On 25/07/18 14:47, Richard Biener wrote: > On Wed, Jul 25, 2018 at 2:41 PM Richard Earnshaw (lists) > wrote: >> >> On 25/07/18 11:36, Richard Biener wrote: >>> On Wed, Jul 25, 2018 at 11:49 AM Richard Earnshaw (lists) >>> wrote: >>>> >>>> On 24/07/18 18:26, Richard Biener wrote: >>>>> On Mon, Jul 9, 2018 at 6:40 PM Richard Earnshaw >>>>> 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); > > if (cond) > ... _b_s_s_v (x); > > where jump-threading might decide to make that to > > if (cond) > { > ... _b_s_s_v (x); > > ... _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. 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.