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Subject: Re: [PATCH 2/2] fs: btrfs: Don't leak memory when failing add fsid
To:     "Tobin C. Harding" <tobin@kernel.org>, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20190513033912.3436-1-tobin@kernel.org>
 <20190513033912.3436-3-tobin@kernel.org>
From:   Nikolay Borisov <nborisov@suse.com>
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Message-ID: <3473fcfa-88cf-b372-3beb-69d59320d50a@suse.com>
Date:   Mon, 13 May 2019 09:04:49 +0300
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On 13.05.19 г. 6:39 ч., Tobin C. Harding wrote:
> A failed call to kobject_init_and_add() must be followed by a call to
> kobject_put().  Currently in the error path when adding fs_devices we
> are missing this call.  This could be fixed by calling
> btrfs_sysfs_remove_fsid() if btrfs_sysfs_add_fsid() returns an error or
> by adding a call to kobject_put() directly in btrfs_sysfs_add_fsid().
> Here we choose the second option because it prevents the slightly
> unusual error path handling requirements of kobject from leaking out
> into btrfs functions.
> 
> Add a call to kobject_put() in the error path of kobject_add_and_init().
> This causes the release method to be called if kobject_init_and_add()
> fails.  open_tree() is the function that calls btrfs_sysfs_add_fsid()
> and the error code in this function is already written with the
> assumption that the release method is called during the error path of
> open_tree() (as seen by the call to btrfs_sysfs_remove_fsid() under the
> fail_fsdev_sysfs label).

I'm not familiar with the internals of kobject but
btrfs_sysfs_remove_fsid calls __btrfs_sysfs_remove_fsid which in turn
does kobject_del followed by kobject_put so its sequence is not exactly
identical with your change. Presumably kobject_del is only required if
you want to dispose of successfully registered sysfs node. This implies
that __btrfs_sysfs_remove_fsid is actually broken when it comes to
handling failed sysfs_add_fsid?

> 
> Signed-off-by: Tobin C. Harding <tobin@kernel.org>
> ---
>  fs/btrfs/sysfs.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
> index 5a5930e3d32b..2f078b77fe14 100644
> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -825,7 +825,12 @@ int btrfs_sysfs_add_fsid(struct btrfs_fs_devices *fs_devs,
>  	fs_devs->fsid_kobj.kset = btrfs_kset;
>  	error = kobject_init_and_add(&fs_devs->fsid_kobj,
>  				&btrfs_ktype, parent, "%pU", fs_devs->fsid);
> -	return error;
> +	if (error) {
> +		kobject_put(&fs_devs->fsid_kobj);
> +		return error;
> +	}
> +
> +	return 0;
>  }
>  
>  int btrfs_sysfs_add_mounted(struct btrfs_fs_info *fs_info)
>