Fix a bug of caching negative dentry

kernel/src/fs/path/dentry.rs

@@ -175,15 +175,8 @@ impl Dentry {
     pub(super) fn lookup_via_fs(&self, name: &str) -> Result<Arc<Dentry>> {
         let children = self.children.upread();
 
-        let inode = match self.inode.lookup(name) {
-            Ok(inode) => inode,
-            Err(e) => {
-                if e.error() == Errno::ENOENT && self.is_dentry_cacheable() {
-                    children.upgrade().insert_negative(String::from(name));
-                }
-                return Err(e);
-            }
-        };
+        // TODO: Add a right implementation to cache negative dentry.
+        let inode = self.inode.lookup(name)?;
         let name = String::from(name);
         let target = Self::new(inode, DentryOptions::Leaf((name.clone(), self.this())));
 
@@ -446,6 +439,7 @@ impl DentryChildren {
     }
 
     /// Inserts a negative dentry.
+    #[expect(dead_code)]
     fn insert_negative(&mut self, name: String) {
         let _ = self.dentries.insert(name, None);
     }

test/src/apps/procfs/dentry_cache.c

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#define _GNU_SOURCE
+
+#include <unistd.h>
+#include <fcntl.h>
+#include <wait.h>
+
+#include "../test.h"
+
+FN_TEST(negative_cache_pid)
+{
+	pid_t pid, pid2;
+	char path[20];
+	int i, status;
+
+	pid = TEST_SUCC(getpid());
+
+	// These paths may not yet exist, but we cannot cache negative results here.
+	for (i = 0; i < 100; ++i) {
+		snprintf(path, sizeof(path), "/proc/%d", pid + i);
+		(void)!access(path, F_OK);
+	}
+
+	pid2 = TEST_SUCC(fork());
+	if (pid2 == 0) {
+		CHECK(access("/proc/self/", F_OK));
+
+		usleep(200 * 1000);
+		exit(EXIT_SUCCESS);
+	}
+
+	snprintf(path, sizeof(path), "/proc/%d", pid2);
+	TEST_SUCC(access(path, F_OK));
+
+	TEST_RES(wait(&status), _ret == pid2 && WIFEXITED(status) &&
+					WEXITSTATUS(status) == EXIT_SUCCESS);
+
+	snprintf(path, sizeof(path), "/proc/%d", pid2);
+	TEST_ERRNO(access(path, F_OK), ENOENT);
+}
+END_TEST()
+
+FN_TEST(negative_cache_fd)
+{
+	TEST_ERRNO(access("/proc/self/fdinfo/100", F_OK), ENOENT);
+
+	TEST_SUCC(dup2(0, 100));
+	TEST_SUCC(access("/proc/self/fdinfo/100", F_OK));
+
+	TEST_SUCC(close(100));
+	TEST_ERRNO(access("/proc/self/fdinfo/100", F_OK), ENOENT);
+}
+END_TEST()

test/src/apps/scripts/process.sh

@@ -44,6 +44,7 @@ process/group_session
 process/job_control
 process/pidfd
 process/wait4
+procfs/dentry_cache
 procfs/pid_mem
 pseudofs/pseudo_inode
 pseudofs/memfd_access_err