Check for existence before permissions

Return EEXIST when overwritting a file as long as the caller has exec
permission on the parent directory, even if the caller doesn't have
write permission.

Also reordered the mount write check, which happens before permission
is checked.

Closes #5164

PiperOrigin-RevId: 351868123

1 files changed, 33 insertions, 0 deletions

diff --git a/test/syscalls/linux/mkdir.cc b/test/syscalls/linux/mkdir.cc
index 27758203d..11fbfa5c5 100644
--- a/test/syscalls/linux/mkdir.cc
+++ b/test/syscalls/linux/mkdir.cc
@@ -82,6 +82,39 @@ TEST_F(MkdirTest, FailsOnDirWithoutWritePerms) {
               SyscallFailsWithErrno(EACCES));
 }
 
+TEST_F(MkdirTest, DirAlreadyExists) {
+  // Drop capabilities that allow us to override file and directory permissions.
+  ASSERT_NO_ERRNO(SetCapability(CAP_DAC_OVERRIDE, false));
+  ASSERT_NO_ERRNO(SetCapability(CAP_DAC_READ_SEARCH, false));
+
+  ASSERT_THAT(mkdir(dirname_.c_str(), 0777), SyscallSucceeds());
+  auto dir = JoinPath(dirname_.c_str(), "foo");
+  EXPECT_THAT(mkdir(dir.c_str(), 0777), SyscallSucceeds());
+
+  struct {
+    int mode;
+    int err;
+  } tests[] = {
+      {.mode = 0000, .err = EACCES},  // No perm
+      {.mode = 0100, .err = EEXIST},  // Exec only
+      {.mode = 0200, .err = EACCES},  // Write only
+      {.mode = 0300, .err = EEXIST},  // Write+exec
+      {.mode = 0400, .err = EACCES},  // Read only
+      {.mode = 0500, .err = EEXIST},  // Read+exec
+      {.mode = 0600, .err = EACCES},  // Read+write
+      {.mode = 0700, .err = EEXIST},  // All
+  };
+  for (const auto& t : tests) {
+    printf("mode: 0%o\n", t.mode);
+    EXPECT_THAT(chmod(dirname_.c_str(), t.mode), SyscallSucceeds());
+    EXPECT_THAT(mkdir(dir.c_str(), 0777), SyscallFailsWithErrno(t.err));
+  }
+
+  // Clean up.
+  EXPECT_THAT(chmod(dirname_.c_str(), 0777), SyscallSucceeds());
+  ASSERT_THAT(rmdir(dir.c_str()), SyscallSucceeds());
+}
+
 TEST_F(MkdirTest, MkdirAtEmptyPath) {
   ASSERT_THAT(mkdir(dirname_.c_str(), 0777), SyscallSucceeds());
   auto fd =