Message-ID: <CAG48ez0fsB_XTmNfE-2tuabH7JHyQdih8bu7Qwu9HGWJXti7tQ@mail.gmail.com>
Date:   Wed, 19 Feb 2020 22:01:47 +0100
From:   Jann Horn <jannh@...gle.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/19] afs: Support fsinfo() [ver #16]

On Tue, Feb 18, 2020 at 6:07 PM David Howells <dhowells@...hat.com> wrote:
> Add fsinfo support to the AFS filesystem.
[...]
>  static const struct super_operations afs_super_ops = {
>         .statfs         = afs_statfs,
> +#ifdef CONFIG_FSINFO
> +       .fsinfo_attributes = afs_fsinfo_attributes,
> +#endif
> +       .alloc_inode    = afs_alloc_inode,
> +       .drop_inode     = afs_drop_inode,
> +       .destroy_inode  = afs_destroy_inode,
> +       .free_inode     = afs_free_inode,
> +       .evict_inode    = afs_evict_inode,
> +       .show_devname   = afs_show_devname,
> +       .show_options   = afs_show_options,
> +};
> +
> +static const struct super_operations afs_dyn_super_ops = {
> +       .statfs         = afs_statfs,
> +#ifdef CONFIG_FSINFO
> +       .fsinfo_attributes = afs_dyn_fsinfo_attributes,
> +#endif
>         .alloc_inode    = afs_alloc_inode,
>         .drop_inode     = afs_drop_inode,
>         .destroy_inode  = afs_destroy_inode,
[...]
> @@ -432,9 +454,12 @@ static int afs_fill_super(struct super_block *sb, struct afs_fs_context *ctx)
>         sb->s_blocksize_bits    = PAGE_SHIFT;
>         sb->s_maxbytes          = MAX_LFS_FILESIZE;
>         sb->s_magic             = AFS_FS_MAGIC;
> -       sb->s_op                = &afs_super_ops;
> -       if (!as->dyn_root)
> +       if (!as->dyn_root) {
> +               sb->s_op        = &afs_super_ops;
>                 sb->s_xattr     = afs_xattr_handlers;
> +       } else {
> +               sb->s_op        = &afs_dyn_super_ops;
> +       }

Ewww. So basically, having one static set of .fsinfo_attributes is not
sufficiently flexible for everyone, but instead of allowing the
filesystem to dynamically provide a list of supported attributes, you
just duplicate the super_operations? Seems to me like it'd be cleaner
to add a function pointer to the super_operations that can dynamically
fill out the supported fsinfo attributes.

It seems to me like the current API is going to be a dead end if you
ever want to have decent passthrough of these things for e.g. FUSE, or
overlayfs, or VirtFS?

>         ret = super_setup_bdi(sb);
>         if (ret)
>                 return ret;
> @@ -444,7 +469,7 @@ static int afs_fill_super(struct super_block *sb, struct afs_fs_context *ctx)
>         if (as->dyn_root) {
>                 inode = afs_iget_pseudo_dir(sb, true);
>         } else {
> -               sprintf(sb->s_id, "%llu", as->volume->vid);
> +               sprintf(sb->s_id, "%llx", as->volume->vid);

(This is technically a (small) UAPI change for audit logging of AFS
filesystems, right? You may want to note that in the commit message.)

>                 afs_activate_volume(as->volume);
>                 iget_data.fid.vid       = as->volume->vid;
>                 iget_data.fid.vnode     = 1;
[...]
> +static int afs_fsinfo_get_supports(struct path *path, struct fsinfo_context *ctx)
> +{
> +       struct fsinfo_supports *sup = ctx->buffer;
> +
> +       sup = ctx->buffer;

Duplicate assignment to "sup".

> +       sup->stx_mask = (STATX_TYPE | STATX_MODE |
> +                        STATX_NLINK |
> +                        STATX_UID | STATX_GID |
> +                        STATX_MTIME | STATX_INO |
> +                        STATX_SIZE);
> +       sup->stx_attributes = STATX_ATTR_AUTOMOUNT;
> +       return sizeof(*sup);
> +}
[...]
> +static int afs_fsinfo_get_server_address(struct path *path, struct fsinfo_context *ctx)
> +{
> +       struct fsinfo_afs_server_address *addr = ctx->buffer;
> +       struct afs_server_list *slist;
> +       struct afs_super_info *as = AFS_FS_S(path->dentry->d_sb);
> +       struct afs_addr_list *alist;
> +       struct afs_volume *volume = as->volume;
> +       struct afs_server *server;
> +       struct afs_net *net = afs_d2net(path->dentry);
> +       unsigned int i;
> +       int ret = -ENODATA;
> +
> +       read_lock(&volume->servers_lock);
> +       slist = afs_get_serverlist(volume->servers);
> +       read_unlock(&volume->servers_lock);
> +
> +       if (ctx->Nth >= slist->nr_servers)
> +               goto put_slist;
> +       server = slist->servers[ctx->Nth].server;
> +
> +       read_lock(&server->fs_lock);
> +       alist = afs_get_addrlist(rcu_access_pointer(server->addresses));

Documentation for rcu_access_pointer() says:

 * Return the value of the specified RCU-protected pointer, but omit the
 * lockdep checks for being in an RCU read-side critical section.  This is
 * useful when the value of this pointer is accessed, but the pointer is
 * not dereferenced, for example, when testing an RCU-protected pointer
 * against NULL.  Although rcu_access_pointer() may also be used in cases
 * where update-side locks prevent the value of the pointer from changing,
 * you should instead use rcu_dereference_protected() for this use case.
 *
 * It is also permissible to use rcu_access_pointer() when read-side
 * access to the pointer was removed at least one grace period ago, as
 * is the case in the context of the RCU callback that is freeing up
 * the data, or after a synchronize_rcu() returns.  This can be useful
 * when tearing down multi-linked structures after a grace period
 * has elapsed.

> +       read_unlock(&server->fs_lock);

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