[asan] Allow getpwnam(NULL) for binary compatibily

[kubamracek]

Summary

Calling getpwnam(NULL) is probably a bug, but at least on Darwin, such a call succeeds without segfaulting. I have some existing code that relies on that. To maintain binary compatibility, ASan should also survive a call to getpwnam with NULL.

lib/sanitizer_common/sanitizer_common_interceptors.inc

#endif  // SANITIZER_INTERCEPT_GETPWNAM_AND_FRIENDS ||
         // SANITIZER_INTERCEPT_GETPWENT || SANITIZER_INTERCEPT_FGETPWENT ||
         // SANITIZER_INTERCEPT_GETPWENT_R ||
         // SANITIZER_INTERCEPT_GETPWNAM_R_AND_FRIENDS
 
 #if SANITIZER_INTERCEPT_GETPWNAM_AND_FRIENDS
 INTERCEPTOR(__sanitizer_passwd *, getpwnam, const char *name) {
   void *ctx;
   COMMON_INTERCEPTOR_ENTER(ctx, getpwnam, name);
+  if (name)
     COMMON_INTERCEPTOR_READ_RANGE(ctx, name, REAL(strlen)(name) + 1);
   __sanitizer_passwd *res = REAL(getpwnam)(name);
   if (res) unpoison_passwd(ctx, res);
   return res;
 }
 INTERCEPTOR(__sanitizer_passwd *, getpwuid, u32 uid) {
   void *ctx;
   COMMON_INTERCEPTOR_ENTER(ctx, getpwuid, uid);
   __sanitizer_passwd *res = REAL(getpwuid)(uid);

test/asan/TestCases/Darwin/getpwnam.c

+// RUN: %clang_asan %s -o %t
+// RUN: %run %t 2>&1 | FileCheck %s
+
+#include <pwd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+
+int main(int argc, const char * argv[]) {
+  getpwnam(NULL);
+  fprintf(stderr, "Finished.\n");
+  return 0;
+}
+
+// CHECK: Finished.