Re: [PATCH] gdbstub: Implement qqemu.Pid packet

[Dominik Czarnota]

Hi,

We need the `qqemu.Pid` packet only for the QEMU system part in order to figure out the page tables and to do it efficiently. We do have a fallback to using `monitor info mem` but it is slow, (iirc) doesn't provide all the necessary information and (iirc) isn't implemented on all architectures.

Also, is the QEMU own pid leak such a problem? There are worse issues than that available with current features though I believe they are not really security issues. I will detail this in a private email to you.

Best,
Dominik 'Disconnect3d' Czarnota

On Fri, 2 May 2025 at 15:18, Alex Bennée <alex.bennee@linaro.org> wrote:

Dominik 'Disconnect3d' Czarnota <dominik.b.czarnota@gmail.com> writes:

> This patch adds support for the `qqemu.Pid` packet to the qemu
> gdbstub which can be used by clients to get the QEMU process PID.
>
> This is useful for plugins like Pwndbg [0] or gdb-pt-dump in order to
> inspect the QEMU process memory through the /proc/self/{maps,mem}
> interfaces. Without this feature, they have to rely on doing an
> unreliable pgrep/ps output processing.

That seems a little thin a reason for QEMU to expose its own PID. For
user-mode you can already get that detail through anything using
gdb_append_thread_id().

For system-mode leaking QEMU's own pid seems like an information leak at
best. There are modes like semihosting which give a remote even more
power but you need to at least opt in to that.

>
> This patch has been developed by Patryk, who I included in the
> Co-authored-by and who asked me to send the patch.
>
> [0] https://github.com/pwndbg/pwndbg
> [1] https://github.com/martinradev/gdb-pt-dump
>
> Co-authored-by: Patryk 'patryk4815' Sondej <patryk.sondej@gmail.com>
> Signed-off-by: Dominik 'Disconnect3d' Czarnota <dominik.b.czarnota@gmail.com>
> ---
>  gdbstub/gdbstub.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/gdbstub/gdbstub.c b/gdbstub/gdbstub.c
> index 282e13e163..a077c2c5ed 100644
> --- a/gdbstub/gdbstub.c
> +++ b/gdbstub/gdbstub.c
> @@ -1746,6 +1746,12 @@ static void handle_query_qemu_supported(GArray *params, void *user_ctx)
>      gdb_put_strbuf();
>  }
> 
> +static void handle_query_qemu_pid(GArray *params, void *user_ctx)
> +{
> +    g_string_printf(gdbserver_state.str_buf, "F%x", getpid());
> +    gdb_put_strbuf();
> +}
> +
>  static const GdbCmdParseEntry gdb_gen_query_set_common_table[] = {
>      /* Order is important if has same prefix */
>      {
> @@ -1902,6 +1908,10 @@ static const GdbCmdParseEntry gdb_gen_query_table[] = {
>          .handler = handle_query_qemu_supported,
>          .cmd = "qemu.Supported",
>      },
> +    {
> +        .handler = handle_query_qemu_pid,
> +        .cmd = "qemu.Pid",
> +    },
>  #ifndef CONFIG_USER_ONLY
>      {
>          .handler = gdb_handle_query_qemu_phy_mem_mode,

--
Alex Bennée
Virtualisation Tech Lead @ Linaro