Re: [PATCH] gdbstub: Correct misparsing of vCont C/S requests

[Alex Bennée]

Peter Maydell <peter.maydell@linaro.org> writes:

> In the vCont packet, two of the command actions (C and S) take an
> argument specifying the signal to be sent to the process/thread, which is
> sent as an ASCII string of two hex digits which immediately follow the
> 'C' or 'S' character.
>
> Our code for parsing this packet accidentally skipped the first of the
> two bytes of the signal value, because it started parsing the hex string
> at 'p + 1' when the preceding code had already moved past the 'C' or
> 'S' with "cur_action = *p++".
>
> This meant that we would only do the right thing for signals below
> 10, and would misinterpret the rest.  For instance, when the debugger
> wants to send the process a SIGPROF (27 on x86-64) we mangle this into
> a SIGSEGV (11).
>
> Remove the accidental double increment.
>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1773743
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

LGTM

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

> ---
> Haven't really given this enough testing to want to put it into 5.2,
> I think (though it does fix the repro in the bug report).
> The bug has been present since commit 544177ad1cfd78 from 2017.

I'd be happy including it. I don't have any gdbstub patches queued at
the moment but I could put together one if you want or you could just
include it directly if you are now happy to.

>
>  gdbstub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gdbstub.c b/gdbstub.c
> index f19f98ab1ab..d99bc0bf2ea 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -1243,7 +1243,7 @@ static int gdb_handle_vcont(const char *p)
>          cur_action = *p++;
>          if (cur_action == 'C' || cur_action == 'S') {
>              cur_action = qemu_tolower(cur_action);
> -            res = qemu_strtoul(p + 1, &p, 16, &tmp);
> +            res = qemu_strtoul(p, &p, 16, &tmp);
>              if (res) {
>                  goto out;
>              }


-- 
Alex Bennée