[Bug other/105335] New: libiberty does not handle script exit codes correctly.

["lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105335

            Bug ID: 105335
           Summary: libiberty does not handle script exit codes correctly.
           Product: gcc
           Version: 11.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

Created attachment 52847
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52847&action=edit
proposed patch

A few days ago I was playing with linker plugins. I created a bug in the LTO
plugin that it segfaulted when the process was about to terminate. While I was
attempt to bootstrap GCC with this modified piece of code, it failed during
stage1 with a mysterious message:

    ```
    configure:3566:
/d/lh_mouse/GitHub/MINGW-packages-dev/mingw-w64-gcc-git/src/build-x86_64-w64-mingw32/./gcc/xgcc
   
-B/d/lh_mouse/GitHub/MINGW-packages-dev/mingw-w64-gcc-git/src/build-x86_64-w64-mingw32/./gcc/
-L/mingw64/x86_64-w64-mingw32/lib
    -L/mingw64/lib -isystem /mingw64/x86_64-w64-mingw32/include -isystem
/mingw64/include -B/mingw64/x86_64-w64-mingw32/bin/
    -B/mingw64/x86_64-w64-mingw32/lib/ -isystem
/mingw64/x86_64-w64-mingw32/include -isystem
/mingw64/x86_64-w64-mingw32/sys-include
    -fno-checking -o conftest -g -march=x86-64 -mtune=generic -O2 -pipe  -pipe
conftest.c  >&5
    C:\MSYS2\mingw64\x86_64-w64-mingw32\bin\ld.exe: cannot find -lgcc: No such
file or directory
    C:\MSYS2\mingw64\x86_64-w64-mingw32\bin\ld.exe: cannot find -lgcc_eh: No
such file or directory
    C:\MSYS2\mingw64\x86_64-w64-mingw32\bin\ld.exe: cannot find -lgcc: No such
file or directory
    C:\MSYS2\mingw64\x86_64-w64-mingw32\bin\ld.exe: cannot find -lgcc_eh: No
such file or directory
    configure:3669: error: C compiler cannot create executables
    ```

Later I found that it was because configure couldn't tell whether the stage1
compiler should fail, as it always returned zero, indicating success.

But how? Apparently ld failed with errors, and GCC should not have exited with
zero. I used GDB to dig for a while. There didn't seem anything wrong with GCC.
It was collect2 that returned zero.

collect2 seemed to invoke ld indirectly, so I set a breakpoint on
`GetExitCodeProcess()`. This of course got the exit code of ld (I thought so,
but it was wrong! see below), which was 2816, so it failed, but in liberty
`pex_win32_wait()` took merely its lowest byte and consider it the exit status,
which was zero, which was incorrect.

Later I set a breakpoint on `CreateProcessA()` and realized the aforementioned
process that exited with 2816 was actually created by `spawn_script()`, which
attempted to parse the shebang of `collect-ld`, which should denote `/bin/sh`.
I had a working MSYS2 shell at the moment, and it correctly detected the
segfault and returned 2816 = (11 << 8), indicating the signal 11 = SIGSEGV.


So far we can conclude that there is a bug in libiberty: It only takes the
lowest byte as the child process's exit status. If the child (bash) fails with
2816 then it is mistaken as having exited successfully.

Untested patch attached.