tracing: Check field value in hist_field_name()

The function hist_field_name() cannot handle being passed a NULL field
parameter. It should never be NULL, but due to a previous bug, NULL was
passed to the function and the kernel crashed due to a NULL dereference.
Mark Rutland reported this to me on IRC.

The bug was fixed, but to prevent future bugs from crashing the kernel,
check the field and add a WARN_ON() if it is NULL.

Link: https://lkml.kernel.org/r/[email protected]

Cc: [email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Andrew Morton <[email protected]>
Reported-by: Mark Rutland <[email protected]>
Fixes: c6afad4 ("tracing: Add hist trigger 'sym' and 'sym-offset' modifiers")
Tested-by: Mark Rutland <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/trace_events_hist.c

@@ -1331,6 +1331,9 @@ static const char *hist_field_name(struct hist_field *field,
 {
 	const char *field_name = "";
 
+	if (WARN_ON_ONCE(!field))
+		return field_name;
+
 	if (level > 1)
 		return field_name;