[PATCH v2 4/4] trace-cmd: Do not try to update parent's memory from a fork()-ed child

["Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com>]

When trace-cmd runs a command, specified with the "-F" flag, it forks a
child process and executes the command in its context. This child process
receives a full copy of the parents memory at the moment of fork(). When
it modifies this copy, the parent memory is not affected. Calling the
function update_task_filter() in the child context will operate on a valid
data, but will not update anything in the parent's databases.

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
---
 tracecmd/trace-record.c | 64 +++++++++++++++++++++++++++++++++--------
 1 file changed, 52 insertions(+), 12 deletions(-)

diff --git a/tracecmd/trace-record.c b/tracecmd/trace-record.c
index 1e4d38fa..ae8a5745 100644
--- a/tracecmd/trace-record.c
+++ b/tracecmd/trace-record.c
@@ -11,6 +11,7 @@
 #include <stdarg.h>
 #include <getopt.h>
 #include <time.h>
+#include <semaphore.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/time.h>
@@ -1187,10 +1188,12 @@ static void get_filter_pid_maps(void)
 	}
 }
 
-static void update_task_filter(void)
+static void update_task_filter(int pid)
 {
 	struct buffer_instance *instance;
-	int pid = getpid();
+
+	if (pid < 0)
+		pid = getpid();
 
 	if (no_filter)
 		return;
@@ -1487,18 +1490,40 @@ static int change_user(const char *user)
 	return 0;
 }
 
+#define TRACE_RUN_SEM	"trace_cmd_semaphore_run"
+#define TRACE_INIT_SEM	"trace_cmd_semaphore_init"
 static void run_cmd(enum trace_type type, const char *user, int argc, char **argv)
 {
+	sem_t *sem_init = SEM_FAILED;
+	sem_t *sem_run = SEM_FAILED;
+	char *die_log = NULL;
 	int status;
 	int pid;
 
-	if ((pid = fork()) < 0)
-		die("failed to fork");
+	sem_init = sem_open(TRACE_INIT_SEM, O_CREAT, S_IRUSR|S_IWUSR, 0);
+	if (sem_init == SEM_FAILED) {
+		die_log = "failed to init trace_cmd init semaphore";
+		goto out;
+	}
+
+	sem_run = sem_open(TRACE_RUN_SEM, O_CREAT, S_IRUSR|S_IWUSR, 0);
+	if (sem_run == SEM_FAILED) {
+		die_log = "failed to init trace_cmd run semaphore";
+		goto out;
+
+	}
+
+	if ((pid = fork()) < 0) {
+		die_log = "failed to fork";
+		goto out;
+	}
 	if (!pid) {
 		/* child */
-		update_task_filter();
+		sem_wait(sem_init);
 		tracecmd_enable_tracing();
 		enable_ptrace();
+		sem_post(sem_run);
+
 		/*
 		 * If we are using stderr for stdout, switch
 		 * it back to the saved stdout for the code we run.
@@ -1519,12 +1544,27 @@ static void run_cmd(enum trace_type type, const char *user, int argc, char **arg
 			die("Failed to exec %s", argv[0]);
 		}
 	}
-	if (do_ptrace) {
-		add_filter_pid(pid, 0);
-		ptrace_attach(pid);
+	update_task_filter(pid);
+	sem_post(sem_init);
+	sem_wait(sem_run);
+
+	if (do_ptrace)
 		ptrace_wait(type);
-	} else
+	else
 		trace_waitpid(type, pid, &status, 0);
+
+out:
+	if (sem_init != SEM_FAILED) {
+		sem_close(sem_init);
+		sem_unlink(TRACE_INIT_SEM);
+	}
+	if (sem_run != SEM_FAILED) {
+		sem_close(sem_run);
+		sem_unlink(TRACE_RUN_SEM);
+	}
+
+	if (die_log)
+		die(die_log);
 }
 
 static void
@@ -6285,7 +6325,7 @@ static void record_trace(int argc, char **argv,
 		if (!latency)
 			start_threads(type, ctx);
 	} else {
-		update_task_filter();
+		update_task_filter(-1);
 		tracecmd_enable_tracing();
 		exit(0);
 	}
@@ -6293,11 +6333,11 @@ static void record_trace(int argc, char **argv,
 	if (ctx->run_command) {
 		run_cmd(type, ctx->user, (argc - optind) - 1, &argv[optind + 1]);
 	} else if (ctx->instance && is_agent(ctx->instance)) {
-		update_task_filter();
+		update_task_filter(-1);
 		tracecmd_enable_tracing();
 		tracecmd_msg_wait_close(ctx->instance->msg_handle);
 	} else {
-		update_task_filter();
+		update_task_filter(-1);
 		tracecmd_enable_tracing();
 		/* We don't ptrace ourself */
 		if (do_ptrace && filter_pids) {
-- 
2.26.2