forked from OSchip/llvm-project
536b0ee40a
Summary: Quote from http://eel.is/c++draft/expr.add#4: ``` 4 When an expression J that has integral type is added to or subtracted from an expression P of pointer type, the result has the type of P. (4.1) If P evaluates to a null pointer value and J evaluates to 0, the result is a null pointer value. (4.2) Otherwise, if P points to an array element i of an array object x with n elements ([dcl.array]), the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) array element i+j of x if 0≤i+j≤n and the expression P - J points to the (possibly-hypothetical) array element i−j of x if 0≤i−j≤n. (4.3) Otherwise, the behavior is undefined. ``` Therefore, as per the standard, applying non-zero offset to `nullptr` (or making non-`nullptr` a `nullptr`, by subtracting pointer's integral value from the pointer itself) is undefined behavior. (*if* `nullptr` is not defined, i.e. e.g. `-fno-delete-null-pointer-checks` was *not* specified.) To make things more fun, in C (6.5.6p8), applying *any* offset to null pointer is undefined, although Clang front-end pessimizes the code by not lowering that info, so this UB is "harmless". Since rL369789 (D66608 `[InstCombine] icmp eq/ne (gep inbounds P, Idx..), null -> icmp eq/ne P, null`) LLVM middle-end uses those guarantees for transformations. If the source contains such UB's, said code may now be miscompiled. Such miscompilations were already observed: * https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190826/687838.html * https://github.com/google/filament/pull/1566 Surprisingly, UBSan does not catch those issues ... until now. This diff teaches UBSan about these UB's. `getelementpointer inbounds` is a pretty frequent instruction, so this does have a measurable impact on performance; I've addressed most of the obvious missing folds (and thus decreased the performance impact by ~5%), and then re-performed some performance measurements using my [[ https://github.com/darktable-org/rawspeed | RawSpeed ]] benchmark: (all measurements done with LLVM ToT, the sanitizer never fired.) * no sanitization vs. existing check: average `+21.62%` slowdown * existing check vs. check after this patch: average `22.04%` slowdown * no sanitization vs. this patch: average `48.42%` slowdown Reviewers: vsk, filcab, rsmith, aaron.ballman, vitalybuka, rjmccall, #sanitizers Reviewed By: rsmith Subscribers: kristof.beyls, nickdesaulniers, nikic, ychen, dtzWill, xbolva00, dberris, arphaman, rupprecht, reames, regehr, llvm-commits, cfe-commits Tags: #clang, #sanitizers, #llvm Differential Revision: https://reviews.llvm.org/D67122 llvm-svn: 374293 |
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| .. | ||
| CommandGuide | ||
| analyzer | ||
| tools | ||
| AddressSanitizer.rst | ||
| AttributeReference.rst | ||
| AutomaticReferenceCounting.rst | ||
| Block-ABI-Apple.rst | ||
| Block-ABI-Apple.txt | ||
| BlockLanguageSpec.rst | ||
| CMakeLists.txt | ||
| ClangCheck.rst | ||
| ClangCommandLineReference.rst | ||
| ClangFormat.rst | ||
| ClangFormatStyleOptions.rst | ||
| ClangPlugins.rst | ||
| ClangStaticAnalyzer.rst | ||
| ClangTools.rst | ||
| ConstantInterpreter.rst | ||
| ControlFlowIntegrity.rst | ||
| ControlFlowIntegrityDesign.rst | ||
| CrossCompilation.rst | ||
| DataFlowSanitizer.rst | ||
| DataFlowSanitizerDesign.rst | ||
| DiagnosticsReference.rst | ||
| DriverArchitecture.png | ||
| DriverInternals.rst | ||
| ExternalClangExamples.rst | ||
| FAQ.rst | ||
| HardwareAssistedAddressSanitizerDesign.rst | ||
| HowToSetupToolingForLLVM.rst | ||
| InternalsManual.rst | ||
| IntroductionToTheClangAST.rst | ||
| ItaniumMangleAbiTags.rst | ||
| JSONCompilationDatabase.rst | ||
| LTOVisibility.rst | ||
| LanguageExtensions.rst | ||
| LeakSanitizer.rst | ||
| LibASTImporter.rst | ||
| LibASTMatchers.rst | ||
| LibASTMatchersReference.html | ||
| LibASTMatchersTutorial.rst | ||
| LibFormat.rst | ||
| LibTooling.rst | ||
| MSVCCompatibility.rst | ||
| Makefile.sphinx | ||
| MemorySanitizer.rst | ||
| Modules.rst | ||
| ObjectiveCLiterals.rst | ||
| OpenMPSupport.rst | ||
| PCHInternals.rst | ||
| PCHLayout.graffle | ||
| PCHLayout.png | ||
| RAVFrontendAction.rst | ||
| README.txt | ||
| RefactoringEngine.rst | ||
| ReleaseNotes.rst | ||
| SafeStack.rst | ||
| SanitizerCoverage.rst | ||
| SanitizerSpecialCaseList.rst | ||
| SanitizerStats.rst | ||
| ShadowCallStack.rst | ||
| SourceBasedCodeCoverage.rst | ||
| ThinLTO.rst | ||
| ThreadSafetyAnalysis.rst | ||
| ThreadSanitizer.rst | ||
| Toolchain.rst | ||
| Tooling.rst | ||
| UndefinedBehaviorSanitizer.rst | ||
| UsersManual.rst | ||
| conf.py | ||
| doxygen-mainpage.dox | ||
| doxygen.cfg.in | ||
| index.rst | ||
| make.bat | ||
README.txt
See llvm/docs/README.txt