Dangerous Optimizations! in C++ - Part Deux
summary
Shows concrete compiler behaviors that break causality: a loop-invariant remainder hoisting undefined behavior on MSVC moves a division fault earlier, GCC O0 vs O2 reproduces the shifted crash, and integer promotions lead to integer promotion overflow that lets the optimizer remove a negative-branch. Demonstrates a Linux kernel exploit where null-deref plus mmap(0) executes attacker code and explains the fix (move the dereference or build with fno-delete-null-pointer-checks). Slides and examples live on the provided GitHub.
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chapters (12)
- 0:00Introduction
- 1:24Dangerous optimizations premise
- 3:08Time travel UB in compilers
- 12:23C committee on UB
- 23:45UB types defined
- 26:14Compiler UB optimizations
- 31:55Integer overflow pitfalls
- 43:02Strict aliasing impact
- 52:07Effective type violations
- 1:01:47Null pointer risks
- 1:13:17Linux kernel exploit
- 1:22:39Best practices
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