fs: Push mnt_want_write() outside of i_mutex

Currently, mnt_want_write() is sometimes called with i_mutex held and sometimes
without it. This isn't really a problem because mnt_want_write() is a
non-blocking operation (essentially has a trylock semantics) but when the
function starts to handle also frozen filesystems, it will get a full lock
semantics and thus proper lock ordering has to be established. So move
all mnt_want_write() calls outside of i_mutex.

One non-trivial case needing conversion is kern_path_create() /
user_path_create() which didn't include mnt_want_write() but now needs to
because it acquires i_mutex.  Because there are virtual file systems which
don't bother with freeze / remount-ro protection we actually provide both
versions of the function - one which calls mnt_want_write() and one which does
not.

[AV: scratch the previous, mnt_want_write() has been moved to kern_path_create()
by now]

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 25 insertions, 21 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 35291ac6f42b..1b464390dde8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2975,6 +2975,7 @@ struct dentry *kern_path_create(int dfd, const char *pathname, struct path *path
 {
         struct dentry *dentry = ERR_PTR(-EEXIST);
         struct nameidata nd;
+        int err2;
         int error = do_path_lookup(dfd, pathname, LOOKUP_PARENT, &nd);
         if (error)
                 return ERR_PTR(error);
@@ -2988,6 +2989,8 @@ struct dentry *kern_path_create(int dfd, const char *pathname, struct path *path
         nd.flags &= ~LOOKUP_PARENT;
         nd.flags |= LOOKUP_CREATE | LOOKUP_EXCL;
 
+        /* don't fail immediately if it's r/o, at least try to report other errors */
+        err2 = mnt_want_write(nd.path.mnt);
         /*
          * Do the final lookup.
          */
@@ -3009,9 +3012,10 @@ struct dentry *kern_path_create(int dfd, const char *pathname, struct path *path
                 error = -ENOENT;
                 goto fail;
         }
-        error = mnt_want_write(nd.path.mnt);
-        if (error)
+        if (unlikely(err2)) {
+                error = err2;
                 goto fail;
+        }
         *path = nd.path;
         return dentry;
 fail:
@@ -3019,6 +3023,8 @@ fail:
         dentry = ERR_PTR(error);
 unlock:
         mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+        if (!err2)
+                mnt_drop_write(nd.path.mnt);
 out:
         path_put(&nd.path);
         return dentry;
@@ -3266,6 +3272,9 @@ static long do_rmdir(int dfd, const char __user *pathname)
         }
 
         nd.flags &= ~LOOKUP_PARENT;
+        error = mnt_want_write(nd.path.mnt);
+        if (error)
+                goto exit1;
 
         mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
         dentry = lookup_hash(&nd);
@@ -3276,19 +3285,15 @@ static long do_rmdir(int dfd, const char __user *pathname)
                 error = -ENOENT;
                 goto exit3;
         }
-        error = mnt_want_write(nd.path.mnt);
-        if (error)
-                goto exit3;
         error = security_path_rmdir(&nd.path, dentry);
         if (error)
-                goto exit4;
+                goto exit3;
         error = vfs_rmdir(nd.path.dentry->d_inode, dentry);
-exit4:
-        mnt_drop_write(nd.path.mnt);
 exit3:
         dput(dentry);
 exit2:
         mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+        mnt_drop_write(nd.path.mnt);
 exit1:
         path_put(&nd.path);
         putname(name);
@@ -3355,6 +3360,9 @@ static long do_unlinkat(int dfd, const char __user *pathname)
                 goto exit1;
 
         nd.flags &= ~LOOKUP_PARENT;
+        error = mnt_want_write(nd.path.mnt);
+        if (error)
+                goto exit1;
 
         mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
         dentry = lookup_hash(&nd);
@@ -3367,21 +3375,17 @@ static long do_unlinkat(int dfd, const char __user *pathname)
                 if (!inode)
                         goto slashes;
                 ihold(inode);
-                error = mnt_want_write(nd.path.mnt);
-                if (error)
-                        goto exit2;
                 error = security_path_unlink(&nd.path, dentry);
                 if (error)
-                        goto exit3;
+                        goto exit2;
                 error = vfs_unlink(nd.path.dentry->d_inode, dentry);
-exit3:
-                mnt_drop_write(nd.path.mnt);
-        exit2:
+exit2:
                 dput(dentry);
         }
         mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
         if (inode)
                 iput(inode);    /* truncate the inode here */
+        mnt_drop_write(nd.path.mnt);
 exit1:
         path_put(&nd.path);
         putname(name);
@@ -3753,6 +3757,10 @@ SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
         if (newnd.last_type != LAST_NORM)
                 goto exit2;
 
+        error = mnt_want_write(oldnd.path.mnt);
+        if (error)
+                goto exit2;
+
         oldnd.flags &= ~LOOKUP_PARENT;
         newnd.flags &= ~LOOKUP_PARENT;
         newnd.flags |= LOOKUP_RENAME_TARGET;
@@ -3788,23 +3796,19 @@ SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
         if (new_dentry == trap)
                 goto exit5;
 
-        error = mnt_want_write(oldnd.path.mnt);
-        if (error)
-                goto exit5;
         error = security_path_rename(&oldnd.path, old_dentry,
                                      &newnd.path, new_dentry);
         if (error)
-                goto exit6;
+                goto exit5;
         error = vfs_rename(old_dir->d_inode, old_dentry,
                                    new_dir->d_inode, new_dentry);
-exit6:
-        mnt_drop_write(oldnd.path.mnt);
 exit5:
         dput(new_dentry);
 exit4:
         dput(old_dentry);
 exit3:
         unlock_rename(new_dir, old_dir);
+        mnt_drop_write(oldnd.path.mnt);
 exit2:
         path_put(&newnd.path);
         putname(to);