Re: [Qemu-devel] [PATCH] gdbstub.c: fix GDB connection segfault caused by empty machines

[Thomas Huth <thuth@redhat.com>]

On 17.01.2017 16:19, Ziyue Yang wrote:
> From: Ziyue Yang <yzylivezh@hotmail.com>
> 
> This patch is to fix the segmentation fault caused by attaching
> GDB to a QEMU instance initialized with "-M none" option.
> 
> The bug can be reproduced by
> 
>> ./qemu-system-x86_64 -M none -nographic -S -s
> 
> and attach a GDB to it by
> 
>> gdb -ex 'target remote :1234
> 
> The segmentation fault was originally caused by trying to read
> the information about CPU when communicating with GDB. However,
> it's impossible for any control flow to exist on an empty machine,
> nor can CPU's be hot plugged to an empty machine later by QOM
> commands. So I think simply disabling GDB connections on empty
> machines makes sense.

Yes, this sounds like a proper and easy fix for the problem.

> Signed-off-by: Ziyue Yang <skiver.cloud.yzy@gmail.com>
> ---
>  gdbstub.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/gdbstub.c b/gdbstub.c
> index de62d26..413e817 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -1731,6 +1731,12 @@ int gdbserver_start(const char *device)
>      CharDriverState *mon_chr;
>      ChardevCommon common = { 0 };
>  
> +    if (!first_cpu) {
> +        fprintf(stderr, "gdbstub: meaningless to attach gdb to a "
> +                "machine without any CPU.\n");
> +        return -1;
> +    }

Could you maybe rather use error_report() instead of fprintf()? I think
that's the preferred way to print out an error in QEMU nowadays.

 Thomas