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>

5 files changed, 49 insertions, 26 deletions

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 1f32e4edf490..b171778e3863 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1027,7 +1027,9 @@ static int blk_log_action_seq(struct trace_seq *s, const struct blk_io_trace *t,
 
 static int blk_log_generic(struct trace_seq *s, const struct trace_entry *ent)
 {
-        const char *cmd = trace_find_cmdline(ent->pid);
+        char cmd[TASK_COMM_LEN];
+
+        trace_find_cmdline(ent->pid, cmd);
 
         if (t_sec(ent))
                 return trace_seq_printf(s, "%llu + %u [%s]\n",
@@ -1057,19 +1059,30 @@ static int blk_log_remap(struct trace_seq *s, const struct trace_entry *ent)
 
 static int blk_log_plug(struct trace_seq *s, const struct trace_entry *ent)
 {
-        return trace_seq_printf(s, "[%s]\n", trace_find_cmdline(ent->pid));
+        char cmd[TASK_COMM_LEN];
+
+        trace_find_cmdline(ent->pid, cmd);
+
+        return trace_seq_printf(s, "[%s]\n", cmd);
 }
 
 static int blk_log_unplug(struct trace_seq *s, const struct trace_entry *ent)
 {
-        return trace_seq_printf(s, "[%s] %llu\n", trace_find_cmdline(ent->pid),
-                                get_pdu_int(ent));
+        char cmd[TASK_COMM_LEN];
+
+        trace_find_cmdline(ent->pid, cmd);
+
+        return trace_seq_printf(s, "[%s] %llu\n", cmd, get_pdu_int(ent));
 }
 
 static int blk_log_split(struct trace_seq *s, const struct trace_entry *ent)
 {
+        char cmd[TASK_COMM_LEN];
+
+        trace_find_cmdline(ent->pid, cmd);
+
         return trace_seq_printf(s, "%llu / %llu [%s]\n", t_sector(ent),
-                                get_pdu_int(ent), trace_find_cmdline(ent->pid));
+                                get_pdu_int(ent), cmd);
 }
 
 /*
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index efe3202c0209..2796bd2b17e4 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -770,25 +770,29 @@ static void trace_save_cmdline(struct task_struct *tsk)
         __raw_spin_unlock(&trace_cmdline_lock);
 }
 
-char *trace_find_cmdline(int pid)
+void trace_find_cmdline(int pid, char comm[])
 {
-        char *cmdline = "<...>";
         unsigned map;
 
-        if (!pid)
-                return "<idle>";
+        if (!pid) {
+                strcpy(comm, "<idle>");
+                return;
+        }
 
-        if (pid > PID_MAX_DEFAULT)
-                goto out;
+        if (pid > PID_MAX_DEFAULT) {
+                strcpy(comm, "<...>");
+                return;
+        }
 
+        __raw_spin_lock(&trace_cmdline_lock);
         map = map_pid_to_cmdline[pid];
         if (map >= SAVED_CMDLINES)
                 goto out;
 
-        cmdline = saved_cmdlines[map];
+        strcpy(comm, saved_cmdlines[map]);
 
  out:
-        return cmdline;
+        __raw_spin_unlock(&trace_cmdline_lock);
 }
 
 void tracing_record_cmdline(struct task_struct *tsk)
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 56ce34d90b03..b0ecad8ecc34 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -547,7 +547,7 @@ struct tracer_switch_ops {
 };
 #endif /* CONFIG_CONTEXT_SWITCH_TRACER */
 
-extern char *trace_find_cmdline(int pid);
+extern void trace_find_cmdline(int pid, char comm[]);
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 extern unsigned long ftrace_update_tot_cnt;
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 4c388607ed67..6004ccac2dd7 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -190,15 +190,15 @@ print_graph_cpu(struct trace_seq *s, int cpu)
 static enum print_line_t
 print_graph_proc(struct trace_seq *s, pid_t pid)
 {
-        int i;
-        int ret;
-        int len;
-        char comm[8];
-        int spaces = 0;
+        char comm[TASK_COMM_LEN];
         /* sign + log10(MAX_INT) + '\0' */
         char pid_str[11];
+        int spaces = 0;
+        int ret;
+        int len;
+        int i;
 
-        strncpy(comm, trace_find_cmdline(pid), 7);
+        trace_find_cmdline(pid, comm);
         comm[7] = '\0';
         sprintf(pid_str, "%d", pid);
 
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,