Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths

[Linus Torvalds]

On Wed, Jan 17, 2018 at 6:17 AM, Alan Cox <alan@xxxxxxxxxxxxxxx> wrote:
>
> Can we kill off the remaining users of set_fs() ?

I would love to, but it's not going to happen short-term. If ever.

Some could be removed today: the code in arch/x86/net/bpf_jit_comp.c
seems to be literally the ramblings of a diseased mind. There's no
reason for the set_fs(), there's no reason for the
flush_icache_range() (it's a no-op on x86 anyway), and the smp_wmb()
looks bogus too.

I have no idea how that braindamage happened, but I assume it got
copied from some broken source.

But there are about ~100 set_fs() calls in generic code, and some of
those really are pretty fundamental. Doing things like "kernel_read()"
without set_fs() is basically impossible.

We've had set_fs() since the beginning. The naming is obviously very
historical. We have it for a very good reason, and I don't really see
that reason going away.

So realistically, we want to _minimize_ set_fs(), and we might want to
make sure that it's done only in limited settings (it might, for
example, be a good idea and a realistic goal to make sure that drivers
and modules can't do it, and use proper helper functions like that
"read_kernel()").

But getting rid of the concept entirely? Doesn't seem likely.

             Linus