Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

[Andy Lutomirski]

On Wed, Mar 18, 2015 at 11:25 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 03/18, Cyrill Gorcunov wrote:
>>
>> On Wed, Mar 18, 2015 at 11:06:00AM -0700, Andy Lutomirski wrote:
>> > > --- a/arch/x86/crtools.c
>> > > +++ b/arch/x86/crtools.c
>> > > @@ -475,6 +475,7 @@ int restore_gpregs(struct rt_sigframe *f, UserX86RegsEntry *r)
>> > >         CPREG2(rip, ip);
>> > >         CPREG2(eflags, flags);
>> > >         CPREG1(cs);
>> > > +       CPREG1(ss);
>> > >         CPREG1(gs);
>> > >         CPREG1(fs);
>> >
>> > Huh?  Is CRIU actually trying to build an entire sigcontext from
>> > scratch here?  I don't see how this can reliably work across kernel
>> > versions or CPU versions.
>>
>> Yes we are. And why it can't work reliably? As to CPU -- we're
>> testing that cpu features saved in image should match ones
>> provided by the kernel runtime, ie on the machine where we're
>> restoring.
>
> But, say, __USER_CS can be changed in kernel, and nobody should notice this.

This actually happens on a regular basis.  Has anyone tried
checkpointing on Xen and restoring on native?  Xen does interesting
things to cs and possibly to ss as well on x86.

You get somewhat of a free pass on ds and es because the kernel's
context switch code is very tolerant of failures there.

--Andy
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