Re: [PATCH] vfs: check dentry is still valid in get_link()

[Al Viro]

On Mon, Jan 10, 2022 at 05:11:31PM +0800, Ian Kent wrote:
> When following a trailing symlink in rcu-walk mode it's possible for
> the dentry to become invalid between the last dentry seq lock check
> and getting the link (eg. an unlink) leading to a backtrace similar
> to this:
> 
> crash> bt
> PID: 10964  TASK: ffff951c8aa92f80  CPU: 3   COMMAND: "TaniumCX"
> …
>  #7 [ffffae44d0a6fbe0] page_fault at ffffffff8d6010fe
>     [exception RIP: unknown or invalid address]
>     RIP: 0000000000000000  RSP: ffffae44d0a6fc90  RFLAGS: 00010246
>     RAX: ffffffff8da3cc80  RBX: ffffae44d0a6fd30  RCX: 0000000000000000
>     RDX: ffffae44d0a6fd98  RSI: ffff951aa9af3008  RDI: 0000000000000000
>     RBP: 0000000000000000   R8: ffffae44d0a6fb94   R9: 0000000000000000
>     R10: ffff951c95d8c318  R11: 0000000000080000  R12: ffffae44d0a6fd98
>     R13: ffff951aa9af3008  R14: ffff951c8c9eb840  R15: 0000000000000000
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>  #8 [ffffae44d0a6fc90] trailing_symlink at ffffffff8cf24e61
>  #9 [ffffae44d0a6fcc8] path_lookupat at ffffffff8cf261d1
> #10 [ffffae44d0a6fd28] filename_lookup at ffffffff8cf2a700
> #11 [ffffae44d0a6fe40] vfs_statx at ffffffff8cf1dbc4
> #12 [ffffae44d0a6fe98] __do_sys_newstat at ffffffff8cf1e1f9
> #13 [ffffae44d0a6ff38] do_syscall_64 at ffffffff8cc0420b
> 
> Most of the time this is not a problem because the inode is unchanged
> while the rcu read lock is held.
> 
> But xfs can re-use inodes which can result in the inode ->get_link()
> method becoming invalid (or NULL).

Without an RCU delay?  Then we have much worse problems...

Details, please.