linux-ext4 - Re: [PATCH 1/3] ext4: introduce macro sbi_array_rcu

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Message-ID: <6db70858fc2e9792cff585650891f5f9896cf28a.camel@amazon.com>
Date:   Wed, 19 Feb 2020 03:16:30 +0000
From:   "Jitindar SIngh, Suraj" <surajjs@...zon.com>
To:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
CC:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "tytso@....edu" <tytso@....edu>,
        "Singh, Balbir" <sblbir@...zon.com>
Subject: Re: [PATCH 1/3] ext4: introduce macro sbi_array_rcu_deref() to access
 rcu protected fields

+Cc stable
(correctly this time)

On Tue, 2020-02-18 at 19:08 -0800, Suraj Jitindar Singh wrote:
> The s_group_desc field in the super block info (sbi) is protected by
> rcu to
> prevent access to an invalid pointer during online resize operations.
> There are 2 other arrays in sbi, s_group_info and s_flex_groups,
> which
> require similar rcu protection which is introduced in the subsequent
> patches. Introduce a helper macro sbi_array_rcu_deref() to be used to
> provide rcu protected access to such fields.
> 
> Also update the current s_group_desc access site to use the macro.
> 
> Signed-off-by: Suraj Jitindar Singh <surajjs@...zon.com>
Cc: stable@...r.kernel.org
> Cc: stable@...r-kernel.org
> ---
>  fs/ext4/balloc.c | 11 +++++------
>  fs/ext4/ext4.h   | 17 +++++++++++++++++
>  2 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 5368bf67300b..8fd0b3cdab4c 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -281,14 +281,13 @@ struct ext4_group_desc *
> ext4_get_group_desc(struct super_block *sb,
>  
>  	group_desc = block_group >> EXT4_DESC_PER_BLOCK_BITS(sb);
>  	offset = block_group & (EXT4_DESC_PER_BLOCK(sb) - 1);
> -	rcu_read_lock();
> -	bh_p = rcu_dereference(sbi->s_group_desc)[group_desc];
> +	bh_p = sbi_array_rcu_deref(sbi, s_group_desc, group_desc);
>  	/*
> -	 * We can unlock here since the pointer being dereferenced
> won't be
> -	 * dereferenced again. By looking at the usage in add_new_gdb()
> the
> -	 * value isn't modified, just the pointer, and so it remains
> valid.
> +	 * sbi_array_rcu_deref returns with rcu unlocked, this is ok
> since
> +	 * the pointer being dereferenced won't be dereferenced again.
> By
> +	 * looking at the usage in add_new_gdb() the value isn't
> modified,
> +	 * just the pointer, and so it remains valid.
>  	 */
> -	rcu_read_unlock();
>  	if (!bh_p) {
>  		ext4_error(sb, "Group descriptor not loaded - "
>  			   "block_group = %u, group_desc = %u, desc =
> %u",
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 149ee0ab6d64..236fc6500340 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1576,6 +1576,23 @@ static inline int ext4_valid_inum(struct
> super_block *sb, unsigned long ino)
>  		 ino <= le32_to_cpu(EXT4_SB(sb)->s_es-
> >s_inodes_count));
>  }
>  
> +/*
> + * Returns: sbi->field[index]
> + * Used to access an array element from the following sbi fields
> which require
> + * rcu protection to avoid dereferencing an invalid pointer due to
> reassignment
> + * - s_group_desc
> + * - s_group_info
> + * - s_flex_group
> + */
> +#define sbi_array_rcu_deref(sbi, field, index)			
> 	   \
> +({									
>    \
> +	typeof(*((sbi)->field)) _v;					
>    \
> +	rcu_read_lock();						   \
> +	_v = ((typeof((sbi)->field))rcu_dereference((sbi)-
> >field))[index]; \
> +	rcu_read_unlock();						
>    \
> +	_v;								
>    \
> +})
> +
>  /*
>   * Simulate_fail codes
>   */