fs: introduce open_by_handle_at()

Author: bertschingert

src/backend/libc/fs/syscalls.rs

@@ -1911,6 +1911,29 @@ pub(crate) fn name_to_handle_at(
     }
 }
 
+#[cfg(target_os = "linux")]
+pub(crate) fn open_by_handle_at(
+    mount_fd: BorrowedFd<'_>,
+    handle: *const core::ffi::c_void,
+    flags: OFlags,
+) -> io::Result<OwnedFd> {
+    syscall! {
+        fn open_by_handle_at(
+            mount_fd: c::c_int,
+            handle: *const ffi::c_void,
+            flags: u32
+        ) via SYS_open_by_handle_at -> c::c_int
+    }
+
+    unsafe {
+        ret_owned_fd(open_by_handle_at(
+            borrowed_fd(mount_fd),
+            handle,
+            flags.bits(),
+        ))
+    }
+}
+
 #[cfg(target_os = "linux")]
 pub(crate) fn sendfile(
     out_fd: BorrowedFd<'_>,

src/backend/linux_raw/fs/syscalls.rs

@@ -1677,6 +1677,15 @@ pub(crate) fn name_to_handle_at(
     }
 }
 
+#[inline]
+pub(crate) fn open_by_handle_at(
+    mount_fd: BorrowedFd<'_>,
+    handle: *const core::ffi::c_void,
+    flags: OFlags,
+) -> io::Result<OwnedFd> {
+    unsafe { ret_owned_fd(syscall!(__NR_open_by_handle_at, mount_fd, handle, flags)) }
+}
+
 // Some linux_raw_sys structs have unsigned types for values which are
 // interpreted as signed. This defines a utility or casting to the
 // same-sized signed type.

src/fs/filehandle.rs

@@ -73,6 +73,10 @@ impl FileHandle {
     fn as_mut_ptr(&mut self) -> *mut ffi::c_void {
         self.raw.as_mut_ptr() as *mut _
     }
+
+    fn as_ptr(&self) -> *const ffi::c_void {
+        self.raw.as_ptr() as *const _
+    }
 }
 
 /// An identifier for a mount that is returned by [`name_to_handle_at`].
@@ -143,6 +147,20 @@ pub fn name_to_handle_at<Fd: AsFd, P: path::Arg>(
     })
 }
 
+/// `open_by_handle_at(mount_fd, handle, flags)` - Open a file by filehandle.
+///
+/// # References
+///  - [Linux]
+///
+///  [Linux]: https://man7.org/linux/man-pages/man2/open_by_handle_at.2.html
+pub fn open_by_handle_at<Fd: AsFd>(
+    mount_fd: Fd,
+    handle: &FileHandle,
+    flags: OFlags,
+) -> io::Result<OwnedFd> {
+    backend::fs::syscalls::open_by_handle_at(mount_fd.as_fd(), handle.as_ptr(), flags)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;