[for-next][PATCH 1/2] tracing: Simplify & fix saved_tgids logic

[for-next][PATCH 1/2] tracing: Simplify & fix saved_tgids logic

[Steven Rostedt]

From: Paul Burton <paulburton@google.com>

The tgid_map array records a mapping from pid to tgid, where the index
of an entry within the array is the pid & the value stored at that index
is the tgid.

The saved_tgids_next() function iterates over pointers into the tgid_map
array & dereferences the pointers which results in the tgid, but then it
passes that dereferenced value to trace_find_tgid() which treats it as a
pid & does a further lookup within the tgid_map array. It seems likely
that the intent here was to skip over entries in tgid_map for which the
recorded tgid is zero, but instead we end up skipping over entries for
which the thread group leader hasn't yet had its own tgid recorded in
tgid_map.

A minimal fix would be to remove the call to trace_find_tgid, turning:

  if (trace_find_tgid(*ptr))

into:

  if (*ptr)

..but it seems like this logic can be much simpler if we simply let
seq_read() iterate over the whole tgid_map array & filter out empty
entries by returning SEQ_SKIP from saved_tgids_show(). Here we take that
approach, removing the incorrect logic here entirely.

Link: https://lkml.kernel.org/r/20210630003406.4013668-1-paulburton@google.com

Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Burton <paulburton@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/trace/trace.c | 38 +++++++++++++-------------------------
 1 file changed, 13 insertions(+), 25 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 60492464281e..4843076d67d3 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5662,37 +5662,20 @@ static const struct file_operations tracing_readme_fops = {
 
 static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	int *ptr = v;
+	int pid = ++(*pos);
 
-	if (*pos || m->count)
-		ptr++;
-
-	(*pos)++;
-
-	for (; ptr <= &tgid_map[PID_MAX_DEFAULT]; ptr++) {
-		if (trace_find_tgid(*ptr))
-			return ptr;
-	}
+	if (pid > PID_MAX_DEFAULT)
+		return NULL;
 
-	return NULL;
+	return &tgid_map[pid];
 }
 
 static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
 {
-	void *v;
-	loff_t l = 0;
-
-	if (!tgid_map)
+	if (!tgid_map || *pos > PID_MAX_DEFAULT)
 		return NULL;
 
-	v = &tgid_map[0];
-	while (l <= *pos) {
-		v = saved_tgids_next(m, v, &l);
-		if (!v)
-			return NULL;
-	}
-
-	return v;
+	return &tgid_map[*pos];
 }
 
 static void saved_tgids_stop(struct seq_file *m, void *v)
@@ -5701,9 +5684,14 @@ static void saved_tgids_stop(struct seq_file *m, void *v)
 
 static int saved_tgids_show(struct seq_file *m, void *v)
 {
-	int pid = (int *)v - tgid_map;
+	int *entry = (int *)v;
+	int pid = entry - tgid_map;
+	int tgid = *entry;
+
+	if (tgid == 0)
+		return SEQ_SKIP;
 
-	seq_printf(m, "%d %d\n", pid, trace_find_tgid(pid));
+	seq_printf(m, "%d %d\n", pid, tgid);
 	return 0;
 }
 
-- 
2.30.2

[for-next][PATCH 2/2] tracing: Resize tgid_map to pid_max, not PID_MAX_DEFAULT

[Steven Rostedt]

From: Paul Burton <paulburton@google.com>

Currently tgid_map is sized at PID_MAX_DEFAULT entries, which means that
on systems where pid_max is configured higher than PID_MAX_DEFAULT the
ftrace record-tgid option doesn't work so well. Any tasks with PIDs
higher than PID_MAX_DEFAULT are simply not recorded in tgid_map, and
don't show up in the saved_tgids file.

In particular since systemd v243 & above configure pid_max to its
highest possible 1<<22 value by default on 64 bit systems this renders
the record-tgids option of little use.

Increase the size of tgid_map to the configured pid_max instead,
allowing it to cover the full range of PIDs up to the maximum value of
PID_MAX_LIMIT if the system is configured that way.

On 64 bit systems with pid_max == PID_MAX_LIMIT this will increase the
size of tgid_map from 256KiB to 16MiB. Whilst this 64x increase in
memory overhead sounds significant 64 bit systems are presumably best
placed to accommodate it, and since tgid_map is only allocated when the
record-tgid option is actually used presumably the user would rather it
spends sufficient memory to actually record the tgids they expect.

The size of tgid_map could also increase for CONFIG_BASE_SMALL=y
configurations, but these seem unlikely to be systems upon which people
are both configuring a large pid_max and running ftrace with record-tgid
anyway.

Of note is that we only allocate tgid_map once, the first time that the
record-tgid option is enabled. Therefore its size is only set once, to
the value of pid_max at the time the record-tgid option is first
enabled. If a user increases pid_max after that point, the saved_tgids
file will not contain entries for any tasks with pids beyond the earlier
value of pid_max.

Link: https://lkml.kernel.org/r/20210701172407.889626-2-paulburton@google.com

Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Burton <paulburton@google.com>
[ Fixed comment coding style ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/trace/trace.c | 63 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 47 insertions(+), 16 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 4843076d67d3..14f56e9fa001 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2191,8 +2191,15 @@ void tracing_reset_all_online_cpus(void)
 	}
 }
 
+/*
+ * The tgid_map array maps from pid to tgid; i.e. the value stored at index i
+ * is the tgid last observed corresponding to pid=i.
+ */
 static int *tgid_map;
 
+/* The maximum valid index into tgid_map. */
+static size_t tgid_map_max;
+
 #define SAVED_CMDLINES_DEFAULT 128
 #define NO_CMDLINE_MAP UINT_MAX
 static arch_spinlock_t trace_cmdline_lock = __ARCH_SPIN_LOCK_UNLOCKED;
@@ -2468,24 +2475,41 @@ void trace_find_cmdline(int pid, char comm[])
 	preempt_enable();
 }
 
+static int *trace_find_tgid_ptr(int pid)
+{
+	/*
+	 * Pairs with the smp_store_release in set_tracer_flag() to ensure that
+	 * if we observe a non-NULL tgid_map then we also observe the correct
+	 * tgid_map_max.
+	 */
+	int *map = smp_load_acquire(&tgid_map);
+
+	if (unlikely(!map || pid > tgid_map_max))
+		return NULL;
+
+	return &map[pid];
+}
+
 int trace_find_tgid(int pid)
 {
-	if (unlikely(!tgid_map || !pid || pid > PID_MAX_DEFAULT))
-		return 0;
+	int *ptr = trace_find_tgid_ptr(pid);
 
-	return tgid_map[pid];
+	return ptr ? *ptr : 0;
 }
 
 static int trace_save_tgid(struct task_struct *tsk)
 {
+	int *ptr;
+
 	/* treat recording of idle task as a success */
 	if (!tsk->pid)
 		return 1;
 
-	if (unlikely(!tgid_map || tsk->pid > PID_MAX_DEFAULT))
+	ptr = trace_find_tgid_ptr(tsk->pid);
+	if (!ptr)
 		return 0;
 
-	tgid_map[tsk->pid] = tsk->tgid;
+	*ptr = tsk->tgid;
 	return 1;
 }
 
@@ -5225,6 +5249,8 @@ int trace_keep_overwrite(struct tracer *tracer, u32 mask, int set)
 
 int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled)
 {
+	int *map;
+
 	if ((mask == TRACE_ITER_RECORD_TGID) ||
 	    (mask == TRACE_ITER_RECORD_CMD))
 		lockdep_assert_held(&event_mutex);
@@ -5247,10 +5273,19 @@ int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled)
 		trace_event_enable_cmd_record(enabled);
 
 	if (mask == TRACE_ITER_RECORD_TGID) {
-		if (!tgid_map)
-			tgid_map = kvcalloc(PID_MAX_DEFAULT + 1,
-					   sizeof(*tgid_map),
-					   GFP_KERNEL);
+		if (!tgid_map) {
+			tgid_map_max = pid_max;
+			map = kvcalloc(tgid_map_max + 1, sizeof(*tgid_map),
+				       GFP_KERNEL);
+
+			/*
+			 * Pairs with smp_load_acquire() in
+			 * trace_find_tgid_ptr() to ensure that if it observes
+			 * the tgid_map we just allocated then it also observes
+			 * the corresponding tgid_map_max value.
+			 */
+			smp_store_release(&tgid_map, map);
+		}
 		if (!tgid_map) {
 			tr->trace_flags &= ~TRACE_ITER_RECORD_TGID;
 			return -ENOMEM;
@@ -5664,18 +5699,14 @@ static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
 {
 	int pid = ++(*pos);
 
-	if (pid > PID_MAX_DEFAULT)
-		return NULL;
-
-	return &tgid_map[pid];
+	return trace_find_tgid_ptr(pid);
 }
 
 static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
 {
-	if (!tgid_map || *pos > PID_MAX_DEFAULT)
-		return NULL;
+	int pid = *pos;
 
-	return &tgid_map[*pos];
+	return trace_find_tgid_ptr(pid);
 }
 
 static void saved_tgids_stop(struct seq_file *m, void *v)
-- 
2.30.2