[llvm-bugs] [Bug 31775] New: CFI: unclear behavior discrepancy with -O0 vs -Os when libstdc++ is linked statically

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https://llvm.org/bugs/show_bug.cgi?id=31775

            Bug ID: 31775
           Summary: CFI: unclear behavior discrepancy with -O0 vs -Os when
                    libstdc++ is linked statically
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: krasin at google.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 17900
  --> https://llvm.org/bugs/attachment.cgi?id=17900&action=edit
main.cc

Consider the following c++ program:

$ cat main.cc
#include <iostream>

int main(int argc, const char *argv[]) {
  std::cout << "Hello" << std::endl;
}

And the following build script:

$ cat build.sh
#!/bin/bash

set -ue

export CFLAGS="-g -c -flto -fsanitize=cfi -fsanitize-cfi-cross-dso
-fno-sanitize-trap=all -fvisibility=default"
export LDFLAGS="-flto -fuse-ld=gold -fno-sanitize-trap=all -flto -fuse-ld=gold
-fsanitize=cfi -fsanitize-cfi-cross-dso -fvisibility=default -static-libstdc++"

clang++ -O0 -o main_0.o main.cc ${CFLAGS}
clang++ -o main_0 main_0.o ${LDFLAGS}
echo "Running a binary built with -O0:"
./main_0

clang++ -Os -o main_s.o main.cc ${CFLAGS}
clang++ -o main_s main_s.o ${LDFLAGS}
echo "Running a binary built with -Os:"
./main_s

When running the script it outputs:

$ ./build.sh 
Running a binary built with -O0:
Hello
Running a binary built with -Os:
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/ostream:113:9:
runtime error: control flow integrity check for type 'std::basic_ostream<char>
&(std::basic_ostream<char> &)' failed during indirect function call
(/usr/local/google/home/krasin/play/cfi-inline/main_s+0x71b70): note:
std::basic_ostream<char, std::char_traits<char> >& std::endl<char,
std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
defined here
SUMMARY: CFI: undefined-behavior
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/ostream:113:9 in

There are few unclear moments in the report:

1. Why is it only happening at -Os, but not -O0. The answer is likely related
to inlining, but the exact cause is still not found.

2. Why isn't the address symbolized (main_s+0x71b70) and what does it point to?

3. Given that the main linkage unit has CFI and non-CFI together, it's an
unsupported combination. Do we want to do anything in order to support it? It
has been the reason for a major loss of productivity for a user.

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