[CRIU] [PATCH 2/8] x86/kerndat: Add a check for ptrace() bug on Skylake

[Dmitry Safonov]

2018-02-06 1:57 GMT+00:00 Andrei Vagin <avagin at virtuozzo.com>:
> On Tue, Feb 06, 2018 at 01:54:21AM +0000, Dmitry Safonov wrote:
>> 2018-02-06 1:40 GMT+00:00 Andrei Vagin <avagin at virtuozzo.com>:
>> > On Wed, Jan 31, 2018 at 04:58:09PM +0000, Dmitry Safonov wrote:
>> >> We need to know if ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, iov)
>> >> returns xsave without FP state part.
>> >>
>> >> Signed-off-by: Dmitry Safonov <dima at arista.com>
>> >> ---
>> >>  criu/arch/x86/crtools.c              | 71 ++++++++++++++++++++++++++++++++++++
>> >>  criu/arch/x86/include/asm/restorer.h |  2 +
>> >>  criu/include/kerndat.h               |  1 +
>> >>  criu/kerndat.c                       | 18 +++++++++
>> >>  4 files changed, 92 insertions(+)
>> >>
>> >> diff --git a/criu/arch/x86/crtools.c b/criu/arch/x86/crtools.c
>> >> index d7b21e7c9bae..a389e563a886 100644
>> >> --- a/criu/arch/x86/crtools.c
>> >> +++ b/criu/arch/x86/crtools.c
>> >> @@ -7,6 +7,7 @@
>> >>  #include <sys/syscall.h>
>> >>  #include <sys/auxv.h>
>> >>  #include <sys/wait.h>
>> >> +#include <sys/ptrace.h>
>> >>
>> >>  #include "types.h"
>> >>  #include "log.h"
>> >> @@ -30,6 +31,7 @@
>> >>  #include "images/core.pb-c.h"
>> >>  #include "images/creds.pb-c.h"
>> >>
>> >> +/* XXX: Move all kerndat features to per-arch kerndat .c */
>> >>  int kdat_can_map_vdso(void)
>> >>  {
>> >>       pid_t child;
>> >> @@ -188,6 +190,75 @@ int kdat_compatible_cr(void)
>> >>  }
>> >>  #endif
>> >>
>> >> +/*
>> >> + * Pre v4.14 kernels have a bug on Skylake CPUs:
>> >> + * copyout_from_xsaves() creates fpu state for
>> >> + *   ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov)
>> >> + * without MXCSR and MXCSR_FLAGS if there is SSE/YMM state, but no FP state.
>> >> + * That is xfeatures had either/both XFEATURE_MASK_{SSE,YMM} set, but not
>> >> + * XFEATURE_MASK_FP.
>> >> + * But we *really* need to C/R MXCSR & MXCSR_FLAGS if SSE/YMM active,
>> >> + * as mxcsr store part of the state.
>> >> + */
>> >> +int kdat_x86_has_ptrace_fpu_xsave_bug(void)
>> >> +{
>> >> +     user_fpregs_struct_t xsave = { };
>> >> +     struct iovec iov;
>> >> +     int ret = -1;
>> >> +     pid_t child;
>> >> +     int stat;
>> >> +
>> >> +     /* OSXSAVE can't be changed during boot. */
>> >> +     if (!compel_cpu_has_feature(X86_FEATURE_OSXSAVE))
>> >> +             return 0;
>> >> +
>> >> +     child = fork();
>> >> +     if (child < 0) {
>> >> +             pr_perror("%s(): failed to fork()", __func__);
>> >> +             return -1;
>> >> +     }
>> >> +
>> >> +     if (child == 0) {
>> >> +             ptrace(PTRACE_TRACEME, 0, 0, 0);
>> >
>> > It can fail in a case when criu is running under strace. In this case I
>> > would like to get a warning and conside that a kernel doesn't have this
>> > bug.
>>
>> Makes sense, will do.
>>
>> >
>> >> +             kill(getpid(), SIGSTOP);
>> >> +             pr_err("Continue after SIGSTOP.. Urr what?\n");
>> >> +             exit(1);
>> >> +     }
>> >> +
>> >> +     if (waitpid(child, &stat, WUNTRACED) != child) {
>> >> +             /*
>> >> +              * waitpid() may end with ECHILD if SIGCHLD == SIG_IGN,
>> >> +              * and the child has stopped already.
>> >> +              */
>> >> +             if (!(errno == ECHILD && WIFSTOPPED(stat))) {
>> >
>> > I don't understand this expression ^^^^
>>
>> If I'm not mistaken, if criu process has SIG_IGN set for SIGCHLD
>> waitpid will return ECHILD, but stat will be still filled.
>
> No, it is impossible. waitpid will return ECHILD only if someone else
> waited it.

I'm maybe misreading this, eh?

:ERRORS
:       ECHILD (for wait()) The calling process does not have any  unwaited-for
:              children.
:
:       ECHILD (for  waitpid() or waitid()) The process specified by pid (wait‐
:              pid()) or idtype and id (waitid()) does not exist or  is  not  a
:              child  of  the  calling process.  (This can happen for one's own
:              child if the action for SIGCHLD is set to SIG_IGN.  See also the
:              Linux Notes section about threads.)

>
>> (criu doesn't set SIG_IGN at the moment, but I didn't want to rely
>> on that. Having in mind how many modes criu now has..
>> and for each we need kerndat to be initialized)
>> Anyway, I can delete this sub-check if you think it'll be better.
>>



-- 
             Dmitry