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Re: i686-w64-mingw32-gcc -fstack-protector-strong




On 9/8/17, Steven Penny <svnpenn@xxxxxxxxx> wrote:
> On Fri, 8 Sep 2017 10:14:54, Lee wrote:
>> $ i686-w64-mingw32-gcc -o div.exe div.c
>> $ ./div
>>   works
>>
>> $ i686-w64-mingw32-gcc -fstack-protector-strong -o div.exe div.c
>> $ ./div
>> C:/cygwin/home/Lee/t/div.exe: error while loading shared libraries: ?:
>> cannot open shared object file: No such file or directory
>>
>> $ ldd div.exe
>>         ntdll.dll => /cygdrive/c/WINDOWS/SYSTEM32/ntdll.dll
>> (0x7ffef5c50000)
>>         ??? => ??? (0x778f0000)
>>         wow64.dll => /cygdrive/c/WINDOWS/System32/wow64.dll (0x50e50000)
>>         wow64win.dll => /cygdrive/c/WINDOWS/System32/wow64win.dll
>> (0x50dd0000)
>>
>> $ i686-w64-mingw32-gcc --version
>> i686-w64-mingw32-gcc (GCC) 5.4.0
>
> You need to paste or link to div.c, otherwise this seems a pointless
> exercise.
> See, I cannot repro:
>
> $ cat div.c
> #include <stdio.h>
> int main() {
>   puts("hello world");
> }

man gcc

       -fstack-protector
           Emit extra code to check for buffer overflows, such as
stack smashing attacks.  This is done by adding a guard
           variable to functions with vulnerable objects.  This
includes functions that call "alloca", and functions with
           buffers larger than 8 bytes.  The guards are initialized
when a function is entered and then checked when the
           function exits.  If a guard check fails, an error message
is printed and the program exits.

       -fstack-protector-all
           Like -fstack-protector except that all functions are protected.

       -fstack-protector-strong
           Like -fstack-protector but includes additional functions to
be protected --- those that have local array definitions,
           or have references to local frame addresses.


I maybe should have used -fstack-protector-all, but I'm still learning :)

> $ x86_64-w64-mingw32-gcc -o div div.c
> $ ./div
> hello world
>
> $ x86_64-w64-mingw32-gcc -fstack-protector-strong -o div div.c
> $ ./div
> hello world
>
> $ ldd div
> ntdll.dll => /cygdrive/c/Windows/SYSTEM32/ntdll.dll (0x773a0000)
> kernel32.dll => /cygdrive/c/Windows/system32/kernel32.dll (0x77180000)
> KERNELBASE.dll => /cygdrive/c/Windows/system32/KERNELBASE.dll
> (0x7fefd1a0000)
> msvcrt.dll => /cygdrive/c/Windows/system32/msvcrt.dll (0x7fefde00000)

note libssp-0 (or ???) isn't mentioned
you don't have any user defined functions, so it got optimized
out/never called??

Thanks
Lee

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