Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 2 May 2016 10:26:03 -0700 | Subject | Re: ptrace vs FSGSBASE |
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On Mon, May 2, 2016 at 8:35 AM, Oleg Nesterov <oleg@redhat.com> wrote: > 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.
Right. When I said writing to fs_base, I meant using POKEUSER or similar to write to the thing referred to as fs_base via the helpers in arch/x86/kernel/ptrace.c
> >> > 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.
Exactly. Hence my uncertainly as to what to do.
> Nor I can understand the "thread.fs != value" > check before do_arch_prctl(ARCH_SET_FS). Confused.
I think that code was a optimization that doesn't make much sense.
It wouldn't surprise me if almost no one uses any of this functionality right now.
--Andy
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