Kernel+Tests: Allow deleting someone else's file in my sticky directory

This should be allowed according to Dr. POSIX. :^)

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -868,8 +868,13 @@ ErrorOr<void> VirtualFileSystem::rmdir(Credentials const& credentials, StringVie
         return EACCES;
 
     if (parent_metadata.is_sticky()) {
-        if (!credentials.is_superuser() && inode.metadata().uid != credentials.euid())
+        // [EACCES] The S_ISVTX flag is set on the directory containing the file referred to by the path argument
+        //          and the process does not satisfy the criteria specified in XBD Directory Protection.
+        if (!credentials.is_superuser()
+            && inode.metadata().uid != credentials.euid()
+            && parent_metadata.uid != credentials.euid()) {
             return EACCES;
+        }
     }
 
     size_t child_count = 0;

Tests/LibC/TestIo.cpp

@@ -15,6 +15,7 @@
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/uio.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #define EXPECT_ERROR_2(err, syscall, arg1, arg2)                                                                                   \
@@ -356,6 +357,49 @@ TEST_CASE(rmdir_dot_dot)
     EXPECT_EQ(rc, 0);
 }
 
+TEST_CASE(rmdir_someone_elses_directory_in_my_sticky_directory)
+{
+    // NOTE: This test only works when run as root, since it has to chown a directory to someone else.
+    if (getuid() != 0)
+        return;
+
+    // Create /tmp/sticky-dir a sticky directory owned by 12345:12345
+    // Then, create /tmp/sticky-dir/notmine, a normal directory owned by 23456:23456
+    // Then, fork and seteuid to 12345, and try to rmdir the "notmine" directory. This should succeed.
+    // In the parent, waitpid on the child, and finally rmdir /tmp/sticky-dir
+
+    int rc = mkdir("/tmp/sticky-dir", 01777);
+    EXPECT_EQ(rc, 0);
+
+    rc = chown("/tmp/sticky-dir", 12345, 12345);
+    EXPECT_EQ(rc, 0);
+
+    rc = mkdir("/tmp/sticky-dir/notmine", 0700);
+    EXPECT_EQ(rc, 0);
+
+    rc = chown("/tmp/sticky-dir/notmine", 23456, 23456);
+    EXPECT_EQ(rc, 0);
+
+    int pid = fork();
+    EXPECT(pid >= 0);
+
+    if (pid == 0) {
+        // We are in the child.
+        rc = seteuid(12345);
+        EXPECT_EQ(rc, 0);
+
+        rc = rmdir("/tmp/sticky-dir/notmine");
+        EXPECT_EQ(rc, 0);
+        _exit(0);
+    }
+
+    int status = 0;
+    waitpid(pid, &status, 0);
+
+    rc = rmdir("/tmp/sticky-dir");
+    EXPECT_EQ(rc, 0);
+}
+
 TEST_CASE(rmdir_while_inside_dir)
 {
     int rc = mkdir("/home/anon/testdir", 0700);