Re: fs: NULL deref in atime_needs_update

[Al Viro]

On Mon, Feb 29, 2016 at 04:54:54PM +0100, Dmitry Vyukov wrote:

> Regardless of whether reordering is wrong or not, do we see how it can
> fix the WARNINGs/oopses? Because it does seem to. I've tried to revert
> just this part:
> 
> -               *inode = d_backing_inode(dentry);
>                 negative = d_is_negative(dentry);
> +               *inode = d_backing_inode(dentry);
> 
> And got:
> 
> [  976.609688] WARNING: CPU: 0 PID: 12126 at fs/namei.c:1587
> lookup_fast+0x3fa/0x450()
> [  976.626768] WARNING: CPU: 0 PID: 12126 at fs/namei.c:3123
> path_openat+0x12bc/0x1520()
> 
> in 15 minutes.

dentry going from negative to positive			lookup_fast()
							fetch NULL ->d_inode
store non-NULL ->d_inode
store new ->d_flags
							fetch new ->d_flags
							check ->d_seq
bump ->d_seq by 2

Change the order of fetches and you'll get rid of that scenario.

> In particular, applying this on top the previous patch will be
> inconclusive, because I already don't see the warnings.

Apply it with that reordering reversed, please.