Messages in this thread | | | Date | Mon, 2 May 2016 17:35:08 +0200 | From | Oleg Nesterov <> | Subject | Re: ptrace vs FSGSBASE |
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On 05/02, Andy Lutomirski wrote: > > On Mon, May 2, 2016 at 7:27 AM, Oleg Nesterov <oleg@redhat.com> 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.
Oleg.
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