c++cgccextern

add "extern C" as a compiler option for a symbol?

I'm working with FIPS Capable OpenSSL. The source code is sequestered and cannot be changed.

To link to the static version of the OpenSSL library, all we need to do is:

export FIPS_SIG=`find /usr/local/ssl -iname incore`
export CC=`find /usr/local/ssl -iname fipsld`
export FIPSLS_CC=`find /usr/bin -iname gcc`

Then, simply perform:

$CC $CFLAGS <sources> -o myprogram <openssl libs>

The reasons for the gyration is OpenSSL will insert an additional source file - fips_premain.c - and compile it with the program sources. (Some additional steps occur, but the compilation of fips_premain.c is the relevant step).

However, when using g++, a couple of symbols are undefined because they were compiled with the C compiler when OpenSSL was installed, and g++ cannot find them when invoked as above:

/tmp/fips_premain-20db15.o: In function `FINGERPRINT_premain()':
/usr/local/ssl/fips-2.0/lib/fips_premain.c:103: undefined reference to `FIPS_text_start()'
/usr/local/ssl/fips-2.0/lib/fips_premain.c:116: undefined reference to `FIPS_incore_fingerprint(unsigned char*, unsigned int)'

If I add the --no-demangle linker option, here's what is output:

/tmp/fips_premain-be4611.o: In function `_Z19FINGERPRINT_premainv':
/usr/local/ssl/fips-2.0/lib/fips_premain.c:103: undefined reference to `_Z15FIPS_text_startv'
/usr/local/ssl/fips-2.0/lib/fips_premain.c:116: undefined reference to `_Z23FIPS_incore_fingerprintPhj'

Here are the lines of interest in fips_premain.c (around line 85):

extern const void         *FIPS_text_start(),  *FIPS_text_end();
extern const unsigned char FIPS_rodata_start[], FIPS_rodata_end[];
extern unsigned char       FIPS_signature[20];
extern unsigned int        FIPS_incore_fingerprint(unsigned char *,unsigned int);

Is there a way to mark a symbol as extern "C" from the command line?

Solution

  • Supplying the -x c switch with g++ before the file on the compilation command line will force it to be compiled as a C file instead of the default C++.

    From the Fipsld and C++ page:

    Open fipsld++ and find occurrences where fips_premain.c is compiled. It is compiled through the variable {PREMAIN_C} ...

    Change the lines so that -x c preceeds "${PREMAIN_C}", and -x none follows it. 

    ${CC}  ${CANISTER_O_CMD:+"${CANISTER_O_CMD}"} \
    -x c "${PREMAIN_C}" -x none \
    ${_WL_PREMAIN} "$@"

    Forcing C compilation on fips_premain.c applies to all configurations (libraries, shared objects, and executables), so you are safe to apply it to all sections of fipsld++.