Re: RFA: Actual support for tracing forks on GNU/Linux

Daniel Jacobowitz
2003-08-17T18:22:47Z

On Sun, Aug 10, 2003 at 06:11:41PM +0200, Mark Kettenis wrote:
> Date: Thu, 24 Jul 2003 14:48:49 -0400
> From: Daniel Jacobowitz <drow@mvista.com>
>
> Ping.
>
> Sorry Daniel for not responding earlier. I've had almost no time to
> spend on hacking since the second half of june and I've just returned
> from a vacation in Finland. Anyway, I do have a few comments:
>
> * I'm not very enthousiastic about the staggering of
> CHILD_POST_STARTUP_INFERIOR and LINUX_CHILD_POST_STARTUP_INFERIOR,
> especially since it infects i386-nat.c. However, I'm pretty sure
> there isn't a better alternative that doesn't involve redisigning
> the target vector :-(. I'd appreciate though if you could stick in
> a comment somewhere that says this is really ugly.
>
> * The changes to lin-lwp.c seem pretty much OK to me.
>
> So I think this can go in.

Thanks. I agree with your reaction to CHILD_POST_STARTUP_INFERIOR, so
I've clarified the comment. Also needed to move a few prototypes to
linux-nat.h.

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer

2003-08-17 Daniel Jacobowitz <drow@mvista.com>

* Makefile.in (i386-linux-nat.o): Update dependencies.
* config/i386/nm-linux.h (LINUX_CHILD_POST_STARTUP_INFERIOR): Define.
* config/nm-linux.h (CHILD_POST_STARTUP_INFERIOR, CHILD_POST_ATTACH)
(CHILD_FOLLOW_FORK, KILL_INFERIOR): Define.
* i386-linux-nat.c: Include "linux-nat.h".
(child_post_startup_inferior): New function.
* i386-nat.c (child_post_startup_inferior): Wrap in #ifdef.
* infptrace.c (kill_inferior): Wrap in #ifdef.
* lin-lwp.c (lin_lwp_attach_lwp): Call child_post_attach after
attaching to each LWP.
(child_wait, lin_lwp_wait): Call linux_handle_extended_wait.
(init_lin_lwp_ops): Fill in some more operations.
* linux-nat.h (linux_enable_event_reporting)
(linux_handle_extended_wait, linux_child_post_startup_inferior): New
prototypes.
* linux-nat.c (linux_enable_event_reporting): New function.
(child_post_attach, linux_child_post_startup_inferior)
(child_post_startup_inferior, child_follow_fork)
(linux_handle_extended_wait, kill_inferior): New functions.

Index: Makefile.in
===================================================================
RCS file: /cvs/src/src/gdb/Makefile.in,v
retrieving revision 1.429
diff -u -p -r1.429 Makefile.in
--- Makefile.in 16 Aug 2003 17:49:12 -0000 1.429
+++ Makefile.in 17 Aug 2003 17:46:09 -0000
@@ -1843,7 +1843,7 @@ i386-interix-tdep.o: i386-interix-tdep.c
i386-linux-nat.o: i386-linux-nat.c $(defs_h) $(inferior_h) $(gdbcore_h) \
$(regcache_h) $(gdb_assert_h) $(gdb_string_h) $(gregset_h) \
$(i387_tdep_h) $(i386_tdep_h) $(i386_linux_tdep_h) \
- $(gdb_proc_service_h)
+ $(gdb_proc_service_h) $(linux_nat_h)
i386-linux-tdep.o: i386-linux-tdep.c $(defs_h) $(gdbcore_h) $(frame_h) \
$(value_h) $(regcache_h) $(inferior_h) $(reggroups_h) $(symtab_h) \
$(symfile_h) $(objfiles_h) $(solib_svr4_h) $(osabi_h) $(i386_tdep_h) \
Index: i386-linux-nat.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-linux-nat.c,v
retrieving revision 1.47
diff -u -p -r1.47 i386-linux-nat.c
--- i386-linux-nat.c 4 Jun 2003 20:51:29 -0000 1.47
+++ i386-linux-nat.c 17 Aug 2003 17:46:10 -0000
@@ -23,6 +23,7 @@
#include "inferior.h"
#include "gdbcore.h"
#include "regcache.h"
+#include "linux-nat.h"

#include "gdb_assert.h"
#include "gdb_string.h"
@@ -889,6 +890,13 @@ child_resume (ptid_t ptid, int step, enu

if (ptrace (request, pid, 0, target_signal_to_host (signal)) == -1)
perror_with_name ("ptrace");
+}
+
+void
+child_post_startup_inferior (ptid_t ptid)
+{
+ i386_cleanup_dregs ();
+ linux_child_post_startup_inferior (ptid);
}
\f

Index: i386-nat.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-nat.c,v
retrieving revision 1.5
diff -u -p -r1.5 i386-nat.c
--- i386-nat.c 4 Jul 2002 12:32:28 -0000 1.5
+++ i386-nat.c 17 Aug 2003 17:46:10 -0000
@@ -230,6 +230,7 @@ i386_cleanup_dregs (void)
dr_status_mirror = 0;
}

+#ifndef LINUX_CHILD_POST_STARTUP_INFERIOR
/* Reset all debug registers at each new startup
to avoid missing watchpoints after restart. */
void
@@ -237,6 +238,7 @@ child_post_startup_inferior (ptid_t ptid
{
i386_cleanup_dregs ();
}
+#endif /* LINUX_CHILD_POST_STARTUP_INFERIOR */

/* Print the values of the mirrored debug registers.
This is called when maint_show_dr is non-zero. To set that
Index: infptrace.c
===================================================================
RCS file: /cvs/src/src/gdb/infptrace.c,v
retrieving revision 1.26
diff -u -p -r1.26 infptrace.c
--- infptrace.c 22 May 2003 15:46:20 -0000 1.26
+++ infptrace.c 17 Aug 2003 17:46:10 -0000
@@ -208,6 +208,7 @@ ptrace_wait (ptid_t ptid, int *status)
return wstate;
}

+#ifndef KILL_INFERIOR
void
kill_inferior (void)
{
@@ -229,6 +230,7 @@ kill_inferior (void)
ptrace_wait (null_ptid, &status);
target_mourn_inferior ();
}
+#endif /* KILL_INFERIOR */

#ifndef CHILD_RESUME

Index: lin-lwp.c
===================================================================
RCS file: /cvs/src/src/gdb/lin-lwp.c,v
retrieving revision 1.47
diff -u -p -r1.47 lin-lwp.c
--- lin-lwp.c 19 Jun 2003 22:52:03 -0000 1.47
+++ lin-lwp.c 17 Aug 2003 17:46:10 -0000
@@ -324,6 +324,8 @@ lin_lwp_attach_lwp (ptid_t ptid, int ver
gdb_assert (pid == GET_LWP (ptid)
&& WIFSTOPPED (status) && WSTOPSIG (status));

+ child_post_attach (pid);
+
lp->stopped = 1;

if (debug_lin_lwp)
@@ -1067,6 +1069,10 @@ child_wait (ptid_t ptid, struct target_w
return minus_one_ptid;
}

+ /* Handle GNU/Linux's extended waitstatus for trace events. */
+ if (WIFSTOPPED (status) && WSTOPSIG (status) == SIGTRAP && status >> 16 != 0)
+ return linux_handle_extended_wait (pid, status, ourstatus);
+
store_waitstatus (ourstatus, status);
return pid_to_ptid (pid);
}
@@ -1488,6 +1494,14 @@ retry:
else
trap_ptid = null_ptid;

+ /* Handle GNU/Linux's extended waitstatus for trace events. */
+ if (WIFSTOPPED (status) && WSTOPSIG (status) == SIGTRAP && status >> 16 != 0)
+ {
+ linux_handle_extended_wait (ptid_get_pid (trap_ptid),
+ status, ourstatus);
+ return trap_ptid;
+ }
+
store_waitstatus (ourstatus, status);
return (threaded ? lp->ptid : pid_to_ptid (GET_LWP (lp->ptid)));
}
@@ -1657,6 +1671,12 @@ init_lin_lwp_ops (void)
lin_lwp_ops.to_mourn_inferior = lin_lwp_mourn_inferior;
lin_lwp_ops.to_thread_alive = lin_lwp_thread_alive;
lin_lwp_ops.to_pid_to_str = lin_lwp_pid_to_str;
+ lin_lwp_ops.to_post_startup_inferior = child_post_startup_inferior;
+ lin_lwp_ops.to_post_attach = child_post_attach;
+ lin_lwp_ops.to_insert_fork_catchpoint = child_insert_fork_catchpoint;
+ lin_lwp_ops.to_insert_vfork_catchpoint = child_insert_vfork_catchpoint;
+ lin_lwp_ops.to_insert_exec_catchpoint = child_insert_exec_catchpoint;
+
lin_lwp_ops.to_stratum = thread_stratum;
lin_lwp_ops.to_has_thread_control = tc_schedlock;
lin_lwp_ops.to_magic = OPS_MAGIC;
Index: linux-nat.c
===================================================================
RCS file: /cvs/src/src/gdb/linux-nat.c,v
retrieving revision 1.3
diff -u -p -r1.3 linux-nat.c
--- linux-nat.c 19 Jun 2003 22:52:03 -0000 1.3
+++ linux-nat.c 17 Aug 2003 17:46:10 -0000
@@ -56,6 +56,8 @@
#define __WALL 0x40000000 /* Wait for any child. */
#endif

+extern struct target_ops child_ops;
+
struct simple_pid_list
{
int pid;
@@ -189,13 +191,147 @@ linux_supports_tracefork (void)
}

\f
+void
+linux_enable_event_reporting (ptid_t ptid)
+{
+ int pid = ptid_get_pid (ptid);
+ int options;
+
+ if (! linux_supports_tracefork ())
+ return;
+
+ options = PTRACE_O_TRACEFORK;
+
+ ptrace (PTRACE_SETOPTIONS, pid, 0, options);
+}
+
+void
+child_post_attach (int pid)
+{
+ linux_enable_event_reporting (pid_to_ptid (pid));
+}
+
+void
+linux_child_post_startup_inferior (ptid_t ptid)
+{
+ linux_enable_event_reporting (ptid);
+}
+
+#ifndef LINUX_CHILD_POST_STARTUP_INFERIOR
+void
+child_post_startup_inferior (ptid_t ptid)
+{
+ linux_child_post_startup_inferior (ptid);
+}
+#endif
+
int
-child_insert_fork_catchpoint (int pid)
+child_follow_fork (int follow_child)
{
- if (linux_supports_tracefork ())
- error ("Fork catchpoints have not been implemented yet.");
+ ptid_t last_ptid;
+ struct target_waitstatus last_status;
+ int parent_pid, child_pid;
+
+ get_last_target_status (&last_ptid, &last_status);
+ parent_pid = ptid_get_pid (last_ptid);
+ child_pid = last_status.value.related_pid;
+
+ if (! follow_child)
+ {
+ /* We're already attached to the parent, by default. */
+
+ /* Before detaching from the child, remove all breakpoints from
+ it. (This won't actually modify the breakpoint list, but will
+ physically remove the breakpoints from the child.) */
+ detach_breakpoints (child_pid);
+
+ fprintf_filtered (gdb_stdout,
+ "Detaching after fork from child process %d.\n",
+ child_pid);
+
+ ptrace (PTRACE_DETACH, child_pid, 0, 0);
+ }
else
+ {
+ char child_pid_spelling[40];
+
+ /* Needed to keep the breakpoint lists in sync. */
+ detach_breakpoints (child_pid);
+
+ /* Before detaching from the parent, remove all breakpoints from it. */
+ remove_breakpoints ();
+
+ fprintf_filtered (gdb_stdout,
+ "Attaching after fork to child process %d.\n",
+ child_pid);
+
+ target_detach (NULL, 0);
+
+ inferior_ptid = pid_to_ptid (child_pid);
+ push_target (&child_ops);
+
+ /* Reset breakpoints in the child as appropriate. */
+ follow_inferior_reset_breakpoints ();
+ }
+
+ return 0;
+}
+
+ptid_t
+linux_handle_extended_wait (int pid, int status,
+ struct target_waitstatus *ourstatus)
+{
+ int event = status >> 16;
+
+ if (event == PTRACE_EVENT_CLONE)
+ internal_error (__FILE__, __LINE__,
+ "unexpected clone event");
+
+ if (event == PTRACE_EVENT_FORK)
+ {
+ unsigned long new_pid;
+ int ret;
+
+ ptrace (PTRACE_GETEVENTMSG, pid, 0, &new_pid);
+
+ /* If we haven't already seen the new PID stop, wait for it now. */
+ if (! pull_pid_from_list (&stopped_pids, new_pid))
+ {
+ /* The new child has a pending SIGSTOP. We can't affect it until it
+ hits the SIGSTOP, but we're already attached.
+
+ It won't be a clone (we didn't ask for clones in the event mask)
+ so we can just call waitpid and wait for the SIGSTOP. */
+ do {
+ ret = waitpid (new_pid, &status, 0);
+ } while (ret == -1 && errno == EINTR);
+ if (ret == -1)
+ perror_with_name ("waiting for new child");
+ else if (ret != new_pid)
+ internal_error (__FILE__, __LINE__,
+ "wait returned unexpected PID %d", ret);
+ else if (!WIFSTOPPED (status) || WSTOPSIG (status) != SIGSTOP)
+ internal_error (__FILE__, __LINE__,
+ "wait returned unexpected status 0x%x", status);
+ }
+
+ ourstatus->kind = TARGET_WAITKIND_FORKED;
+ ourstatus->value.related_pid = new_pid;
+ return inferior_ptid;
+ }
+
+ internal_error (__FILE__, __LINE__,
+ "unknown ptrace event %d", event);
+}
+
+\f
+int
+child_insert_fork_catchpoint (int pid)
+{
+ if (! linux_supports_tracefork ())
error ("Your system does not support fork catchpoints.");
+
+ return 0;
}

int
@@ -216,4 +352,43 @@ child_insert_exec_catchpoint (int pid)
error ("Your system does not support exec catchpoints.");
}

+void
+kill_inferior (void)
+{
+ int status;
+ int pid = PIDGET (inferior_ptid);
+ struct target_waitstatus last;
+ ptid_t last_ptid;
+ int ret;
+
+ if (pid == 0)
+ return;
+
+ /* If we're stopped while forking and we haven't followed yet, kill the
+ other task. We need to do this first because the parent will be
+ sleeping if this is a vfork. */
+
+ get_last_target_status (&last_ptid, &last);

+ if (last.kind == TARGET_WAITKIND_FORKED
+ || last.kind == TARGET_WAITKIND_VFORKED)
+ {
+ ptrace (PT_KILL, last.value.related_pid);
+ ptrace_wait (null_ptid, &status);
+ }
+
+ /* Kill the current process. */
+ ptrace (PT_KILL, pid, (PTRACE_ARG3_TYPE) 0, 0);
+ ret = ptrace_wait (null_ptid, &status);
+
+ /* We might get a SIGCHLD instead of an exit status. This is
+ aggravated by the first kill above - a child has just died. */
+
+ while (ret == pid && WIFSTOPPED (status))
+ {
+ ptrace (PT_KILL, pid, (PTRACE_ARG3_TYPE) 0, 0);
+ ret = ptrace_wait (null_ptid, &status);
+ }
+
+ target_mourn_inferior ();
+}
Index: linux-nat.h
===================================================================
RCS file: /cvs/src/src/gdb/linux-nat.h,v
retrieving revision 1.2
diff -u -p -r1.2 linux-nat.h
--- linux-nat.h 20 Jun 2003 04:04:43 -0000 1.2
+++ linux-nat.h 17 Aug 2003 17:46:10 -0000
@@ -65,7 +65,12 @@ extern int linux_proc_xfer_memory (CORE_
int write, struct mem_attrib *attrib,
struct target_ops *target);

+/* linux-nat functions for handling fork events. */
extern void linux_record_stopped_pid (int pid);
+extern void linux_enable_event_reporting (ptid_t ptid);
+extern ptid_t linux_handle_extended_wait (int pid, int status,
+ struct target_waitstatus *ourstatus);
+extern void linux_child_post_startup_inferior (ptid_t ptid);

/* Iterator function for lin-lwp's lwp list. */
struct lwp_info *iterate_over_lwps (int (*callback) (struct lwp_info *,
Index: config/nm-linux.h
===================================================================
RCS file: /cvs/src/src/gdb/config/nm-linux.h,v
retrieving revision 1.18
diff -u -p -r1.18 nm-linux.h
--- config/nm-linux.h 19 Jun 2003 22:52:04 -0000 1.18
+++ config/nm-linux.h 17 Aug 2003 17:46:10 -0000
@@ -73,3 +73,7 @@ extern void lin_thread_get_thread_signal
#define CHILD_INSERT_FORK_CATCHPOINT
#define CHILD_INSERT_VFORK_CATCHPOINT
#define CHILD_INSERT_EXEC_CATCHPOINT
+#define CHILD_POST_STARTUP_INFERIOR
+#define CHILD_POST_ATTACH
+#define CHILD_FOLLOW_FORK
+#define KILL_INFERIOR
Index: config/i386/nm-linux.h
===================================================================
RCS file: /cvs/src/src/gdb/config/i386/nm-linux.h,v
retrieving revision 1.19
diff -u -p -r1.19 nm-linux.h
--- config/i386/nm-linux.h 9 Nov 2002 21:31:12 -0000 1.19
+++ config/i386/nm-linux.h 17 Aug 2003 17:46:10 -0000
@@ -82,4 +82,13 @@ extern int cannot_store_register (int re
/* Override child_resume in `infptrace.c'. */
#define CHILD_RESUME

+/* `linux-nat.c' and `i386-nat.c' have their own versions of
+ child_post_startup_inferior. Define this to use the copy in
+ `i386-linux-nat.c' instead, which calls both.
+
+ NOTE drow/2003-08-17: This is ugly beyond words, but properly
+ fixing it will require some serious surgery. Ideally the target
+ stack could be used for this. */
+#define LINUX_CHILD_POST_STARTUP_INFERIOR
+
#endif /* nm-linux.h */