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; }