tracing: protect reader of cmdline output

Impact: fix to one cause of incorrect comm outputs in trace

The spinlock only protected the creation of a comm <=> pid pair.
But it was possible that a reader could look up a pid, and get the
wrong comm because it had no locking.

This also required changing trace_find_cmdline to copy the comm cache
and not just send back a pointer to it.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index ea9d3b410c7a..6a4c9dea191e 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -309,9 +309,9 @@ static int
 lat_print_generic(struct trace_seq *s, struct trace_entry *entry, int cpu)
 {
         int hardirq, softirq;
-        char *comm;
+        char comm[TASK_COMM_LEN];
 
-        comm = trace_find_cmdline(entry->pid);
+        trace_find_cmdline(entry->pid, comm);
         hardirq = entry->flags & TRACE_FLAG_HARDIRQ;
         softirq = entry->flags & TRACE_FLAG_SOFTIRQ;
 
@@ -346,10 +346,12 @@ int trace_print_context(struct trace_iterator *iter)
 {
         struct trace_seq *s = &iter->seq;
         struct trace_entry *entry = iter->ent;
-        char *comm = trace_find_cmdline(entry->pid);
         unsigned long long t = ns2usecs(iter->ts);
         unsigned long usec_rem = do_div(t, USEC_PER_SEC);
         unsigned long secs = (unsigned long)t;
+        char comm[TASK_COMM_LEN];
+
+        trace_find_cmdline(entry->pid, comm);
 
         return trace_seq_printf(s, "%16s-%-5d [%03d] %5lu.%06lu: ",
                                 comm, entry->pid, iter->cpu, secs, usec_rem);
@@ -372,7 +374,10 @@ int trace_print_lat_context(struct trace_iterator *iter)
         rel_usecs = ns2usecs(next_ts - iter->ts);
 
         if (verbose) {
-                char *comm = trace_find_cmdline(entry->pid);
+                char comm[TASK_COMM_LEN];
+
+                trace_find_cmdline(entry->pid, comm);
+
                 ret = trace_seq_printf(s, "%16s %5d %3d %d %08x %08lx [%08lx]"
                                        " %ld.%03ldms (+%ld.%03ldms): ", comm,
                                        entry->pid, iter->cpu, entry->flags,
@@ -577,14 +582,15 @@ static enum print_line_t trace_ctxwake_print(struct trace_iterator *iter,
                                              char *delim)
 {
         struct ctx_switch_entry *field;
-        char *comm;
+        char comm[TASK_COMM_LEN];
         int S, T;
 
+
         trace_assign_type(field, iter->ent);
 
         T = task_state_char(field->next_state);
         S = task_state_char(field->prev_state);
-        comm = trace_find_cmdline(field->next_pid);
+        trace_find_cmdline(field->next_pid, comm);
         if (!trace_seq_printf(&iter->seq,
                               " %5d:%3d:%c %s [%03d] %5d:%3d:%c %s\n",
                               field->prev_pid,