[PULL 15/20] virtiofsd: Add capability to change/restore umask

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Vivek Goyal <vgoyal@redhat.com>

When parent directory has default acl and a file is created in that
directory, then umask is ignored and final file permissions are
determined using default acl instead. (man 2 umask).

Currently, fuse applies the umask and sends modified mode in create
request accordingly. fuse server can set FUSE_DONT_MASK and tell
fuse client to not apply umask and fuse server will take care of
it as needed.

With posix acls enabled, requirement will be that we want umask
to determine final file mode if parent directory does not have
default acl.

So if posix acls are enabled, opt in for FUSE_DONT_MASK. virtiofsd
will set umask of the thread doing file creation. And host kernel
should use that umask if parent directory does not have default
acls, otherwise umask does not take affect.

Miklos mentioned that we already call unshare(CLONE_FS) for
every thread. That means umask has now become property of per
thread and it should be ok to manipulate it in file creation path.

This patch only adds capability to change umask and restore it. It
does not enable it yet. Next few patches will add capability to enable it
based on if user enabled posix_acl or not.

This should fix fstest generic/099.

Reported-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210622150852.1507204-6-vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 tools/virtiofsd/passthrough_ll.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 4dec087bd4..65b2c6fd74 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -122,6 +122,7 @@ struct lo_inode {
 struct lo_cred {
     uid_t euid;
     gid_t egid;
+    mode_t umask;
 };
 
 enum {
@@ -172,6 +173,8 @@ struct lo_data {
     /* An O_PATH file descriptor to /proc/self/fd/ */
     int proc_self_fd;
     int user_killpriv_v2, killpriv_v2;
+    /* If set, virtiofsd is responsible for setting umask during creation */
+    bool change_umask;
 };
 
 static const struct fuse_opt lo_opts[] = {
@@ -1134,7 +1137,8 @@ static void lo_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
  * ownership of caller.
  * TODO: What about selinux context?
  */
-static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
+static int lo_change_cred(fuse_req_t req, struct lo_cred *old,
+                          bool change_umask)
 {
     int res;
 
@@ -1154,11 +1158,14 @@ static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
         return errno_save;
     }
 
+    if (change_umask) {
+        old->umask = umask(req->ctx.umask);
+    }
     return 0;
 }
 
 /* Regain Privileges */
-static void lo_restore_cred(struct lo_cred *old)
+static void lo_restore_cred(struct lo_cred *old, bool restore_umask)
 {
     int res;
 
@@ -1173,6 +1180,9 @@ static void lo_restore_cred(struct lo_cred *old)
         fuse_log(FUSE_LOG_ERR, "setegid(%u): %m\n", old->egid);
         exit(1);
     }
+
+    if (restore_umask)
+        umask(old->umask);
 }
 
 static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
@@ -1202,7 +1212,7 @@ static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
         return;
     }
 
-    saverr = lo_change_cred(req, &old);
+    saverr = lo_change_cred(req, &old, lo->change_umask && !S_ISLNK(mode));
     if (saverr) {
         goto out;
     }
@@ -1211,7 +1221,7 @@ static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
 
     saverr = errno;
 
-    lo_restore_cred(&old);
+    lo_restore_cred(&old, lo->change_umask && !S_ISLNK(mode));
 
     if (res == -1) {
         goto out;
@@ -1917,7 +1927,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
         return;
     }
 
-    err = lo_change_cred(req, &old);
+    err = lo_change_cred(req, &old, lo->change_umask);
     if (err) {
         goto out;
     }
@@ -1928,7 +1938,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
     fd = openat(parent_inode->fd, name, fi->flags | O_CREAT | O_EXCL, mode);
     err = fd == -1 ? errno : 0;
 
-    lo_restore_cred(&old);
+    lo_restore_cred(&old, lo->change_umask);
 
     /* Ignore the error if file exists and O_EXCL was not given */
     if (err && (err != EEXIST || (fi->flags & O_EXCL))) {
-- 
2.31.1