Re: [PATCH 1/7] Add __builtin_speculation_safe_value

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 26/07/18 13:41, Richard Biener wrote:
> On Thu, Jul 26, 2018 at 12:03 PM Richard Earnshaw (lists)
> <Richard.Earnshaw@arm.com> wrote:
>>
>> On 25/07/18 14:47, Richard Biener wrote:
>>> On Wed, Jul 25, 2018 at 2:41 PM Richard Earnshaw (lists)
>>> <Richard.Earnshaw@arm.com> wrote:
>>>>
>>>> On 25/07/18 11:36, Richard Biener wrote:
>>>>> On Wed, Jul 25, 2018 at 11:49 AM Richard Earnshaw (lists)
>>>>> <Richard.Earnshaw@arm.com> wrote:
>>>>>>
>>>>>> On 24/07/18 18:26, Richard Biener wrote:
>>>>>>> On Mon, Jul 9, 2018 at 6:40 PM Richard Earnshaw
>>>>>>> <Richard.Earnshaw@arm.com> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> This patch defines a new intrinsic function
>>>>>>>> __builtin_speculation_safe_value.  A generic default implementation is
>>>>>>>> defined which will attempt to use the backend pattern
>>>>>>>> "speculation_safe_barrier".  If this pattern is not defined, or if it
>>>>>>>> is not available, then the compiler will emit a warning, but
>>>>>>>> compilation will continue.
>>>>>>>>
>>>>>>>> Note that the test spec-barrier-1.c will currently fail on all
>>>>>>>> targets.  This is deliberate, the failure will go away when
>>>>>>>> appropriate action is taken for each target backend.
>>>>>>>
>>>>>>> So given this series is supposed to be backported I question
>>>>>>>
>>>>>>> +rtx
>>>>>>> +default_speculation_safe_value (machine_mode mode ATTRIBUTE_UNUSED,
>>>>>>> +                               rtx result, rtx val,
>>>>>>> +                               rtx failval ATTRIBUTE_UNUSED)
>>>>>>> +{
>>>>>>> +  emit_move_insn (result, val);
>>>>>>> +#ifdef HAVE_speculation_barrier
>>>>>>> +  /* Assume the target knows what it is doing: if it defines a
>>>>>>> +     speculation barrier, but it is not enabled, then assume that one
>>>>>>> +     isn't needed.  */
>>>>>>> +  if (HAVE_speculation_barrier)
>>>>>>> +    emit_insn (gen_speculation_barrier ());
>>>>>>> +
>>>>>>> +#else
>>>>>>> +  warning_at (input_location, 0,
>>>>>>> +             "this target does not define a speculation barrier; "
>>>>>>> +             "your program will still execute correctly, but speculation "
>>>>>>> +             "will not be inhibited");
>>>>>>> +#endif
>>>>>>> +  return result;
>>>>>>>
>>>>>>> which makes all but aarch64 archs warn on __bultin_speculation_safe_value
>>>>>>> uses, even those that do not suffer from Spectre like all those embedded targets
>>>>>>> where implementations usually do not speculate at all.
>>>>>>>
>>>>>>> In fact for those targets the builtin stays in the way of optimization on GIMPLE
>>>>>>> as well so we should fold it away early if neither the target hook is
>>>>>>> implemented
>>>>>>> nor there is a speculation_barrier insn.
>>>>>>>
>>>>>>> So, please make resolve_overloaded_builtin return a no-op on such targets
>>>>>>> which means you can remove the above warning.  Maybe such targets
>>>>>>> shouldn't advertise / initialize the builtins at all?
>>>>>>
>>>>>> I disagree with your approach here.  Why would users not want to know
>>>>>> when the compiler is failing to implement a security feature when it
>>>>>> should?  As for targets that don't need something, they can easily
>>>>>> define the hook as described to suppress the warning.
>>>>>>
>>>>>> Or are you just suggesting moving the warning to resolve overloaded builtin.
>>>>>
>>>>> Well.  You could argue I say we shouldn't even support
>>>>> __builtin_sepeculation_safe_value
>>>>> for archs that do not need it or have it not implemented.  That way users can
>>>>> decide:
>>>>>
>>>>> #if __HAVE_SPECULATION_SAFE_VALUE
>>>>>  ....
>>>>> #else
>>>>> #warning oops // or nothing
>>>>> #endif
>>>>>
>>>>
>>>> So how about removing the predefine of __HAVE_S_S_V when the builtin is
>>>> a nop, but then leaving the warning in if people try to use it anyway?
>>>
>>> Little bit inconsistent but I guess I could live with that.  It still leaves
>>> the question open for how to declare you do not need speculation
>>> barriers at all then.
>>>
>>>>>> Other ports will need to take action, but in general, it can be as
>>>>>> simple as, eg patch 2 or 3 do for the Arm and AArch64 backends - or
>>>>>> simpler still if nothing is needed for that architecture.
>>>>>
>>>>> Then that should be the default.  You might argue we'll only see
>>>>> __builtin_speculation_safe_value uses for things like Firefox which
>>>>> is unlikely built for AVR (just to make an example).  But people
>>>>> are going to test build just on x86 and if they build with -Werror
>>>>> this will break builds on all targets that didn't even get the chance
>>>>> to implement this feature.
>>>>>
>>>>>> There is a test which is intended to fail to targets that have not yet
>>>>>> been patched - I thought that was better than hard-failing the build,
>>>>>> especially given that we want to back-port.
>>>>>>
>>>>>> Port maintainers DO need to decide what to do about speculation, even if
>>>>>> it is explicitly that no mitigation is needed.
>>>>>
>>>>> Agreed.  But I didn't yet see a request for maintainers to decide that?
>>>>>
>>>>
>>>> consider it made, then :-)
>>>
>>> I suspect that drew their attention ;)
>>>
>>> So a different idea would be to produce patches implementing the hook for
>>> each target "empty", CC the target maintainers and hope they quickly
>>> ack if the target doesn't have a speculation problem.  Others then would
>>> get no patch (from you) and thus raise a warning?
>>>
>>> Maybe at least do that for all primary and secondary targets given we do
>>> not want to regress diagnostic-wise (not get new _false_-positives) on
>>> the branch.
>>>
>>>>>>>
>>>>>>> The builtins also have no attributes which mean they are assumed to be
>>>>>>> 1) calling back into the CU via exported functions, 2) possibly throwing
>>>>>>> exceptions, 3) affecting memory state.  I think you at least want
>>>>>>> to use ATTR_NOTHROW_LEAF_LIST.
>>>>>>>
>>>>>>> The builtins are not designed to be optimization or memory barriers as
>>>>>>> far as I can see and should thus be CONST as well.
>>>>>>>
>>>>>>
>>>>>> I think they should be barriers.  They do need to ensure that they can't
>>>>>> be moved past other operations that might depend on the speculation
>>>>>> state.  Consider, for example,
>>>>>
>>>>> That makes eliding them for targets that do not need mitigation even
>>>>> more important.
>>>>>
>>>>>>  ...
>>>>>>  t = untrusted_value;
>>>>>>  ...
>>>>>>  if (t + 5 < limit)
>>>>>>  {
>>>>>>    v = mem[__builtin_speculation_safe_value (untrusted_value)];
>>>>>>    ...
>>>>>>
>>>>>> The compiler must never lift the builtin outside the bounds check as
>>>>>> that is part of the speculation state.
>>>>>
>>>>> OK, so you are relying on the fact that with the current setup GCC has
>>>>> to assume the builtin has side-effects (GCC may not move it to a place that
>>>>> the original location is not post-dominated on).  It doesn't explain
>>>>> why you cannot set ECF_LEAF or why the builtin needs to be
>>>>> considered affecting the memory state.  That is, ECF_NOVOPS
>>>>> or ECF_LOOPING_CONST_OR_PURE (I don't think you can
>>>>> set that manually) would work here, both keep the builtin as
>>>>> having side-effects.
>>>>>
>>>>
>>>> I wish some of this builtin gloop were better documented; at present you
>>>> have to reverse engineer significant amounts of code just to decide
>>>> whether or not you even have to think about whether or not it's relevant...
>>>>
>>>>
>>>>> Btw, if you have an inline function with a pattern like above and
>>>>> you use it multiple times in a row GCC should be able to
>>>>> optimize this?  That is, optimizations like jump-threading also
>>>>> affect the speculation state by modifying the controling
>>>>> conditions.
>>>>
>>>> Ideally, if there's no control flow change, yes.  As soon as you insert
>>>> another branch (in or out) then you might have another speculation path
>>>> to consider.  Not sure how that can easily merging could be done, though.
>>>
>>> The usual case would be
>>>
>>>   if (cond)
>>>    ... _b_s_s_v (x);
>>> <code>
>>>   if (cond)
>>>     ... _b_s_s_v (x);
>>>
>>> where jump-threading might decide to make that to
>>>
>>>   if (cond)
>>>    {
>>>      ... _b_s_s_v (x);
>>>      <copy of code>
>>>      ... _b_s_s_v (x);
>>>    }
>>>
>>> now we might even be able to CSE the 2nd _b_s_s_v (x)
>>> to the first?  That would mean using ECF_CONST|ECF_LOOPING_PURE_OR_CONST
>>> is the best (but we currently have no attribute for the latter).
>>>
>>>>>
>>>>> You didn't answer my question about "what about C++"?
>>>>>
>>>>
>>>> It didn't need a response at this point.  It's a reasonable one, as are
>>>> some of your others...  I was focusing on the the comments that were
>>>> potentially contentious.
>>>>
>>>> BTW, this bit:
>>>>
>>>> +    case BUILT_IN_SPECULATION_SAFE_VALUE_N:
>>>> +      {
>>>> +       int n = speculation_safe_value_resolve_size (function, params);
>>>> +       tree new_function, first_param, result;
>>>> +       enum built_in_function fncode;
>>>> +
>>>> +       if (n == -1)
>>>> +         return error_mark_node;
>>>> +       else if (n == 0)
>>>> +         fncode = (enum built_in_function)((int)orig_code + 1);
>>>> +       else
>>>> +         fncode
>>>> +           = (enum built_in_function)((int)orig_code + exact_log2 (n) + 2);
>>>>
>>>>> resolve_size does that?  Why can that not return the built_in_function
>>>>> itself or BUILT_IN_NONE on error to make that clearer?
>>>>
>>>> is essentially a clone of some existing code that already does it this
>>>> way.  See BUILT_IN_SYNC_LOCK_RELEASE_N etc.  Admittedly, that hunk
>>>> handles multiple origins so would be harder to rewrite as you suggest;
>>>> it just seemed more appropriate to handle the cases similarly.
>>>
>>> Yes, I realized you copied handling from that so I didn't look too closely...
>>>
>>> These days we'd probably use an internal-function and spare us all
>>> the resolving completely (besides a test for validity) ;)
>>>
>>> Richard.
>>>
>>>> R.
>>>>
>>>>> Richard.
>>>>>
>>>>>>
>>>>>>
>>>>>>> BUILT_IN_SPECULATION_SAFE_VALUE_PTR is declared but
>>>>>>> nowhere generated?  Maybe
>>>>>>>
>>>>>>> +    case BUILT_IN_SPECULATION_SAFE_VALUE_N:
>>>>>>> +      {
>>>>>>> +       int n = speculation_safe_value_resolve_size (function, params);
>>>>>>> +       tree new_function, first_param, result;
>>>>>>> +       enum built_in_function fncode;
>>>>>>> +
>>>>>>> +       if (n == -1)
>>>>>>> +         return error_mark_node;
>>>>>>> +       else if (n == 0)
>>>>>>> +         fncode = (enum built_in_function)((int)orig_code + 1);
>>>>>>> +       else
>>>>>>> +         fncode
>>>>>>> +           = (enum built_in_function)((int)orig_code + exact_log2 (n) + 2);
>>>>>>>
>>>>>>> resolve_size does that?  Why can that not return the built_in_function
>>>>>>> itself or BUILT_IN_NONE on error to make that clearer?
>>>>>>>
>>>>>>> Otherwise it looks reasonable but C FE maintainers should comment.
>>>>>>> I miss C++ testcases (or rather testcases should be in c-c++-common).
>>>>>>>
>>>>>>> Richard.
>>>>>>>
>>>>>>>> gcc:
>>>>>>>>         * builtin-types.def (BT_FN_PTR_PTR_VAR): New function type.
>>>>>>>>         (BT_FN_I1_I1_VAR, BT_FN_I2_I2_VAR, BT_FN_I4_I4_VAR): Likewise.
>>>>>>>>         (BT_FN_I8_I8_VAR, BT_FN_I16_I16_VAR): Likewise.
>>>>>>>>         * builtins.def (BUILT_IN_SPECULATION_SAFE_VALUE_N): New builtin.
>>>>>>>>         (BUILT_IN_SPECULATION_SAFE_VALUE_PTR): New internal builtin.
>>>>>>>>         (BUILT_IN_SPECULATION_SAFE_VALUE_1): Likewise.
>>>>>>>>         (BUILT_IN_SPECULATION_SAFE_VALUE_2): Likewise.
>>>>>>>>         (BUILT_IN_SPECULATION_SAFE_VALUE_4): Likewise.
>>>>>>>>         (BUILT_IN_SPECULATION_SAFE_VALUE_8): Likewise.
>>>>>>>>         (BUILT_IN_SPECULATION_SAFE_VALUE_16): Likewise.
>>>>>>>>         * builtins.c (expand_speculation_safe_value): New function.
>>>>>>>>         (expand_builtin): Call it.
>>>>>>>>         * doc/cpp.texi: Document predefine __HAVE_SPECULATION_SAFE_VALUE.
>>>>>>>>         * doc/extend.texi: Document __builtin_speculation_safe_value.
>>>>>>>>         * doc/md.texi: Document "speculation_barrier" pattern.
>>>>>>>>         * doc/tm.texi.in: Pull in TARGET_SPECULATION_SAFE_VALUE.
>>>>>>>>         * doc/tm.texi: Regenerated.
>>>>>>>>         * target.def (speculation_safe_value): New hook.
>>>>>>>>         * targhooks.c (default_speculation_safe_value): New function.
>>>>>>>>         * targhooks.h (default_speculation_safe_value): Add prototype.
>>>>>>>>
>>>>>>>> c-family:
>>>>>>>>         * c-common.c (speculation_safe_resolve_size): New function.
>>>>>>>>         (speculation_safe_resolve_params): New function.
>>>>>>>>         (speculation_safe_resolve_return): New function.
>>>>>>>>         (resolve_overloaded_builtin): Handle __builtin_speculation_safe_value.
>>>>>>>>         * c-cppbuiltin.c (c_cpp_builtins): Add pre-define for
>>>>>>>>         __HAVE_SPECULATION_SAFE_VALUE.
>>>>>>>>
>>>>>>>> testsuite:
>>>>>>>>         * gcc.dg/spec-barrier-1.c: New test.
>>>>>>>>         * gcc.dg/spec-barrier-2.c: New test.
>>>>>>>>         * gcc.dg/spec-barrier-3.c: New test.
>>>>>>>> ---
>>>>>>>>  gcc/builtin-types.def                 |   6 ++
>>>>>>>>  gcc/builtins.c                        |  57 ++++++++++++++
>>>>>>>>  gcc/builtins.def                      |  20 +++++
>>>>>>>>  gcc/c-family/c-common.c               | 143 ++++++++++++++++++++++++++++++++++
>>>>>>>>  gcc/c-family/c-cppbuiltin.c           |   5 +-
>>>>>>>>  gcc/doc/cpp.texi                      |   4 +
>>>>>>>>  gcc/doc/extend.texi                   |  29 +++++++
>>>>>>>>  gcc/doc/md.texi                       |  15 ++++
>>>>>>>>  gcc/doc/tm.texi                       |  20 +++++
>>>>>>>>  gcc/doc/tm.texi.in                    |   2 +
>>>>>>>>  gcc/target.def                        |  23 ++++++
>>>>>>>>  gcc/targhooks.c                       |  27 +++++++
>>>>>>>>  gcc/targhooks.h                       |   2 +
>>>>>>>>  gcc/testsuite/gcc.dg/spec-barrier-1.c |  40 ++++++++++
>>>>>>>>  gcc/testsuite/gcc.dg/spec-barrier-2.c |  19 +++++
>>>>>>>>  gcc/testsuite/gcc.dg/spec-barrier-3.c |  13 ++++
>>>>>>>>  16 files changed, 424 insertions(+), 1 deletion(-)
>>>>>>>>  create mode 100644 gcc/testsuite/gcc.dg/spec-barrier-1.c
>>>>>>>>  create mode 100644 gcc/testsuite/gcc.dg/spec-barrier-2.c
>>>>>>>>  create mode 100644 gcc/testsuite/gcc.dg/spec-barrier-3.c
>>>>>>>>
>>>>>>
>>>>
>>
>> Here's an updated version of this patch, based on these discussions.
>> Notable changes since last time:
>> - __HAVE_SPECULATION_SAFE_VALUE is now only defined if the target has
>> been updated for this feature.
>> - Warnings are only issued if the builtin is used when
>> __HAVE_SPECULATION_SAFE_VALUE is not defined (so the builtin will always
>> generate a workable program, it just might not be protected in this case).
>> - Some of the tests moved to c-c++-common to improve C++ testing.
>> - The builtin is elided early on targets that do not need, or do not
>> provide a specific means to restrict speculative execution.
>>
>> A full bootstrap has completed, but tests are still running.
> 
> Please make the builtins NOVOPS as well by adding
> 
> DEF_ATTR_TREE_LIST (ATTR_NOVOPS_NOTHROW_LEAF_LIST, ATTR_NOVOPS,
> ATTR_NULL, ATTR_NOTHROW_LEAF_LIST)
> 
> to builtin-attrs.def and using that.

This is an optimization right?  If so, can I defer that to a follow-up
patch?  Although the builtin doesn't touch memory, I need think very
carefully about whether or not it is safe to move other memory ops
across it.

> 
> + The default implementation returns true for the first case if the target
> + defines a pattern named @code{speculation_barrier}; for the second case
> + and if the pattern is enabled for the current compilation.
> +@end deftypefn
> 
> I do not understand the last sentence.  I suspect it shold be
> 
> "The default implementation returns false if the target does not define
> a pattern named @code{speculation_barrier}.  Else it returns true
> for the first case and whether the pattern is enabled for the current
> compilation for the second case."

Thanks, I like that wording better than mine.

R.

> 
> Otherwise the middle-end changes look OK to me.  The c-family changes
> need review by a appropriate maintainer still.
> 
> Thanks,
> Richard.
> 
>> gcc:
>>         * builtin-types.def (BT_FN_PTR_PTR_VAR): New function type.
>>         (BT_FN_I1_I1_VAR, BT_FN_I2_I2_VAR, BT_FN_I4_I4_VAR): Likewise.
>>         (BT_FN_I8_I8_VAR, BT_FN_I16_I16_VAR): Likewise.
>>         * builtins.def (BUILT_IN_SPECULATION_SAFE_VALUE_N): New builtin.
>>         (BUILT_IN_SPECULATION_SAFE_VALUE_PTR): New internal builtin.
>>         (BUILT_IN_SPECULATION_SAFE_VALUE_1): Likewise.
>>         (BUILT_IN_SPECULATION_SAFE_VALUE_2): Likewise.
>>         (BUILT_IN_SPECULATION_SAFE_VALUE_4): Likewise.
>>         (BUILT_IN_SPECULATION_SAFE_VALUE_8): Likewise.
>>         (BUILT_IN_SPECULATION_SAFE_VALUE_16): Likewise.
>>         * builtins.c (expand_speculation_safe_value): New function.
>>         (expand_builtin): Call it.
>>         * doc/cpp.texi: Document predefine __HAVE_SPECULATION_SAFE_VALUE.
>>         * doc/extend.texi: Document __builtin_speculation_safe_value.
>>         * doc/md.texi: Document "speculation_barrier" pattern.
>>         * doc/tm.texi.in: Pull in TARGET_SPECULATION_SAFE_VALUE and
>>         TARGET_HAVE_SPECULATION_SAFE_VALUE.
>>         * doc/tm.texi: Regenerated.
>>         * target.def (have_speculation_safe_value, speculation_safe_value): New
>>         hooks.
>>         * targhooks.c (default_have_speculation_safe_value): New function.
>>         (default_speculation_safe_value): New function.
>>         * targhooks.h (default_have_speculation_safe_value): Add prototype.
>>         (default_speculation_safe_value): Add prototype.
>>
>> c-family:
>>         * c-common.c (speculation_safe_resolve_call): New function.
>>         (speculation_safe_resolve_params): New function.
>>         (speculation_safe_resolve_return): New function.
>>         (resolve_overloaded_builtin): Handle __builtin_speculation_safe_value.
>>         * c-cppbuiltin.c (c_cpp_builtins): Add pre-define for
>>         __HAVE_SPECULATION_SAFE_VALUE.
>>
>> testsuite:
>>         * c-c++-common/spec-barrier-1.c: New test.
>>         * c-c++-common/spec-barrier-2.c: New test.
>>         * gcc.dg/spec-barrier-3.c: New test.