tracing/eprobes: Fix reading of string fields

Currently when an event probe (eprobe) hooks to a string field, it does
not display it as a string, but instead as a number. This makes the field
rather useless. Handle the different kinds of strings, dynamic, static,
relational/dynamic etc.

Now when a string field is used, the ":string" type can be used to display
it:

  echo "e:sw sched/sched_switch comm=$next_comm:string" > dynamic_events

Link: https://lkml.kernel.org/r/20220820134400.959640191@goodmis.org
Cc: stable@vger.kernel.org
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Tzvetomir Stoyanov <tz.stoyanov@gmail.com>
Cc: Tom Zanussi <zanussi@kernel.org>
Fixes: 7491e2c44278 ("tracing: Add a probe that attaches to trace events")
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c
index 5506719..a1d3423 100644 (file)
--- a/kernel/trace/trace_eprobe.c
+++ b/kernel/trace/trace_eprobe.c
@@ -311,6 +311,27 @@ static unsigned long get_event_field(struct fetch_insn *code, void *rec)
 
        addr = rec + field->offset;
 
+       if (is_string_field(field)) {
+               switch (field->filter_type) {
+               case FILTER_DYN_STRING:
+                       val = (unsigned long)(rec + (*(unsigned int *)addr & 0xffff));
+                       break;
+               case FILTER_RDYN_STRING:
+                       val = (unsigned long)(addr + (*(unsigned int *)addr & 0xffff));
+                       break;
+               case FILTER_STATIC_STRING:
+                       val = (unsigned long)addr;
+                       break;
+               case FILTER_PTR_STRING:
+                       val = (unsigned long)(*(char *)addr);
+                       break;
+               default:
+                       WARN_ON_ONCE(1);
+                       return 0;
+               }
+               return val;
+       }
+
        switch (field->size) {
        case 1:
                if (field->is_signed)