[StaticAnalyzer] Relax the pre-condition of 'setsockopt' (#130683)

For the unix function
`int setsockopt(int, int, int, const void *, socklen_t);`, the last two
parameters represent a buffer and a size.

In case the size is zero, buffer can be null. Previously, the hard-coded
pre-condition requires the buffer to never be null, which can cause
false positives.

(rdar://146678142)

Author: ziqingluo-90

clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp

@@ -1797,7 +1797,8 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
   auto IsNull = [&](ArgNo ArgN) {
     return std::make_shared<NotNullConstraint>(ArgN, false);
   };
-  auto NotNullBuffer = [&](ArgNo ArgN, ArgNo SizeArg1N, ArgNo SizeArg2N) {
+  auto NotNullBuffer = [&](ArgNo ArgN, ArgNo SizeArg1N,
+                           std::optional<ArgNo> SizeArg2N = std::nullopt) {
     return std::make_shared<NotNullBufferConstraint>(ArgN, SizeArg1N,
                                                      SizeArg2N);
   };
@@ -3365,7 +3366,7 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
         Summary(NoEvalCall)
             .Case(ReturnsZero, ErrnoMustNotBeChecked, GenericSuccessMsg)
             .Case(ReturnsMinusOne, ErrnoNEZeroIrrelevant, GenericFailureMsg)
-            .ArgConstraint(NotNull(ArgNo(3)))
+            .ArgConstraint(NotNullBuffer(ArgNo(3), ArgNo(4)))
             .ArgConstraint(
                 BufferSize(/*Buffer=*/ArgNo(3), /*BufSize=*/ArgNo(4)))
             .ArgConstraint(

clang/test/Analysis/std-c-library-functions-POSIX.c

@@ -237,3 +237,14 @@ void test_readlinkat_bufsize_zero(int fd, char *Buf, size_t Bufsize) {
   else
     clang_analyzer_eval(Bufsize == 0); // expected-warning{{UNKNOWN}}
 }
+
+void test_setsockopt_bufptr_null(int x) {
+  char buf[10] = {0};
+
+  setsockopt(1, 2, 3, 0, 0);
+  setsockopt(1, 2, 3, buf, 10);
+  if (x)
+    setsockopt(1, 2, 3, buf, 11); // expected-warning{{The 4th argument to 'setsockopt' is a buffer with size 10 but should be a buffer with size equal to or greater than the value of the 5th argument (which is 11)}}
+  else
+    setsockopt(1, 2, 3, 0, 10);  // expected-warning{{The 4th argument to 'setsockopt' is NULL but should not be NULL}}
+}