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@amazon.com

Cc: stable@vger.kernel.org

Cc: stable@vger-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

*/