, somehow.
>
> I still wonder why does the read from 0 work at all.  I've tried
> reproducing this, using x86_64 Fedora 23's stock qemu-arm:
>
> $ qemu-arm -version
> qemu-arm version 2.4.1 (qemu-2.4.1-11.fc23), Copyright (c) 2003-2008 Fabrice Bellard
>
> ... but couldn't.  Here's what I get instead:
>
> Terminal 1:
>
>  $ cat crash.c
>  int main ()
>  {
>    volatile int *null = 0;
>
>    return *null;
>  }
>
>  $ arm-linux-gnueabi -static crash.c
>
>  $ qemu-arm -g 9999 a.out
>
> Terminal 2:
>
>  $ gdb a.out -ex "tar rem :9999"
>
>  (gdb) x 0
>  Sending packet: $m0,4#fd...Ack
>  Packet received: E14
>  0x0:    Cannot access memory at address 0x0
>
> IIRC, arm's exception vector is at 0 and that that's
> where execution starts.  But that shouldn't be applicable
> to mmu userspace programs?

I'm getting:

(gdb) x 0
0x0:    0x464c457f

However note that I compiled with arm-none-eabi-gcc so there would be some bits 
of the program (ARM's exception vector indeed) at this address.

>
> I wonder what's different between our environments.

We are targetting ARM bare-metal targets.

>
> Note: I had to use -static, because of:
>
>  $ qemu-arm -g 9999  ./crash.arm
>  /lib/ld-linux.so.3: No such file or directory
>
> I suppose I would need to point qemu at an arm sysroot,
> somehow to get past that.
>
> Is there a dejagnu board file somewhere handy one could steal
> in order to run tests against qemu-user ?

Nothing public that I am aware of I'm afraid. However I can reproduce this issue 
easily by compiling examine-backward.c with the GNU ARM embedded toolchain [1] 
using the following command line:

arm-none-eabi-gcc -o examine-backward.axf 
gdb/testuite/gdb.base/examine-backward.c -specs=rdimon.specs

and then running:

qemu-arm -g 2048 examine-backward.axf &
arm-none-eabi-gdb examine-backward.axf
(gdb) target remote :2048
x/3xb 0x0
x/-6x


[1] https://launchpad.net/gcc-arm-embedded

Thanks for your effort on understanding this Pedro.

Best regards,

Thomas