perf machine: Avoid out of bounds LBR memory read

Running perf top with address sanitizer and "--call-graph=lbr" fails
due to reading sample 0 when no samples exist. Add a guard to prevent
this.

Fixes: e2b2348 ("perf machine: Factor out lbr_callchain_add_lbr_ip()")
Signed-off-by: Ian Rogers <[email protected]>
Cc: K Prateek Nayak <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: German Gomez <[email protected]>
Cc: James Clark <[email protected]>
Cc: Nick Terrell <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: liuwenyu <[email protected]>
Cc: Yang Jihong <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Athira Rajeev <[email protected]>
Cc: Yanteng Si <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

tools/perf/util/machine.c

@@ -2622,16 +2622,18 @@ static int lbr_callchain_add_lbr_ip(struct thread *thread,
 		save_lbr_cursor_node(thread, cursor, i);
 	}
 
-	/* Add LBR ip from first entries.to */
-	ip = entries[0].to;
-	flags = &entries[0].flags;
-	*branch_from = entries[0].from;
-	err = add_callchain_ip(thread, cursor, parent,
-			       root_al, &cpumode, ip,
-			       true, flags, NULL,
-			       *branch_from);
-	if (err)
-		return err;
+	if (lbr_nr > 0) {
+		/* Add LBR ip from first entries.to */
+		ip = entries[0].to;
+		flags = &entries[0].flags;
+		*branch_from = entries[0].from;
+		err = add_callchain_ip(thread, cursor, parent,
+				root_al, &cpumode, ip,
+				true, flags, NULL,
+				*branch_from);
+		if (err)
+			return err;
+	}
 
 	return 0;
 }