Re: [PATCH v3 6/8] gdbstub: Allow late attachment

["Alex Bennée" <alex.bennee@linaro.org>]

Ilya Leoshkevich <iii@linux.ibm.com> writes:

> Allow debugging individual processes in multi-process applications by
> starting them with export QEMU_GDB=/tmp/qemu-%d.sock,suspend=n.
> Currently one would have to attach to every process to ensure the app
> makes progress.
>
> In case suspend=n is not specified, the flow remains unchanged. If it
> is specified, then accepting the client connection is delegated to a
> thread. In the future this machinery may be reused for handling
> reconnections and interruptions.
>
> On accepting a connection, the thread schedules gdb_handlesig() on the
> first CPU and wakes it up with host_interrupt_signal. Note that the
> result of this gdb_handlesig() invocation is handled, as opposed to
> many other existing call sites. These other call sites probably need to
> be fixed separately.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  bsd-user/main.c   |   1 -
>  gdbstub/user.c    | 120 ++++++++++++++++++++++++++++++++++++++++++----
>  linux-user/main.c |   1 -
>  3 files changed, 110 insertions(+), 12 deletions(-)
>
> diff --git a/bsd-user/main.c b/bsd-user/main.c
> index 61ca73c4781..ca4773a3f40 100644
> --- a/bsd-user/main.c
> +++ b/bsd-user/main.c
> @@ -628,7 +628,6 @@ int main(int argc, char **argv)
>  
>      if (gdbstub) {
>          gdbserver_start(gdbstub);
> -        gdb_handlesig(cpu, 0, NULL, NULL, 0);
>      }
>      cpu_loop(env);
>      /* never exits */
> diff --git a/gdbstub/user.c b/gdbstub/user.c
> index c900d0a52fe..6ada0d687b9 100644
> --- a/gdbstub/user.c
> +++ b/gdbstub/user.c
> @@ -10,6 +10,7 @@
>   */
>  
>  #include "qemu/osdep.h"
> +#include <sys/syscall.h>

Whats this needed for? I can build without it.

>  #include "qemu/bitops.h"
>  #include "qemu/cutils.h"
>  #include "qemu/sockets.h"
> @@ -21,6 +22,7 @@
>  #include "gdbstub/user.h"
>  #include "gdbstub/enums.h"
>  #include "hw/core/cpu.h"
> +#include "user/signal.h"
>  #include "trace.h"
>  #include "internals.h"
>  
> @@ -416,11 +418,101 @@ static int gdbserver_open_port(int port)
>      return fd;
>  }
>  
> -int gdbserver_start(const char *port_or_path)
> +static bool gdbserver_accept(int port, int gdb_fd, const char *port_or_path)
>  {
> -    int port = g_ascii_strtoull(port_or_path, NULL, 10);
> +    bool ret;
> +
> +    if (port > 0) {
> +        ret = gdb_accept_tcp(gdb_fd);
> +    } else {
> +        ret = gdb_accept_socket(gdb_fd);
> +        if (ret) {
> +            gdbserver_user_state.socket_path = g_strdup(port_or_path);
> +        }
> +    }
> +
> +    if (!ret) {
> +        close(gdb_fd);
> +    }
> +
> +    return ret;
> +}

Is the clean-up worth it here for the extra level of indirection. Is the
string even port_or_path at this point because if it was a port the int
port > 0?

> +
> +struct {
> +    int port;
>      int gdb_fd;
> +    char *port_or_path;
> +} gdbserver_args;
> +
> +static void do_gdb_handlesig(CPUState *cs, run_on_cpu_data arg)
> +{
> +    int sig;
> +
> +    sig = target_to_host_signal(gdb_handlesig(cs, 0, NULL, NULL, 0));
> +    if (sig >= 1 && sig < NSIG) {
> +        qemu_kill_thread(gdb_get_cpu_index(cs), sig);
> +    }
> +}
> +
> +static void *gdbserver_accept_thread(void *arg)
> +{
> +    if (gdbserver_accept(gdbserver_args.port, gdbserver_args.gdb_fd,
> +                         gdbserver_args.port_or_path)) {
> +        CPUState *cs = first_cpu;
> +
> +        async_safe_run_on_cpu(cs, do_gdb_handlesig, RUN_ON_CPU_NULL);
> +        qemu_kill_thread(gdb_get_cpu_index(cs), host_interrupt_signal);
> +    }
> +
> +    g_free(gdbserver_args.port_or_path);

Should we set gdbserver_args.port_or_path = NULL here to avoid trying to
reference or free it again?

> +
> +    return NULL;
> +}
> +
> +__attribute__((__format__(__printf__, 1, 2)))
> +static void print_usage(const char *format, ...)
> +{
> +    va_list ap;
> +
> +    va_start(ap, format);
> +    vfprintf(stderr, format, ap);
> +    va_end(ap);
> +    fprintf(stderr, "Usage: -g {port|path}[,suspend={y|n}]\n");
> +}
> +
> +#define SUSPEND "suspend="
> +
> +int gdbserver_start(const char *args)
> +{
> +    g_auto(GStrv) argv = g_strsplit(args, ",", 0);
> +    const char *port_or_path = NULL;
> +    bool suspend = true;
> +    int gdb_fd, port;
> +    GStrv arg;
>  
> +    for (arg = argv; *arg; arg++) {
> +        if (g_str_has_prefix(*arg, SUSPEND)) {
> +            const char *val = *arg + strlen(SUSPEND);
> +
> +            suspend = (strcmp(val, "y") == 0);
> +            if (!suspend && (strcmp(val, "n") != 0)) {
> +                print_usage("Bad option value: %s", *arg);
> +                return -1;
> +            }
> +        } else {
> +            if (port_or_path) {
> +                print_usage("Unknown option: %s", *arg);
> +                return -1;
> +            }
> +            port_or_path = *arg;
> +        }
> +    }

We have some useful utility functions to parsing all the bools,
something like:

    for (arg = argv; *arg; arg++) {
        g_auto(GStrv) tokens = g_strsplit(*arg, "=", 2);
        if (g_strcmp0(tokens[0], "suspend") == 0 && tokens[1]) {
            qapi_bool_parse(tokens[0], tokens[1], &suspend, &error_fatal);
        } else {
            if (port_or_path) {
                print_usage("Unknown option: %s", *arg);
                return -1;
            }
            port_or_path = *arg;
        }
    }

which also avoids the #define and strlen messing about as well.

(side note to no one in particular: should qapi_bool_parse being using g_strcmp0()?)

> +    if (!port_or_path) {
> +        print_usage("Port or path not specified");
> +        return -1;
> +    }
> +
> +    port = g_ascii_strtoull(port_or_path, NULL, 10);
>      if (port > 0) {
>          gdb_fd = gdbserver_open_port(port);
>      } else {
> @@ -431,16 +523,24 @@ int gdbserver_start(const char *port_or_path)
>          return -1;
>      }
>  
> -    if (port > 0 && gdb_accept_tcp(gdb_fd)) {
> -        return 0;
> -    } else if (gdb_accept_socket(gdb_fd)) {
> -        gdbserver_user_state.socket_path = g_strdup(port_or_path);
> +    if (suspend) {
> +        if (gdbserver_accept(port, gdb_fd, port_or_path)) {
> +            gdb_handlesig(first_cpu, 0, NULL, NULL, 0);
> +            return 0;
> +        } else {
> +            return -1;
> +        }
> +    } else {
> +        QemuThread thread;
> +
> +        gdbserver_args.port = port;
> +        gdbserver_args.gdb_fd = gdb_fd;
> +        gdbserver_args.port_or_path = g_strdup(port_or_path);
> +        qemu_thread_create(&thread, "gdb-accept",
> +                           &gdbserver_accept_thread, NULL,
> +                           QEMU_THREAD_DETACHED);
>          return 0;
>      }
> -
> -    /* gone wrong */
> -    close(gdb_fd);
> -    return -1;
>  }

Not a problem with this patch in particular but it seems to me
gdbserver_start should probably look like:

  bool gdbserver_start(const char *args, Error **errp)

so we can do the right thing when starting from the command line or via
HMP. I'll see if I can clean that up on gdbstub/next.

>  
>  void gdbserver_fork_start(void)
> diff --git a/linux-user/main.c b/linux-user/main.c
> index b09af8d4365..97245ab37c2 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -1027,7 +1027,6 @@ int main(int argc, char **argv, char **envp)
>                      gdbstub);
>              exit(EXIT_FAILURE);
>          }
> -        gdb_handlesig(cpu, 0, NULL, NULL, 0);
>      }
>  
>  #ifdef CONFIG_SEMIHOSTING

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro