Re: [PATCH 87/87] fs: move i_blocks up a few places in struct inode

[Christian Brauner <brauner@kernel.org>]

On Thu, Sep 28, 2023 at 10:41:34AM -0700, Linus Torvalds wrote:
> On Thu, 28 Sept 2023 at 04:06, Jeff Layton <jlayton@kernel.org> wrote:
> >
> > Move i_blocks up above the i_lock, which moves the new 4 byte hole to
> > just after the timestamps, without changing the size of the structure.
> 
> I'm sure others have mentioned this, but 'struct inode' is marked with
> __randomize_layout, so the actual layout may end up being very
> different.
> 
> I'm personally not convinced the whole structure randomization is
> worth it - it's easy enough to figure out for any distro kernel since
> the seed has to be the same across machines for modules to work, so
> even if the seed isn't "public", any layout is bound to be fairly
> easily discoverable.
> 
> So the whole randomization only really works for private kernel
> builds, and it adds this kind of pain where "optimizing" the structure
> layout is kind of pointless depending on various options.
> 
> I certainly *hope* no distro enables that pointless thing, but it's a worry.

They don't last we checked. Just last cycle we moved stuff in struct
file around to optimize things and we explicitly said we don't give a
damn about struct randomization. Anyone who enables this will bleed
performance pretty badly, I would reckon.