* [Qemu-devel] [1/2] User-mode GDB stub improvements - handle fork
@ 2008-12-12 19:07 Daniel Jacobowitz
2008-12-18 22:55 ` Aurelien Jarno
0 siblings, 1 reply; 2+ messages in thread
From: Daniel Jacobowitz @ 2008-12-12 19:07 UTC (permalink / raw)
To: qemu-devel; +Cc: Paul Brook
From: Daniel Jacobowitz <dan@codesourcery.com>
Close gdbserver in child processes, so that only one stub tries to talk
to GDB at a time. Updated from an earlier patch by Paul Brook.
Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
---
This patch lets GDB debug a parent process, even if it calls fork.
Otherwise you end up with two QEMU processes sharing a single fd
for the GDB socket, and both writing replies to it. This will more or
less stutter along for a while, with GDB discarding lots of errors,
but eventually the two processes will diverge and things will blow up
depending on which response GDB sees first.
Index: linux-user/syscall.c
===================================================================
--- linux-user/syscall.c (revision 5995)
+++ linux-user/syscall.c (working copy)
@@ -2958,17 +2958,17 @@ static int do_fork(CPUState *env, unsign
return -EINVAL;
fork_start();
ret = fork();
-#if defined(USE_NPTL)
- /* There is a race condition here. The parent process could
- theoretically read the TID in the child process before the child
- tid is set. This would require using either ptrace
- (not implemented) or having *_tidptr to point at a shared memory
- mapping. We can't repeat the spinlock hack used above because
- the child process gets its own copy of the lock. */
if (ret == 0) {
+ /* Child Process. */
cpu_clone_regs(env, newsp);
fork_end(1);
- /* Child Process. */
+#if defined(USE_NPTL)
+ /* There is a race condition here. The parent process could
+ theoretically read the TID in the child process before the child
+ tid is set. This would require using either ptrace
+ (not implemented) or having *_tidptr to point at a shared memory
+ mapping. We can't repeat the spinlock hack used above because
+ the child process gets its own copy of the lock. */
if (flags & CLONE_CHILD_SETTID)
put_user_u32(gettid(), child_tidptr);
if (flags & CLONE_PARENT_SETTID)
@@ -2977,14 +2977,10 @@ static int do_fork(CPUState *env, unsign
if (flags & CLONE_SETTLS)
cpu_set_tls (env, newtls);
/* TODO: Implement CLONE_CHILD_CLEARTID. */
+#endif
} else {
fork_end(0);
}
-#else
- if (ret == 0) {
- cpu_clone_regs(env, newsp);
- }
-#endif
}
return ret;
}
Index: linux-user/main.c
===================================================================
--- linux-user/main.c (revision 5995)
+++ linux-user/main.c (working copy)
@@ -162,6 +162,7 @@ void fork_end(int child)
pthread_cond_init(&exclusive_cond, NULL);
pthread_cond_init(&exclusive_resume, NULL);
pthread_mutex_init(&tb_lock, NULL);
+ gdbserver_fork(thread_env);
} else {
pthread_mutex_unlock(&exclusive_lock);
pthread_mutex_unlock(&tb_lock);
@@ -254,6 +255,9 @@ void fork_start(void)
void fork_end(int child)
{
+ if (child) {
+ gdbserver_fork(thread_env);
+ }
}
#endif
Index: gdbstub.c
===================================================================
--- gdbstub.c (revision 5995)
+++ gdbstub.c (working copy)
@@ -1996,6 +1996,18 @@ int gdbserver_start(int port)
gdb_accept();
return 0;
}
+
+/* Disable gdb stub for child processes. */
+void gdbserver_fork(CPUState *env)
+{
+ GDBState *s = gdbserver_state;
+ if (s->fd < 0)
+ return;
+ close(s->fd);
+ s->fd = -1;
+ cpu_breakpoint_remove_all(env, BP_GDB);
+ cpu_watchpoint_remove_all(env, BP_GDB);
+}
#else
static int gdb_chr_can_receive(void *opaque)
{
Index: gdbstub.h
===================================================================
--- gdbstub.h (revision 5995)
+++ gdbstub.h (working copy)
@@ -13,6 +13,7 @@ void gdb_set_stop_cpu(CPUState *env);
int gdb_handlesig (CPUState *, int);
void gdb_exit(CPUState *, int);
int gdbserver_start(int);
+void gdbserver_fork(CPUState *);
#else
int gdbserver_start(const char *port);
#endif
--
Daniel Jacobowitz
CodeSourcery
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [Qemu-devel] [1/2] User-mode GDB stub improvements - handle fork
2008-12-12 19:07 [Qemu-devel] [1/2] User-mode GDB stub improvements - handle fork Daniel Jacobowitz
@ 2008-12-18 22:55 ` Aurelien Jarno
0 siblings, 0 replies; 2+ messages in thread
From: Aurelien Jarno @ 2008-12-18 22:55 UTC (permalink / raw)
To: qemu-devel
On Fri, Dec 12, 2008 at 02:07:56PM -0500, Daniel Jacobowitz wrote:
> From: Daniel Jacobowitz <dan@codesourcery.com>
>
> Close gdbserver in child processes, so that only one stub tries to talk
> to GDB at a time. Updated from an earlier patch by Paul Brook.
>
> Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Thanks, applied.
> ---
>
> This patch lets GDB debug a parent process, even if it calls fork.
> Otherwise you end up with two QEMU processes sharing a single fd
> for the GDB socket, and both writing replies to it. This will more or
> less stutter along for a while, with GDB discarding lots of errors,
> but eventually the two processes will diverge and things will blow up
> depending on which response GDB sees first.
>
> Index: linux-user/syscall.c
> ===================================================================
> --- linux-user/syscall.c (revision 5995)
> +++ linux-user/syscall.c (working copy)
> @@ -2958,17 +2958,17 @@ static int do_fork(CPUState *env, unsign
> return -EINVAL;
> fork_start();
> ret = fork();
> -#if defined(USE_NPTL)
> - /* There is a race condition here. The parent process could
> - theoretically read the TID in the child process before the child
> - tid is set. This would require using either ptrace
> - (not implemented) or having *_tidptr to point at a shared memory
> - mapping. We can't repeat the spinlock hack used above because
> - the child process gets its own copy of the lock. */
> if (ret == 0) {
> + /* Child Process. */
> cpu_clone_regs(env, newsp);
> fork_end(1);
> - /* Child Process. */
> +#if defined(USE_NPTL)
> + /* There is a race condition here. The parent process could
> + theoretically read the TID in the child process before the child
> + tid is set. This would require using either ptrace
> + (not implemented) or having *_tidptr to point at a shared memory
> + mapping. We can't repeat the spinlock hack used above because
> + the child process gets its own copy of the lock. */
> if (flags & CLONE_CHILD_SETTID)
> put_user_u32(gettid(), child_tidptr);
> if (flags & CLONE_PARENT_SETTID)
> @@ -2977,14 +2977,10 @@ static int do_fork(CPUState *env, unsign
> if (flags & CLONE_SETTLS)
> cpu_set_tls (env, newtls);
> /* TODO: Implement CLONE_CHILD_CLEARTID. */
> +#endif
> } else {
> fork_end(0);
> }
> -#else
> - if (ret == 0) {
> - cpu_clone_regs(env, newsp);
> - }
> -#endif
> }
> return ret;
> }
> Index: linux-user/main.c
> ===================================================================
> --- linux-user/main.c (revision 5995)
> +++ linux-user/main.c (working copy)
> @@ -162,6 +162,7 @@ void fork_end(int child)
> pthread_cond_init(&exclusive_cond, NULL);
> pthread_cond_init(&exclusive_resume, NULL);
> pthread_mutex_init(&tb_lock, NULL);
> + gdbserver_fork(thread_env);
> } else {
> pthread_mutex_unlock(&exclusive_lock);
> pthread_mutex_unlock(&tb_lock);
> @@ -254,6 +255,9 @@ void fork_start(void)
>
> void fork_end(int child)
> {
> + if (child) {
> + gdbserver_fork(thread_env);
> + }
> }
> #endif
>
> Index: gdbstub.c
> ===================================================================
> --- gdbstub.c (revision 5995)
> +++ gdbstub.c (working copy)
> @@ -1996,6 +1996,18 @@ int gdbserver_start(int port)
> gdb_accept();
> return 0;
> }
> +
> +/* Disable gdb stub for child processes. */
> +void gdbserver_fork(CPUState *env)
> +{
> + GDBState *s = gdbserver_state;
> + if (s->fd < 0)
> + return;
> + close(s->fd);
> + s->fd = -1;
> + cpu_breakpoint_remove_all(env, BP_GDB);
> + cpu_watchpoint_remove_all(env, BP_GDB);
> +}
> #else
> static int gdb_chr_can_receive(void *opaque)
> {
> Index: gdbstub.h
> ===================================================================
> --- gdbstub.h (revision 5995)
> +++ gdbstub.h (working copy)
> @@ -13,6 +13,7 @@ void gdb_set_stop_cpu(CPUState *env);
> int gdb_handlesig (CPUState *, int);
> void gdb_exit(CPUState *, int);
> int gdbserver_start(int);
> +void gdbserver_fork(CPUState *);
> #else
> int gdbserver_start(const char *port);
> #endif
>
> --
> Daniel Jacobowitz
> CodeSourcery
>
>
>
--
.''`. Aurelien Jarno | GPG: 1024D/F1BCDB73
: :' : Debian developer | Electrical Engineer
`. `' aurel32@debian.org | aurelien@aurel32.net
`- people.debian.org/~aurel32 | www.aurel32.net
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