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Subject:	Re: problem with i686-w64-mingw32-gcc -fstack-protector-all
To:	cygwin AT cygwin DOT com
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From:	Christian Franke <Christian DOT Franke AT t-online DOT de>
Message-ID:	<7a27bdaa-b3ad-1d33-5206-f32fabeb10c1@t-online.de>
Date:	Sun, 8 Oct 2017 14:58:52 +0200
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Lee wrote:
>
>>> + ./ssp.exe testtestx
>>> ./doit: line 11:  9128 Illegal instruction     ./ssp.exe testtestx
>>> + echo -e '\n\n'
>>>
>> The *** stack smashing detected *** message from MinGW runtime is only
>> visible if stdio is attached to a Windows console.
> Yes!
> But even after re-reading https://github.com/mintty/mintty/wiki/Tips &
> https://github.com/mintty/mintty/issues/56  I'm still missing why _no_
> output is displayed before the 'Illegal instruction' output line.

Stdio streams are usually full buffered if not attached to a 
tty/console. Buffers are flushed on regular exit(), but not on abnormal 
termination. Add fflush() calls to fix.


>> Works for me if one more overflow char is added:
>>
>> Cygwin mintty:
>>
>> $ ./ssp testtestx
>> main: argv[1]=testtestx
>> doit: s="testtestx"   buf="12345678"  i=1
>> doit: s="testtestx"   buf="testtestx"  i=1
>> main: exit
> Interesting.  I have Windows 10 & i686-w64-mingw32-gcc (GCC) 6.3.0   you?

Same (Win10.0.15063 x64 German).


> ...
>
> Anyway.. Thank you!!  It's been driving me nuts trying to figure out
> why all I get is 'Illegal instruction'

The 'Illegal instruction' is printed by the Cygwin shell because the 
program fails with STATUS_ILLEGAL_INSTRUCTION which is mapped to SIGILL.

The libspp code[1] shows that the program is terminated with 
__builtin_trap(). GCC then generates the x86 instruction UD2 ("defined" 
as "undefined instruction" :-)
In the MinGW case, stack error messages are written to CONOUT$ or are 
not written if no console is attached (Cygwin and others: /dev/tty or 
syslog()).

Christian

[1]
https://gcc.gnu.org/viewcvs/gcc/trunk/libssp/ssp.c?revision=233253&view=markup


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