Add support for /dev/fd

src/libos/src/fs/dev_fs/dev_fd.rs

@@ -0,0 +1,48 @@
+use super::*;
+pub struct DevFd;
+
+// Implement /dev/fd as a symlink to /proc/self/fd
+impl INode for DevFd {
+    fn read_at(&self, offset: usize, buf: &mut [u8]) -> vfs::Result<usize> {
+        let proc_fd = "/proc/self/fd";
+        for (tgt, src) in buf.iter_mut().zip(proc_fd.as_bytes().iter()) {
+            *tgt = *src;
+        }
+        Ok(proc_fd.len())
+    }
+
+    fn write_at(&self, offset: usize, buf: &[u8]) -> vfs::Result<usize> {
+        Err(vfs::FsError::PermError)
+    }
+
+    fn poll(&self) -> vfs::Result<vfs::PollStatus> {
+        Ok(vfs::PollStatus {
+            read: true,
+            write: true,
+            error: false,
+        })
+    }
+
+    fn metadata(&self) -> vfs::Result<Metadata> {
+        Ok(Metadata {
+            dev: 1,
+            inode: 0,
+            size: 0,
+            blk_size: 0,
+            blocks: 0,
+            atime: Timespec { sec: 0, nsec: 0 },
+            mtime: Timespec { sec: 0, nsec: 0 },
+            ctime: Timespec { sec: 0, nsec: 0 },
+            type_: vfs::FileType::SymLink,
+            mode: 0o777,
+            nlinks: 1,
+            uid: 0,
+            gid: 0,
+            rdev: 0,
+        })
+    }
+
+    fn as_any_ref(&self) -> &dyn Any {
+        self
+    }
+}

src/libos/src/fs/dev_fs/mod.rs

@@ -6,12 +6,14 @@ use rcore_fs_devfs::DevFS;
 use rcore_fs_mountfs::MountFS;
 use rcore_fs_ramfs::RamFS;
 
+use self::dev_fd::DevFd;
 use self::dev_null::DevNull;
 use self::dev_random::DevRandom;
 use self::dev_sgx::DevSgx;
 use self::dev_shm::DevShm;
 use self::dev_zero::DevZero;
 
+mod dev_fd;
 mod dev_null;
 mod dev_random;
 mod dev_sgx;
@@ -33,6 +35,8 @@ pub fn init_devfs() -> Result<Arc<MountFS>> {
     devfs.add("sgx", dev_sgx)?;
     let dev_shm = Arc::new(DevShm) as _;
     devfs.add("shm", dev_shm)?;
+    let dev_fd = Arc::new(DevFd) as _;
+    devfs.add("fd", dev_fd);
     let mountable_devfs = MountFS::new(devfs);
     // Mount the ramfs at '/shm'
     let ramfs = RamFS::new();

test/device/main.c

@@ -4,6 +4,7 @@
 #include <poll.h>
 #include <unistd.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include "test_fs.h"
 
 // ============================================================================
@@ -143,6 +144,47 @@ int test_dev_shm() {
     return 0;
 }
 
+int test_dev_fd() {
+    char *file_path = "/root/hello_world";
+    char *greetings = "hello";
+    int fd = open(file_path, O_RDWR | O_CREAT | O_TRUNC, 00666);
+    if (fd < 0) {
+        THROW_ERROR("failed to open a file to write");
+    }
+
+    // Generate dev_fd path
+    char dev_fd_path[20] = "/dev/fd/";
+    char *fd_str = calloc(1, 4); // 4 bytes would be enough here.
+    if (fd_str == NULL) {
+        THROW_ERROR("calloc failed");
+    }
+    asprintf(&fd_str, "%d", fd);
+    strcat(dev_fd_path, fd_str);
+    int dev_fd = open(dev_fd_path, O_WRONLY, 0666);
+    if (dev_fd < 0) {
+        THROW_ERROR("failed to open %s", dev_fd_path);
+    }
+
+    int len = write(dev_fd, greetings, strlen(greetings));
+    if (len < 0) {
+        THROW_ERROR("failed to write to %s", dev_fd_path);
+    }
+
+    char buf[10] = {0};
+    len = read(fd, buf, len);
+    if (len < 0) {
+        THROW_ERROR("failed to read from %s", file_path);
+    }
+
+    if (strcmp(buf, greetings)) {
+        THROW_ERROR("file content is wrong");
+    }
+
+    free(fd_str);
+    close(dev_fd);
+    close(fd);
+    return 0;
+}
 // ============================================================================
 // Test suite
 // ============================================================================
@@ -156,6 +198,7 @@ static test_case_t test_cases[] = {
     TEST_CASE(test_dev_urandom_poll),
     TEST_CASE(test_dev_arandom),
     TEST_CASE(test_dev_shm),
+    TEST_CASE(test_dev_fd),
 };
 
 int main() {