vhost_user_fs: Add the WRITE_KILL_PRIV write flag.

Add the WRITE_KILL_PRIV write flag, corresponding to
FUSE_WRITE_KILL_PRIV introduced in 7.31, and use to only remove the
setuid and setgid bits (by switching credentials) conditionally.

Signed-off-by: Sergio Lopez <slp@redhat.com>

vhost_user_fs/src/filesystem.rs

@@ -682,11 +682,11 @@ pub trait FileSystem {
     /// implementation did not return a `Handle` from `open` then the contents of `handle` are
     /// undefined.
     ///
-    /// If the `FsOptions::HANDLE_KILLPRIV` feature is not enabled then then the file system is
-    /// expected to clear the setuid and setgid bits.
-    ///
     /// If `delayed_write` is true then it indicates that this is a write for buffered data.
     ///
+    /// If `kill_priv` is true then it indicates that the file system is expected to clear the
+    /// setuid and setgid bits.
+    ///
     /// This method should return exactly the number of bytes requested by the kernel, except in the
     /// case of error. An exception to this rule is if the file was opened with the "direct I/O"
     /// option (`libc::O_DIRECT`), in which case the kernel will forward the return code from this
@@ -702,6 +702,7 @@ pub trait FileSystem {
         offset: u64,
         lock_owner: Option<u64>,
         delayed_write: bool,
+        kill_priv: bool,
         flags: u32,
     ) -> io::Result<usize> {
         Err(io::Error::from_raw_os_error(libc::ENOSYS))

vhost_user_fs/src/fuse.rs

@@ -373,6 +373,9 @@ pub const WRITE_CACHE: u32 = 1;
 /// `lock_owner` field is valid.
 pub const WRITE_LOCKOWNER: u32 = 2;
 
+/// Kill suid and sgid bits
+pub const WRITE_KILL_PRIV: u32 = 4;
+
 // Read flags.
 pub const READ_LOCKOWNER: u32 = 2;
 

vhost_user_fs/src/passthrough.rs

@@ -998,11 +998,15 @@ impl FileSystem for PassthroughFs {
         offset: u64,
         _lock_owner: Option<u64>,
         _delayed_write: bool,
+        kill_priv: bool,
         _flags: u32,
     ) -> io::Result<usize> {
-        // We need to change credentials during a write so that the kernel will remove setuid or
-        // setgid bits from the file if it was written to by someone other than the owner.
-        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;
+        if kill_priv {
+            // We need to change credentials during a write so that the kernel will remove setuid
+            // or setgid bits from the file if it was written to by someone other than the owner.
+            let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;
+        }
+
         let data = self
             .handles
             .read()

vhost_user_fs/src/server.rs

@@ -636,6 +636,7 @@ impl<F: FileSystem + Sync> Server<F> {
         };
 
         let delayed_write = write_flags & WRITE_CACHE != 0;
+        let kill_priv = write_flags & WRITE_KILL_PRIV != 0;
 
         let data_reader = ZCReader(r);
 
@@ -648,6 +649,7 @@ impl<F: FileSystem + Sync> Server<F> {
             offset,
             owner,
             delayed_write,
+            kill_priv,
             flags,
         ) {
             Ok(count) => {