PING: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack register

PING: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack register

[H.J. Lu]

On Fri, Apr 6, 2018 at 1:26 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Apr 6, 2018 at 5:59 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Thu, Apr 5, 2018 at 9:46 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>>> On 03/30/2018 12:41 PM, H.J. Lu wrote:
>>>> On Thu, Mar 29, 2018 at 1:20 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>>>> * H. J. Lu:
>>>>>
>>>>>> On Thu, Mar 29, 2018 at 1:15 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>>>>>> * H. J. Lu:
>>>>>>>
>>>>>>>> You need to make a choice.  You either don't introduce a new symbol
>>>>>>>> version or don't save shadow stack for thread cancellation.  You
>>>>>>>> can't have both.
>>>>>>> I don't understand.  We have room to save the shadow stack pointer in
>>>>>>> the existing struct.
>>>>>> No, we don't have room in struct pthread_unwind_buf:
>>>>>>
>>>>>> Note: There is an unused pointer space in pthread_unwind_buf_data.  But
>>>>>> it isn't suitable for saving and restoring shadow stack register since
>>>>>> x32 is a 64-bit process with 32-bit software pointer and kernel may
>>>>>> place x32 shadow stack above 4GB.  We need to save and restore 64-bit
>>>>>> shadow stack register for x32.
>>>>> We have for void * fields.  They are subsequently overwritten by
>>>>> __pthread_register_cancel.  But __sigsetjmp can write to them first
>>>>> without causing any harm.  We just need a private __longjmp_cancel
>>>>> that doesn't restore the shadow stack pointer.
>>>> Here is the patch which does that.   Any comments?
>>>
>>> OK, so I have reviewed https://github.com/hjl-tools/glibc/ hjl/cet/master,
>>> please confirm that this is the correct branch of the required implementation
>>> for CET. It really helps to review the rest of the patch set, you should be
>>> preparing this as a patch set instead of having it reviewed one-at-a-time.
>>> This issue was already raised in the thread with Zack.
>>
>> Thanks for your feedbacks.
>>
>>> Architecture:
>>>
>>> * We avoid a "flag day" with feature_1 TCB flag to switch to a new ABI,
>>>   which we have discussed is a fragile process which should be avoided if
>>>   a supportable alternative solution exists.
>>>
>>> * We avoid versioned symbols, this makes CET backportable, and this has a
>>>   bigger benefit for long-term stable distributions.
>>>
>>> * A key design problem has been that cancellation jump buffers within glibc
>>>   are truncated to optimize on-stack size, and this means that setjmp could
>>>   write beyond the structure because setjmp now tries to save the shadowstack
>>>   pointer into space that the cancellation jump buffer did not allocate.
>>>   For the record this optimization seems premature and I'm sad we did it, this
>>>   is a lesson we should learn from.
>>>
>>> * We have all agreed to the following concepts:
>>>
>>>   * The cancellation process, in particular the unwinder, never returns from
>>>     any of the functions we call, it just keeps calling into the unwinder to
>>>     jump to the next unwound cancellation function all the way to the thread
>>>     start routine. Therefore because we never return from one of these functions
>>>     we never need to restore the shadow stack, and consequently wherever it is
>>>     stored in the cancellation jump buffer can be overwritten if we need the
>>>     space (it's a dead store).
>>>
>>>     * The corollary to this is that function calls made from cancellation handlers
>>>       will continue to advance the shadowstack from the deepest point at which
>>>       cancellation is initiated from. This means that the depth of the shadowstack
>>>       doesn't match the depth of the real stack while we are unwinding. I don't
>>>       know if this will have consequences on analysis tooling or not, or debug
>>>       tools during unwinding. It's a fairly advanced situation and corner case,
>>>       and restoring the shadowstack is not useful becuase we don't need it and
>>>       simplifies the implementation.
>>>
>>>   * The cancellation jump buffer has private data used for chaining the cancel
>>>     jump buffers together such that the custom unwinder can follow them and
>>>     call them in sequence. This space constitutes 4 void *'s which is space
>>>     that setjmp can write to, because we will just overwrite it when we register
>>>     the cancel handler.
>>>
>>>   * If the new shadowstack-enabled setjmp stores the shadowstack pointer into
>>>     the space taken by the 4 void*'s then we won't overflow the stack, and we
>>>     don't need to change the layout of the cancellation jump buffer. The 4 void*'s
>>>     are sufficient, even for x32 to write a 64-bit shadow stack address.
>>>
>>> * After fixing the cancellation jump buffers the following work needs to be reviewed:
>>>
>>>   * Add feature_1 in tcb head to track CET status and make it easily available
>>>     to runtime for checking.
>>>
>>>   * Save and restore shadowstack in setjmp/longjmp.
>>>
>>>   * Add CET support to ld.so et. al. and track runtime status.
>>>
>>>   * Adjust vfork for shadow stack usage.
>>>
>>>   * Add ENDBR or NOTRACK where required in assembly.
>>>
>>>   * CET and makecontext incompatible.
>>>     - Probably need to discuss which default is appropriate.
>>>     - Should the user get CET automatically disabled in makecontext() et. al. silently?
>>>     - Should your current solution, which is to error out during the build, and require
>>>       flag changes, be the default? This forces the user to review the security for their
>>>       application.
>>
>> I'd like to reserve 4 slots in ucontext for shadow stack:
>>
>> https://github.com/hjl-tools/glibc/commit/9bf6aefa8fb45f8df140d42ce9cf890bb24076e1
>>
>> It should be binary backward compatible.  I will investigate if there is a way
>> to support shadow stack with existing API.  Otherwise, we need to add a new
>> API for ucontext functions with shadow stack.
>>
>>>   * prctl for CET.
>>
>> We have been experimenting different approaches to get the best implementation.
>> I am expecting that this patch may change as we collect more data.
>>
>>> * The work to review after this patch appears to be less contentious in terms of
>>>   the kinds of changes that are required. Most of the changes are internal details
>>>   of enabling CET and not ABI details, with the exception of the possible pain we
>>>   might cause with makecontext() being unsupported and what default position to take
>>>   there.
>>>
>>> Design:
>>>
>>> * Overall the implementation looks exactly how I might expect it to look, but some
>>>   of the math that places the shadowstack pointer appears to need either commenting
>>>   or fixing because I don't understand it. You need to make it easy for me to see
>>>   that we have placed the shadowstack pointer into the 4 pad words.
>>>
>>> Details:
>>>
>>> * One comment needs filling out a bit more, noted below.
>>>
>>>> 0001-x86-Use-pad-in-pthread_unwind_buf-to-preserve-shadow.patch
>>>>
>>>>
>>>> From f222537447f5ec879427f318b7c0396362b7453a Mon Sep 17 00:00:00 2001
>>>> From: "H.J. Lu" <hjl.tools@gmail.com>
>>>> Date: Sat, 24 Feb 2018 17:23:54 -0800
>>>> Subject: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack
>>>>  register
>>>>
>>>> The pad array in struct pthread_unwind_buf is used by setjmp to save
>>>> shadow stack register.  We assert that size of struct pthread_unwind_buf
>>>> is no less than offset of shadow stack pointer + shadow stack pointer
>>>> size.
>>>>
>>>
>>> OK.
>>>
>>>> Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
>>>> these with thread cancellation, call setjmp, but never return after
>>>> __libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
>>>> __libc_longjmp on x86, doesn't need to restore shadow stack register.
>>>
>>> OK.
>>>
>>>> __libc_longjmp, which is a private interface for thread cancellation
>>>> implementation in libpthread, is changed to call __longjmp_cancel,
>>>> instead of __longjmp.  __longjmp_cancel is a new internal function
>>>> in libc, which is similar to __longjmp, but doesn't restore shadow
>>>> stack register.
>>>
>>> OK. Good. I like the use of a __longjmp_cancel name to call out what's
>>> going on in the API (clear semantics).
>>>
>>>>
>>>> The compatibility longjmp and siglongjmp in libpthread.so are changed
>>>> to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
>>>> restore shadow stack register.
>>>
>>> OK.
>>>
>>>>
>>>> Tested with build-many-glibcs.py.
>>>>
>>>>       * nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
>>>>       handlers after setjmp.
>>>>       * setjmp/longjmp.c (__libc_longjmp): Don't define alias if
>>>>       defined.
>>>>       * sysdeps/unix/sysv/linux/x86/setjmpP.h (_JUMP_BUF_SIGSET_NSIG):
>>>>       Changed to 97.
>>>>       * sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
>>>>       * sysdeps/x86/__longjmp_cancel.S: New file.
>>>>       * sysdeps/x86/longjmp.c: Likewise.
>>>>       * sysdeps/x86/nptl/pt-longjmp.c: Likewise.
>>>
>>> This looks much better.
>>>
>>>> ---
>>>>  nptl/pthread_create.c                 |  9 ++--
>>>>  setjmp/longjmp.c                      |  2 +
>>>>  sysdeps/unix/sysv/linux/x86/setjmpP.h |  4 +-
>>>>  sysdeps/x86/Makefile                  |  4 ++
>>>>  sysdeps/x86/__longjmp_cancel.S        | 20 ++++++++
>>>>  sysdeps/x86/longjmp.c                 | 45 ++++++++++++++++
>>>>  sysdeps/x86/nptl/pt-longjmp.c         | 97 +++++++++++++++++++++++++++++++++++
>>>>  7 files changed, 176 insertions(+), 5 deletions(-)
>>>>  create mode 100644 sysdeps/x86/__longjmp_cancel.S
>>>>  create mode 100644 sysdeps/x86/longjmp.c
>>>>  create mode 100644 sysdeps/x86/nptl/pt-longjmp.c
>>>>
>>>> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
>>>> index caaf07c134..1c5b3780c6 100644
>>>> --- a/nptl/pthread_create.c
>>>> +++ b/nptl/pthread_create.c
>>>> @@ -427,12 +427,15 @@ START_THREAD_DEFN
>>>>       compilers without that support we do use setjmp.  */
>>>>    struct pthread_unwind_buf unwind_buf;
>>>>
>>>> -  /* No previous handlers.  */
>>>> +  int not_first_call;
>>>> +  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
>>>> +
>>>> +  /* No previous handlers.  NB: This must be done after setjmp since
>>>> +     the same space may be used by setjmp to store extra data which
>>>> +     should never be used by __libc_unwind_longjmp.  */
>>>
>>> Suggest:
>>> ~~~
>>> No previous handlers.  NB: This must be done after setjmp since
>>> the private space in the unwind jump buffer may overlap space
>>> used by setjmp to store extra architecture-specific information
>>> which is never be used by the cancellation-specific
>>> __libc_unwind_longjmp.
>>>
>>> The private space is allowed to overlap because the unwinder never
>>> has to return through any of the jumped-to call frames, and thus
>>> only a minimum amount of saved data need be stored, and for example,
>>> need not include the process signal mask information. This is all
>>> an optimization to reduce stack usage when pushing cancellation
>>> handlers.
>>> ~~~
>>
>> Will fix it.
>>
>>>>    unwind_buf.priv.data.prev = NULL;
>>>>    unwind_buf.priv.data.cleanup = NULL;
>>>>
>>>> -  int not_first_call;
>>>> -  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
>>>
>>> OK.
>>>
>>>>    if (__glibc_likely (! not_first_call))
>>>>      {
>>>>        /* Store the new cleanup handler info.  */
>>>> diff --git a/setjmp/longjmp.c b/setjmp/longjmp.c
>>>> index a2a7065a85..453889e103 100644
>>>> --- a/setjmp/longjmp.c
>>>> +++ b/setjmp/longjmp.c
>>>> @@ -40,9 +40,11 @@ __libc_siglongjmp (sigjmp_buf env, int val)
>>>>  }
>>>>
>>>>  #ifndef __libc_siglongjmp
>>>> +# ifndef __libc_longjmp
>>>>  /* __libc_longjmp is a private interface for cancellation implementation
>>>>     in libpthread.  */
>>>>  strong_alias (__libc_siglongjmp, __libc_longjmp)
>>>> +# endif
>>>
>>> OK.
>>>
>>>>  weak_alias (__libc_siglongjmp, _longjmp)
>>>>  weak_alias (__libc_siglongjmp, longjmp)
>>>>  weak_alias (__libc_siglongjmp, siglongjmp)
>>>> diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
>>>> index c0ed767a0d..90a6bbcf32 100644
>>>> --- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
>>>> +++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
>>>> @@ -22,8 +22,8 @@
>>>>  #include <bits/types/__sigset_t.h>
>>>>
>>>>  /* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
>>>> -   Define it to 513 to leave some rooms for future use.  */
>>>> -#define _JUMP_BUF_SIGSET_NSIG        513
>>>> +   Define it to 97 to leave some rooms for future use.  */
>>>
>>> OK.
>>>
>>>> +#define _JUMP_BUF_SIGSET_NSIG        97
>>>
>>> Please provide proof in the way of a comment or rewriting this constant
>>> to show that it places the shadow stack pointer on both x86_64 and x32
>>> into the range of the private pad.
>>
>> sysdeps/x86/nptl/pt-longjmp.c has
>>
>> _Static_assert ((sizeof (struct pthread_unwind_buf)
>>>= (SHADOW_STACK_POINTER_OFFSET
>>      + SHADOW_STACK_POINTER_SIZE)),
>> "size of struct pthread_unwind_buf < "
>> "(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");
>>
>> If shadow stack pointer is saved in the offset bigger than the size of
>> struct pthread_unwind_buf, assert will trigger at compile-time.
>>
>>> Also, from commit  f33632ccd1dec3217583fcfdd965afb62954203c,
>>> where did this math come from?
>>>
>>> ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>>>
>>> Why the +7?
>>
>> _JUMP_BUF_SIGSET_NSIG is the biggest signal number + 1.
>> _JUMP_BUF_SIGSET_NSIG - 1 gives the biggest signal number.
>> _JUMP_BUF_SIGSET_NSIG - 1 + 7 rounds up to the number of bytes
>> which are needed to store the biggest signal number.
>>
>>>>  /* Number of longs to hold all signals.  */
>>>>  #define _JUMP_BUF_SIGSET_NWORDS \
>>>>    ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>>>> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
>>>> index 0d0326c21a..d25d6f0ae4 100644
>>>> --- a/sysdeps/x86/Makefile
>>>> +++ b/sysdeps/x86/Makefile
>>>> @@ -8,3 +8,7 @@ sysdep-dl-routines += dl-get-cpu-features
>>>>  tests += tst-get-cpu-features
>>>>  tests-static += tst-get-cpu-features-static
>>>>  endif
>>>> +
>>>> +ifeq ($(subdir),setjmp)
>>>> +sysdep_routines += __longjmp_cancel
>>>> +endif
>>>
>>> OK.
>>>
>>>> diff --git a/sysdeps/x86/__longjmp_cancel.S b/sysdeps/x86/__longjmp_cancel.S
>>>> new file mode 100644
>>>> index 0000000000..b57dbfa376
>>>> --- /dev/null
>>>> +++ b/sysdeps/x86/__longjmp_cancel.S
>>>> @@ -0,0 +1,20 @@
>>>> +/* __longjmp_cancel for x86.
>>>> +   Copyright (C) 2018 Free Software Foundation, Inc.
>>>> +   This file is part of the GNU C Library.
>>>> +
>>>> +   The GNU C Library is free software; you can redistribute it and/or
>>>> +   modify it under the terms of the GNU Lesser General Public
>>>> +   License as published by the Free Software Foundation; either
>>>> +   version 2.1 of the License, or (at your option) any later version.
>>>> +
>>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> +   Lesser General Public License for more details.
>>>> +
>>>> +   You should have received a copy of the GNU Lesser General Public
>>>> +   License along with the GNU C Library; if not, see
>>>> +   <http://www.gnu.org/licenses/>.  */
>>>> +
>>>> +#define __longjmp __longjmp_cancel
>>>> +#include <__longjmp.S>
>>>
>>> OK.
>>>
>>>> diff --git a/sysdeps/x86/longjmp.c b/sysdeps/x86/longjmp.c
>>>> new file mode 100644
>>>> index 0000000000..a53f31e1dd
>>>> --- /dev/null
>>>> +++ b/sysdeps/x86/longjmp.c
>>>> @@ -0,0 +1,45 @@
>>>> +/* __libc_siglongjmp for x86.
>>>> +   Copyright (C) 2018 Free Software Foundation, Inc.
>>>> +   This file is part of the GNU C Library.
>>>> +
>>>> +   The GNU C Library is free software; you can redistribute it and/or
>>>> +   modify it under the terms of the GNU Lesser General Public
>>>> +   License as published by the Free Software Foundation; either
>>>> +   version 2.1 of the License, or (at your option) any later version.
>>>> +
>>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> +   Lesser General Public License for more details.
>>>> +
>>>> +   You should have received a copy of the GNU Lesser General Public
>>>> +   License along with the GNU C Library; if not, see
>>>> +   <http://www.gnu.org/licenses/>.  */
>>>> +
>>>> +#define __libc_longjmp __redirect___libc_longjmp
>>>> +#include <setjmp/longjmp.c>
>>>> +#undef __libc_longjmp
>>>> +
>>>> +extern void __longjmp_cancel (__jmp_buf __env, int __val)
>>>> +     __attribute__ ((__noreturn__)) attribute_hidden;
>>>> +
>>>> +/* Since __libc_longjmp is a private interface for cancellation
>>>> +   implementation in libpthread, there is no need to restore shadow
>>>> +   stack register.  */
>>>> +
>>>> +void
>>>> +__libc_longjmp (sigjmp_buf env, int val)
>>>> +{
>>>> +  /* Perform any cleanups needed by the frames being unwound.  */
>>>> +  _longjmp_unwind (env, val);
>>>
>>> OK.
>>>
>>>> +
>>>> +  if (env[0].__mask_was_saved)
>>>> +    /* Restore the saved signal mask.  */
>>>> +    (void) __sigprocmask (SIG_SETMASK,
>>>> +                       (sigset_t *) &env[0].__saved_mask,
>>>> +                       (sigset_t *) NULL);
>>>
>>> OK.
>>>
>>>> +
>>>> +  /* Call the machine-dependent function to restore machine state
>>>> +     without shadow stack.  */
>>>> +  __longjmp_cancel (env[0].__jmpbuf, val ?: 1);
>>>
>>> OK.
>>>
>>>> +}
>>>> diff --git a/sysdeps/x86/nptl/pt-longjmp.c b/sysdeps/x86/nptl/pt-longjmp.c
>>>> new file mode 100644
>>>> index 0000000000..7eb8651cfe
>>>> --- /dev/null
>>>> +++ b/sysdeps/x86/nptl/pt-longjmp.c
>>>> @@ -0,0 +1,97 @@
>>>> +/* ABI compatibility for 'longjmp' and 'siglongjmp' symbols in libpthread ABI.
>>>> +   X86 version.
>>>> +   Copyright (C) 18 Free Software Foundation, Inc.
>>>> +   This file is part of the GNU C Library.
>>>> +
>>>> +   The GNU C Library is free software; you can redistribute it and/or
>>>> +   modify it under the terms of the GNU Lesser General Public
>>>> +   License as published by the Free Software Foundation; either
>>>> +   version 2.1 of the License, or (at your option) any later version.
>>>> +
>>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> +   Lesser General Public License for more details.
>>>> +
>>>> +   You should have received a copy of the GNU Lesser General Public
>>>> +   License along with the GNU C Library; if not, see
>>>> +   <http://www.gnu.org/licenses/>.  */
>>>> +
>>>> +/* <nptl/descr.h> has
>>>> +
>>>> +struct pthread_unwind_buf
>>>> +{
>>>> +  struct
>>>> +  {
>>>> +    __jmp_buf jmp_buf;
>>>> +    int mask_was_saved;
>>>> +  } cancel_jmp_buf[1];
>>>> +
>>>> +  union
>>>> +  {
>>>> +    void *pad[4];
>>>> +    struct
>>>> +    {
>>>> +      struct pthread_unwind_buf *prev;
>>>> +      struct _pthread_cleanup_buffer *cleanup;
>>>> +      int canceltype;
>>>> +    } data;
>>>> +  } priv;
>>>> +};
>>>> +
>>>> +  The pad array in struct pthread_unwind_buf is used by setjmp to save
>>>
>>> This appears to be SHADOW_STACK_POINTER_OFFSET in subsequent patches.
>>>
>>> However on your hjl/cet/master branch it appears that this offset is not
>>> defined to be *just after* the mask_was_saved?
>>
>> sysdeps/unix/sysv/linux/x86/setjmpP.h has
>>
>> typedef struct
>>   {
>>     unsigned long int __val[_JUMP_BUF_SIGSET_NWORDS];
>>   } __jmp_buf_sigset_t;
>>
>> typedef union
>>   {
>>     __sigset_t __saved_mask_compat;
>>     struct
>>       {
>>          __jmp_buf_sigset_t __saved_mask;
>>           /* Used for shadow stack pointer.  */
>>          unsigned long int __shadow_stack_pointer;
>>       } __saved;
>>   } __jmpbuf_arch_t;
>>
>> __shadow_stack_pointer is placed after __saved_mask, aka mask_was_saved.
>>
>>>> +  shadow stack register.  Assert that size of struct pthread_unwind_buf
>>>> +  is no less than offset of shadow stack pointer plus shadow stack
>>>> +  pointer size.
>>>> +
>>>> +  NB: setjmp is called in libpthread to save shadow stack register.  But
>>>> +  __libc_unwind_longjmp doesn't restore shadow stack register since they
>>>> +  never return after longjmp.  */
>>>> +
>>>> +#include <pthreadP.h>
>>>> +#include <jmp_buf-ssp.h>
>>>> +
>>>> +#ifdef __x86_64__
>>>> +# define SHADOW_STACK_POINTER_SIZE 8
>>>> +#else
>>>> +# define SHADOW_STACK_POINTER_SIZE 4
>>>> +#endif
>>>> +
>>>> +_Static_assert ((sizeof (struct pthread_unwind_buf)
>>>> +              >= (SHADOW_STACK_POINTER_OFFSET
>>>> +                  + SHADOW_STACK_POINTER_SIZE)),
>>>> +             "size of struct pthread_unwind_buf < "
>>>> +             "(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");
>>>
>>> OK.
>>>
>>>> +
>>>> +#include <shlib-compat.h>
>>>> +
>>>> +/* libpthread once had its own longjmp (and siglongjmp alias), though there
>>>> +   was no apparent reason for it.  There is no use in having a separate
>>>> +   symbol in libpthread, but the historical ABI requires it.  For static
>>>> +   linking, there is no need to provide anything here--the libc version
>>>> +   will be linked in.  For shared library ABI compatibility, there must be
>>>> +   longjmp and siglongjmp symbols in libpthread.so.
>>>> +
>>>> +   With an IFUNC resolver, it would be possible to avoid the indirection,
>>>> +   but the IFUNC resolver might run before the __libc_longjmp symbol has
>>>> +   been relocated, in which case the IFUNC resolver would not be able to
>>>> +   provide the correct address.  */
>>>> +
>>>> +#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_22)
>>>> +
>>>> +static void __attribute__ ((noreturn, used))
>>>> +longjmp_compat (jmp_buf env, int val)
>>>> +{
>>>> +  /* NB: We call __libc_siglongjmp,  instead of __libc_longjmp, since
>>>> +     __libc_longjmp is a private interface for cancellation which
>>>> +     doesn't restore shadow stack register.  */
>>>> +  __libc_siglongjmp (env, val);
>>>
>>> OK.
>>>
>>>> +}
>>>> +
>>>> +strong_alias (longjmp_compat, longjmp_alias)
>>>> +compat_symbol (libpthread, longjmp_alias, longjmp, GLIBC_2_0);
>>>> +
>>>> +strong_alias (longjmp_alias, siglongjmp_alias)
>>>> +compat_symbol (libpthread, siglongjmp_alias, siglongjmp, GLIBC_2_0);
>>>> +
>>>> +#endif
>>>> -- 2.14.3
>>>
>>>
>
> Here is the updated patch.  OK for master?
>
> I will submit a CET patch set after this patch is merged.
>

PING:

https://sourceware.org/ml/libc-alpha/2018-04/msg00156.html


-- 
H.J.