linux-kernel - Re: [PATCH] virtio-fs: Query rootmode during mount

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Message-ID: <CAJfpeguWjwXtM4VJYP2+-0KK5Jkz80eKpWc-ST+yMuKL6Be0=w@mail.gmail.com>
Date: Thu, 7 Nov 2024 09:58:02 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Hanna Czenczek <hreitz@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	virtualization@...ts.linux.dev, Miklos Szeredi <mszeredi@...hat.com>, 
	German Maglione <gmaglione@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] virtio-fs: Query rootmode during mount

On Thu, 24 Oct 2024 at 18:47, Hanna Czenczek <hreitz@...hat.com> wrote:

> To be able to issue INIT (and GETATTR), we need to at least partially
> initialize the super_block structure, which is currently done via
> fuse_fill_super_common().

What exactly is needed to be initialized?

> @@ -1762,18 +1801,12 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
>         sb->s_d_op = &fuse_dentry_operations;
>
>         mutex_lock(&fuse_mutex);
> -       err = -EINVAL;
> -       if (ctx->fudptr && *ctx->fudptr)
> -               goto err_unlock;
> -
>         err = fuse_ctl_add_conn(fc);
>         if (err)
>                 goto err_unlock;
>
>         list_add_tail(&fc->entry, &fuse_conn_list);
>         sb->s_root = root_dentry;
> -       if (ctx->fudptr)
> -               *ctx->fudptr = fud;

This is wrong, because we need the fuse_mutex protection for checking
and setting the private_data on the fuse device file.

If this split is needed (which I'm not sure) then fud allocation
should probably be moved to part2 instead of moving the *ctx->fudptr
setup to part1.


> @@ -1635,8 +1657,16 @@ static void virtio_kill_sb(struct super_block *sb)
>         struct fuse_mount *fm = get_fuse_mount_super(sb);
>         bool last;
>
> -       /* If mount failed, we can still be called without any fc */
> -       if (sb->s_root) {
> +       /*
> +        * Only destroy the connection after full initialization, i.e.
> +        * once s_root is set (see commit d534d31d6a45d).
> +        * One exception: For virtio-fs, we call INIT before s_root is
> +        * set so we can determine the root node's mode.  We must call
> +        * DESTROY after INIT.  So if an error occurs during that time
> +        * window (specifically in fuse_make_root_inode()), we still
> +        * need to call virtio_fs_conn_destroy() here.
> +        */
> +       if (sb->s_root || (fm->fc && fm->fc->initialized && !fm->submount)) {

How could fm->submount be set if sb->s_root isn't?  Or sb->s_root set
and fc->initialized isn't?

Seems it would be sufficient to check fm->fc->initialized, no?

Thanks,
Miklos