[LLVMbugs] [Bug 18403] New: Sigsegv in exception::what() after r

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18403

            Bug ID: 18403
           Summary: Sigsegv in exception::what() after r
           Product: libc++
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: hhinnant at apple.com
          Reporter: jonathan.sauer at gmx.de
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The following program, when compiled with clang r198596 and libc++ r198635
(which was also compiled with clang r198596), crashes with a SIGSEGV when run:

#include <cstdio>
#include <stdexcept>

int main()
{
    try
    {
        throw std::runtime_error("Error");
    }
    catch (const std::exception& e)
    {
        std::printf("%s\n", e.what());
    }
}


This results in:

% ~/LLVM/build/Release+Asserts/bin/clang++ -std=c++11 -stdlib=libc++ clang.cpp
% gdb ./a.out
[...]
(gdb) r
[...]
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x00000001000425e6 in std::runtime_error::what ()
(gdb) bt
#0  0x00000001000425e6 in std::runtime_error::what ()
#1  0x0000000100000da3 in main ()


After reverting to r198504 (before the last change to stdexcept.cpp), the code
ran fine.

I suspect the cause of the crash lies in the following change in
stdexcept.cpp[*]:

 runtime_error::runtime_error(const runtime_error& le) _NOEXCEPT
 {
-    __libcpp_nmstr& s = (__libcpp_nmstr&)__imp_;
-    ::new(&s) __libcpp_nmstr((const __libcpp_nmstr&)le.__imp_);
+    __libcpp_nmstr *s = static_cast<__libcpp_nmstr *>(__imp_);
+    const __libcpp_nmstr *s2 = static_cast<const __libcpp_nmstr *>(le.__imp_);
+    ::new(s) __libcpp_nmstr(*s2);
 }

Before the new "__libcpp_nmstr" was created inside "__imp_" (as "s" was a
reference to it, and taking the address of a reference means taking the address
of the referenced variable, i.e. "__imp_"). Now the new string is created
inside "s" and never gets stored in "__imp_".

Proposed fix: Remove "s" and just say:

  ::new(__imp_) __libcpp_nmstr(*s2);

(same in the other constructors as well)

[*]
<http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/stdexcept.cpp?r1=198505&r2=198504&pathrev=198505>

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