Added support for x86 in setcanary like getcanary

[sudhackar]

cmake-build-multiarch.sh fails for setcanary for x86. Adding the support for x86 similar to what getcanary does.

[sudhackar]

$ tail -n 35 z.c

#include <stdio.h>
#include <stdint.h>

#ifdef __x86_64__
#define canary_t     uint64_t
#define INSN_READ    "movq %%fs:0x28, %0;"
#define FMT          "Found canary: %#lx\n"

#elif __i386__
#define canary_t     uint32_t
#define INSN_READ    "movl %%gs:0x14, %0;"
#define FMT          "Found canary: %#x\n"
#endif

canary_t read_canary()
{
    canary_t val = 0;

    __asm__(INSN_READ
        : "=r"(val)
        :
        :);

    return val;
}

int main(int argc, char **argv)
{
    printf(FMT, read_canary());
    return 0;
}

$ gcc -no-pie -fno-pic z.c -o z64
$ gcc -m32 -no-pie -fno-pic z.c -o z32
$ PREENY_DEBUG=1 PREENY_INFO=1 PREENY_ERROR=1 CANARY=280267669825 LD_PRELOAD=~/tools/preeny/build_x64/lib/libsetcanary.so:~/tools/preeny/build_x64/lib/libgetcanary.so ./z64
--- Found canary: 0xd09868c5f39cd900
+++ Overwriting canary with 0x41414141...
Found canary: 0x4141414141
$ PREENY_DEBUG=1 PREENY_INFO=1 PREENY_ERROR=1 CANARY=1094795585 LD_PRELOAD=~/tools/preeny/build_x86/lib/libsetcanary.so:~/tools/preeny/build_x86/lib/libgetcanary.so ./z32
--- Found canary: 0x2526d900
+++ Overwriting canary with 0x41414141...
Found canary: 0x41414141

If it helps I can make a PR with tests for setcanary

[zardus]

Thanks! Tests would be great, of course :-)