ManLang'17: Lenient Execution of C on a Java Virtual Machine

Transcript

1. 1.

   Lenient Execution of C on a Java Virtual Machine Manuel

   Rigger1, Roland Schatz2, Matthias Grimmer2, Hanspeter Mössenböck1 ManLang, September 27, 2017 1Johannes Kepler University Linz 2Oracle Labs
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   Background: Undefined Behavior in C 2 int* number = malloc(sizeof(int));

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   Background: Undefined Behavior in C 3 int* number = malloc(sizeof(int));

   *number = 5;
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   Background: Undefined Behavior in C 4 int* number = malloc(sizeof(int));

   *number = 5; free(number);
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   Background: Undefined Behavior in C 5 int* number = malloc(sizeof(int));

   *number = 5; free(number); printf("%d\n", *number);
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   Background: Undefined Behavior (UB) in C 6 int* number =

   malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); UB
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   Background: Undefined Behavior (UB) in C 6 int* number =

   malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); 5 UB prints
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   Background: Undefined Behavior (UB) in C 6 int* number =

   malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); 5 0 UB prints
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   Background: Undefined Behavior (UB) in C 6 int* number =

   malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); 5 0 ManLang’17 UB prints
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    Background: Undefined Behavior (UB) in C 6 int* number =

    malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); 5 0 ManLang’17 rm –rf / UB prints
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    Background: Undefined Behavior (UB) in C 6 int* number =

    malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); 5 0 ManLang’17 rm –rf / ✓ UB prints
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    Background: Undefined Behavior (UB) in C 6 int* number =

    malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number); 5 0 ManLang’17 rm –rf / ✓ UB prints UB renders the whole program invalid!
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    Problems with Undefined Behavior • Exploited by compiler optimizations •

    Different behavior between –O0 and –O3 • Time bombs • Memory errors  exploitable by attackers • Behavior differs between platforms 7
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    Is Addressing UB Relevant? • 6/9 of SPEC CINT 2006

    benchmarks contain undefined integer operations (Dietz 2012) • 40% of the Debian packages contain unstable code (Wang 2013) • Experts are unclear about many UB aspects (Memarian 2016) 8
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    Our Approach Provide a user-friendly environment for C programmers on

    the JVM 9
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    Why the JVM? • Clear semantics for Java bytecodes •

    Useful services/features • Garbage collection • Automatic checks  memory and type safety 10
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    Our Approach 11 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    Our Approach 12 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    Our Approach 13 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    Our Approach 14 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    Our Approach 15 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    Our Approach 16 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    Our Approach 17 Clang program.c LLVM IR Truffle Java Virtual

    Machine LLVM IR Interpreter compile to runs on Graal compiler libc.c
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    What to do about Undefined Behavior? 18

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    What to do about Undefined Behavior? 18 Terminate the program

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    What to do about Undefined Behavior? 18 Terminate the program

    Continue execution
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    What to do about Undefined Behavior? 19 Terminate the program

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    What to do about Undefined Behavior? 20 int* number =

    malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number);
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    What to do about Undefined Behavior? 20 UNDEFINED BEHAVIOR …

    according to §2 7.22.3.3 of ISO/IEC 9899:2011 int* number = malloc(sizeof(int)); *number = 5; free(number); printf("%d\n", *number);
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    Object Hierarchy in Safe Sulong 21 ManagedObject Address pointee: ManagedObject

    offset: int I32Array values: int[] Function id: long I32 value: int DoubleArray values: double[]
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    Memory Allocation in Safe Sulong 22 int* number = malloc(sizeof(int));

    Address offset = 0 pointee I32 value = null
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    Memory Allocation in Safe Sulong 23 int* number = malloc(sizeof(int));

    *number = 5; Address offset = 0 pointee I32 value Integer value = 5
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    Memory Allocation in Safe Sulong 24 int* number = malloc(sizeof(int));

    *number = 5; free(number); Address offset = 0 pointee I32 value = null
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    Memory Allocation in Safe Sulong 25 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); Address offset = 0 pointee I32 value = null
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    Memory Allocation in Safe Sulong 25 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); Address offset = 0 pointee I32 value = null NullPointerException causes
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    Memory Allocation in Safe Sulong 25 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); Address offset = 0 pointee I32 value = null causes
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    Memory Allocation in Safe Sulong 25 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); Address offset = 0 pointee I32 value = null causes _________ /_ ___ \ /@ \/@ \ \ \__/\___/ / \_\/______/ / /\\\\\ | |\\\\\\ \ \\\\\\\ \______/\\\\\ _||_||_ UNDEFINED BEHAVIOR ...according to §2 7.22.3.3 of ISO/IEC 9899:2011
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    Expectations 26 Project maintainers can fix the bugs

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    Reality 27 Programmers do not always care about or understand

    the bugs
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    28 https://github.com/aidan-n/prime-number-generator/pull/1

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    What could go wrong? 29

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    What could go wrong? 29 If a side effect on

    a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. §2 6.5
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    UB in the Linux Kernel 30 For the kernel, we

    already really ignore some of the more idiotic C standard rules that introduce pointless undefined behavior: things like the strict aliasing rules are just insane, and the "overflow is u[nd]efined" is bad too. So we use -fno-strict-aliasing -fno-strict-overflow -fno-delete-null-pointer-checks to basically say "those optimizations are fundamentally stupid and wrong, and only encourage compilers to generate random code that doesn't actually match the source code". Linus 2017
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    What to do about Undefined Behavior? 31 Continue execution

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    Be Lenient with the User 32

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    Be Lenient with the User 32 The responsibility of tolerance

    lies with those who have the wider vision. – George Eliot
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    Be Lenient with the User 32 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); 5 The responsibility of tolerance lies with those who have the wider vision. – George Eliot prints
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    Lenient C • 26 rules that supersede the C11 standard

    • Restricted to the core language • Goals • Eliminate UB from the language • Do what the user might expect • Benefit from the JVM/Java semantics 33
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    Memory Allocation in Safe Sulong 34 int* number = malloc(sizeof(int));

    *number = 5; free(number); Address offset = 0 pointee I32 value Integer value = 5
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    Memory Allocation in Safe Sulong 34 int* number = malloc(sizeof(int));

    *number = 5; free(number); Address offset = 0 pointee I32 value Integer value = 5
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    Memory Allocation in Safe Sulong 35 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); Address offset = 0 pointee I32 value Integer value = 5
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    Memory Allocation in Safe Sulong 35 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); 5 Address offset = 0 pointee I32 value Integer value = 5 prints
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    Memory Allocation in Safe Sulong 35 int* number = malloc(sizeof(int));

    *number = 5; free(number); printf("%d\n", *number); 5 Address offset = 0 pointee I32 value Integer value = 5 prints The GC will collect the object if it is no longer referenced
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    Advantages Programmer • Is not required to “fix” the code

    • Easier-understandable C rules Employment • Can run the code 36
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    Example: Signed Integer Overflow 37 int a = 1, b

    = INT_MAX; int val = a + b; printf("%d\n", val); UB
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    Example: Signed Integer Overflow 37 int a = 1, b

    = INT_MAX; int val = a + b; printf("%d\n", val); Most compilers UB
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    Example: Signed Integer Overflow 37 int a = 1, b

    = INT_MAX; int val = a + b; printf("%d\n", val); mov edi, .L.str mov esi, -2147483648 call printf Most compilers UB
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    Example: Signed Integer Overflow 37 int a = 1, b

    = INT_MAX; int val = a + b; printf("%d\n", val); mov edi, .L.str mov esi, -2147483648 call printf Most compilers UB But: integer overflow is not handled consistenly
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    Example: Signed Integer Overflow 37 int a = 1, b

    = INT_MAX; int val = a + b; printf("%d\n", val); mov edi, .L.str mov esi, -2147483648 call printf Most compilers UB Lenient C: Signed integer overflow as wraparound semantics (-fno-strict-overflow)
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    Example: Shifts 38 int a = 1, b = 63;

    int val = a << b; printf("%d\n", val); UB
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    Example: Shifts 38 int a = 1, b = 63;

    int val = a << b; printf("%d\n", val); -O3 ??? UB mov edi, .L.str mov esi, <some value> call printf
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    Example: Shifts 38 int a = 1, b = 63;

    int val = a << b; printf("%d\n", val); -O3 ??? x86 2147483648 UB mov edi, .L.str mov esi, <some value> call printf
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    Example: Shifts 38 int a = 1, b = 63;

    int val = a << b; printf("%d\n", val); -O3 ??? x86 2147483648 PowerPC UB mov edi, .L.str mov esi, <some value> call printf 0
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    Example: Shifts 38 int a = 1, b = 63;

    int val = a << b; printf("%d\n", val); Consensus is impossible! -O3 ??? x86 2147483648 PowerPC UB mov edi, .L.str mov esi, <some value> call printf 0
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    Example: Shifts 38 int a = 1, b = 63;

    int val = a << b; printf("%d\n", val); Consensus is impossible! -O3 ??? x86 2147483648 PowerPC UB mov edi, .L.str mov esi, <some value> call printf 0 Lenient C: Invalid shift values have x86 shift semantics
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    Example: Relational Operators 39 void* memmove(void *dest , void const

    *src , size_t n ) { char *dp = dest; char const *sp = src; if (dp < sp) { while (n-- > 0) *dp++ = *sp++; } else { dp += n; sp += n; while (n-- > 0) *--dp = *--sp ; } return dest; }
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    Example: Relational Operators 40 void* memmove(void *dest , void const

    *src , size_t n ) { char *dp = dest; char const *sp = src; if (dp < sp) { while (n-- > 0) *dp++ = *sp++; } else { dp += n; sp += n; while (n-- > 0) *--dp = *--sp ; } return dest; } Using < to compare pointers to two different objects is UB! UB
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    Example: Relational Operators 41 void* memmove(void *dest , void const

    *src , size_t n ) { char *dp = dest; char const *sp = src; if (dp < sp) { while (n-- > 0) *dp++ = *sp++; } else { dp += n; sp += n; while (n-- > 0) *--dp = *--sp ; } return dest; } Using < to compare pointers to two different objects is UB! UB
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    Example: Relational Operators 41 Lenient C: Pointers are compared using

    their integer representation void* memmove(void *dest , void const *src , size_t n ) { char *dp = dest; char const *sp = src; if (dp < sp) { while (n-- > 0) *dp++ = *sp++; } else { dp += n; sp += n; while (n-- > 0) *--dp = *--sp ; } return dest; } Using < to compare pointers to two different objects is UB! UB
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    Example: Relational Operators 42 a: Address offset pointee b: Address

    offset pointee < integer_rep(a) < integer_rep(b)
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    Integer Representation 43 integer_rep(a) = a.offset

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    Integer Representation 43 integer_rep(a) = a.offset✓

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    Integer Representation 43 integer_rep(a) = a.offset✓ … what about pointers

    to different objects? 
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    Integer Representation 44 integer_rep(a) = (long) System.identityHashCode(a.pointee) << 32 |

    offset
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    Integer Representation 44 integer_rep(a) = (long) System.identityHashCode(a.pointee) << 32 |

    offset Breaks antisymmetry as different objects might have the same hash code! 
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    Integer Representation 45 integer_rep(a) = a.id

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    Integer Representation 45 integer_rep(a) = a.id Need to assign distinct

    IDs ☺
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    Example: Side Effects 46 void func(int *a) { printf("%d", *a);

    if (a == NULL) { abort(); } }
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    Example: Side Effects 46 void func(int *a) { printf("%d", *a);

    if (a == NULL) { abort(); } } func(int*): mov esi, DWORD PTR [rdi] xor eax, eax mov edi, OFFSET FLAT:.LC0 jmp printf GCC –O2
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    Example: Side Effects 46 void func(int *a) { printf("%d", *a);

    if (a == NULL) { abort(); } } func(int*): mov esi, DWORD PTR [rdi] xor eax, eax mov edi, OFFSET FLAT:.LC0 jmp printf Lenient C: NULL-dereferences (-fno- delete-null-pointer-checks), division by 0, etc. have visible side effects GCC –O2
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    int func(int *a , long *b) { *a = 5;

    *b = 8; return *a; } int main(){ long val = 3; printf("%d\n", func((int*) &val, &val)); } Examples: Strict-aliasing rules 47
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    int func(int *a , long *b) { *a = 5;

    *b = 8; return *a; } int main(){ long val = 3; printf("%d\n", func((int*) &val, &val)); } Examples: Strict-aliasing rules 47 5 prints
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    int func(int *a , long *b) { *a = 5;

    *b = 8; return *a; } int main(){ long val = 3; printf("%d\n", func((int*) &val, &val)); } Examples: Strict-aliasing rules 47 5 func(int*, long*): # @func(int*, long*) mov dword ptr [rdi], 5 mov qword ptr [rsi], 8 mov eax, 5 ret Clang or GCC –O2 prints
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    Examples: Strict-aliasing rules 48 Lenient C: Pointers can be dereferenced

    using any type (-fno-strict-aliasing)
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    Examples: Strict-aliasing rules 48 a: Address offset pointee Long value

    = 5 b: Address offset pointee Lenient C: Pointers can be dereferenced using any type (-fno-strict-aliasing)
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    Future Work • Extend Lenient C to cover multithreading, compile-time

    issues, library functions, … • Which parts of Lenient C apply to static compilers? 49
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    Related work • Boringcc by Bernstein • Friendly C by

    Cuoq et al. • C* by Ertl • C-like languages 50
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    Related work • Boringcc by Bernstein • Friendly C by

    Cuoq et al. • C* by Ertl • C-like languages 51 • Boring compiler for crypto software • Clear semantics • No concrete proposal
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    Related work • Boringcc by Bernstein • Friendly C by

    Cuoq et al. • C* by Ertl • C-like languages 52 • Replaces many occurrences of “X has undefined behavior” with “X results in an unspecified value” • Addresses 14 points
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    Related work • Boringcc by Bernstein • Friendly C by

    Cuoq et al. • C* by Ertl • C-like languages 53 • C* specifies language elements according to the hardware features • Behavior might be different for every platform
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    Related work • Boringcc by Bernstein • Friendly C by

    Cuoq et al. • C* by Ertl • C-like languages 54 • Cyclone • Polymorphic C • CCured  Not source-compatible
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    Conclusion Bug-finding mode to detect UB 55 Managed Execution on

    the JVM ManagedObject Address pointee: ManagedObject offset: int I32Array values: int[] Function id: long I32 value: int DoubleArray values: double[] Lenient C as a user-friendly C dialect
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    Bibliography • Linus 2017: Linus Torvalds, https://lkml.org/lkml/2017/7/5/486 • Dietz 2012:

    Will Dietz, Peng Li, John Regehr, and Vikram Adve. 2012. Understanding integer overflow in C/C++. In Proceedings of the 34th International Conference on Software Engineering (ICSE '12). • Wang 2013: Xi Wang, Nickolai Zeldovich, M. Frans Kaashoek, and Armando Solar-Lezama. 2013. Towards optimization-safe systems: analyzing the impact of undefined behavior. In Proceedings of the Twenty-Fourth ACM Symposium on Operating Systems Principles (SOSP '13). • Memarian 2016: Kayvan Memarian, Justus Matthiesen, James Lingard, Kyndylan Nienhuis, David Chisnall, Robert N. M. Watson, and Peter Sewell. 2016. Into the depths of C: elaborating the de facto standards. In Proceedings of the 37th ACM SIGPLAN Conference on Programming Language Design and Implementation (PLDI ‘16) 56
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