Thursday, August 20, 2020

C++ Std::Condition_Variable Null Pointer Derreference


This story is about a bug generated by g++ and clang compilers (at least)
The condition_variables is a feature on the standard library of c++ (libstdc++), when its compiled statically a weird asm code is generated.


Any example on the link below will crash if its compiled statically:
 https://en.cppreference.com/w/cpp/thread/condition_variable



In this case the condition_variable.wait() crashed, but this happens with other methods, a simple way to trigger it:




If this program is compiled dynamically the crash doesn't occur:

Looking the dissasembly there is a surprise created by the compiler:


Compilers:
    g++  9.2.1+20200130-2
    clang++ v9

Both compilers are generating the "call 0x00"

If we check this call in a dynamic compiled:




The implementation of condition_variable in github:
https://github.com/gcc-mirror/gcc/blob/b7c9bd36eaacac42631b882dc67a6f0db94de21c/libstdc%2B%2B-v3/include/std/condition_variable


The compilers can't copile well this code in static,  and same happens on  other condition_variable methods.
I would say the _lock is being assembled improperly in static, is not exacly a null pointer derreference but the effects are the same, executing code at address 0x00 which on linux is a crash on most of cases.

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