[analyzer] sprintf is a taint propagator not a source

Due to a typo, `sprintf()` was recognized as a taint source instead of a
taint propagator. It was because an empty taint source list - which is
the first parameter of the `TaintPropagationRule` - encoded the
unconditional taint sources.
This typo effectively turned the `sprintf()` into an unconditional taint
source.

This patch fixes that typo and demonstrated the correct behavior with
tests.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D112558

clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp

@@ -514,7 +514,7 @@ GenericTaintChecker::TaintPropagationRule::getTaintPropagationRule(
     if (OneOf("snprintf"))
       return {{1}, {0, ReturnValueIndex}, VariadicType::Src, 3};
     if (OneOf("sprintf"))
-      return {{}, {0, ReturnValueIndex}, VariadicType::Src, 2};
+      return {{1}, {0, ReturnValueIndex}, VariadicType::Src, 2};
     if (OneOf("strcpy", "stpcpy", "strcat"))
       return {{1}, {0, ReturnValueIndex}};
     if (OneOf("bcopy"))

clang/test/Analysis/taint-generic.c

@@ -341,6 +341,16 @@ void constraintManagerShouldTreatAsOpaque(int rhs) {
     *(volatile int *) 0; // no-warning
 }
 
+int sprintf_is_not_a_source(char *buf, char *msg) {
+  int x = sprintf(buf, "%s", msg); // no-warning
+  return 1 / x; // no-warning: 'sprintf' is not a taint source
+}
+
+int sprintf_propagates_taint(char *buf, char *msg) {
+  scanf("%s", msg);
+  int x = sprintf(buf, "%s", msg); // propagate taint!
+  return 1 / x; // expected-warning {{Division by a tainted value, possibly zero}}
+}
 
 // Test configuration
 int mySource1();