Handle open(NULL).

Fixes #3865

CMakeLists.txt

@@ -1203,6 +1203,7 @@ set(BASIC_TESTS
   numa
   x86/old_fork
   orphan_process
+  open_null
   openat2
   packet_mmap_disable
   x86/patch_syscall_restart

src/preload/syscallbuf.c

@@ -2532,7 +2532,7 @@ static long sys_open(struct syscall_info* call) {
 
   assert(syscallno == call->no);
 
-  if (!supported_open(pathname, flags)) {
+  if (!pathname || !supported_open(pathname, flags)) {
     return traced_raw_syscall(call);
   }
 

src/record_syscall.cc

@@ -6861,8 +6861,9 @@ static void rec_process_syscall_arch(RecordTask* t,
       Registers r = t->regs();
       if (r.syscall_failed()) {
         uintptr_t path = syscallno == Arch::open ? r.orig_arg1() : r.arg2();
-        string pathname = t->read_c_str(remote_ptr<char>(path));
-        if (is_gcrypt_deny_file(pathname.c_str())) {
+        bool ok = false;
+        string pathname = t->read_c_str(remote_ptr<char>(path), &ok);
+        if (ok && is_gcrypt_deny_file(pathname.c_str())) {
           fake_gcrypt_file(t, &r);
           t->set_regs(r);
         }

src/test/open_null.c

@@ -0,0 +1,12 @@
+/* -*- Mode: C; tab-width: 8; c-basic-offset: 2; indent-tabs-mode: nil; -*- */
+#include "util.h"
+#include <unistd.h>
+
+int main(void) {
+  // Use syscall(2) because the glibc prototype for open(2) might enforce that
+  // it's nonnull.
+  int fd = syscall(SYS_open, NULL, O_RDONLY);
+  test_assert(fd == -1);
+  atomic_puts("EXIT-SUCCESS");
+  return 0;
+}