tracing: Fix potential null-pointer-access of entry in list 'tr->err_log'

Entries in list 'tr->err_log' will be reused after entry number
exceed TRACING_LOG_ERRS_MAX.

The cmd string of the to be reused entry will be freed first then
allocated a new one. If the allocation failed, then the entry will
still be in list 'tr->err_log' but its 'cmd' field is set to be NULL,
later access of 'cmd' is risky.

Currently above problem can cause the loss of 'cmd' information of first
entry in 'tr->err_log'. When execute `cat /sys/kernel/tracing/error_log`,
reproduce logs like:
  [   37.495100] trace_kprobe: error: Maxactive is not for kprobe(null) ^
  [   38.412517] trace_kprobe: error: Maxactive is not for kprobe
    Command: p4:myprobe2 do_sys_openat2
            ^

Link: https://lore.kernel.org/linux-trace-kernel/20221114104632.3547266-1-zhengyejian1@huawei.com

Fixes: 1581a88 ("tracing: Remove size restriction on tracing_log_err cmd strings")
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Zheng Yejian

kernel/trace/trace.c

@@ -7803,6 +7803,7 @@ static struct tracing_log_err *get_tracing_log_err(struct trace_array *tr,
 						   int len)
 {
 	struct tracing_log_err *err;
+	char *cmd;
 
 	if (tr->n_err_log_entries < TRACING_LOG_ERRS_MAX) {
 		err = alloc_tracing_log_err(len);
@@ -7811,12 +7812,12 @@ static struct tracing_log_err *get_tracing_log_err(struct trace_array *tr,
 
 		return err;
 	}
-
+	cmd = kzalloc(len, GFP_KERNEL);
+	if (!cmd)
+		return ERR_PTR(-ENOMEM);
 	err = list_first_entry(&tr->err_log, struct tracing_log_err, list);
 	kfree(err->cmd);
-	err->cmd = kzalloc(len, GFP_KERNEL);
-	if (!err->cmd)
-		return ERR_PTR(-ENOMEM);
+	err->cmd = cmd;
 	list_del(&err->list);
 
 	return err;