Re: ptrace vs FSGSBASE

From: Oleg Nesterov
Date: Mon May 02 2016 - 12:37:19 EST

On 05/02, Andy Lutomirski wrote:
> On Mon, May 2, 2016 at 7:27 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >>
> >> 1. I read fs_base using ptrace. I think I should get the actual
> >> fs_base without any nonsense.
> >
> > Which fs_base? The member of user_regs_struct? But this structure/layout
> > is just the ABI, so to me it seems correct that getreg() tries to look
> > at ->fs and/or ->fsindex.
> Yeah, the member of user_regs_struct.

Still can't understand this... user_regs_struct is just the set of offsets
we use to "name" the registers for getreg/putreg. We simply do not have
"the actual fs_base" we could use in getreg(), we need to calculate it.

> > I can't understand what does "atomically" mean in this context.
> I mean "change fs and fs_base to these two values in a single syscall
> so that the kernel can do something intelligent."
> Let me give some background:
> [... snip ...]

Thanks Andy. I need to re-read your explanation, but it seems I am starting
to understand. And yes, I didn't bother to look at putreg() when I wrote
my reply.

> If you write, say, 0x2b to
> fs and 12345 to fs_base using the ptrace API, you'd end up with FS ==
> 0x2b and FSBASE == 0,

Hmm. I can be easily wrong again but afaics in this case do_arch_prctl()
will change fs/fs_base first and set

fsindex = FS_TLS_SEL
fs = 0

and then... and then I simply can't understand what set_segment_reg(fs)
will/should do in this case. Nor I can understand the "thread.fs != value"
check before do_arch_prctl(ARCH_SET_FS). Confused.