Re: [PATCH] fs: prevent data-race due to missing inode_lock when calling vfs_getattr

[Jeongjun Park]

Hello,

> Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 
> On Mon, Nov 18, 2024 at 01:37:19AM +0900, Jeongjun Park wrote:
>> Many filesystems lock inodes before calling vfs_getattr, so there is no
>> data-race for inodes. However, some functions in fs/stat.c that call
>> vfs_getattr do not lock inodes, so the data-race occurs.
>> 
>> Therefore, we need to apply a patch to remove the long-standing data-race
>> for inodes in some functions that do not lock inodes.
> 
> Why do we care?  Slapping even a shared lock on a _very_ hot path, with
> possible considerable latency, would need more than "theoretically it's
> a data race".

All the functions that added lock in this patch are called only via syscall,
so in most cases there will be no noticeable performance issue. And
this data-race is not a problem that only occurs in theory. It is
a bug that syzbot has been reporting for years. Many file systems that
exist in the kernel lock inode_lock before calling vfs_getattr, so
data-race does not occur, but only fs/stat.c has had a data-race
for years. This alone shows that adding inode_lock to some
functions is a good way to solve the problem without much 
performance degradation.

Regards,

Jeongjun Park