overwriting glibc library functions

10 may 2011

motivation

this posting shows how to alter certain functions in a library. it might be of interest to fix some issues i found in an earlier posting where i wanted to ‘fix’ the date function to force GCC timestamps to be always the same date. still i think the better way of doing so is to add a new function to GCC which gives the user the freedom to fix the date to a constant, maybe by something like this:

GCC_DATE="Tue May 10 11:37:44 CEST 2011" gcc -c main.c -o main.o

anyway here is the code written by linus and other [kernel] developers, i quote the whole posting #55 from Magnus Glantz:

the memcpy(…) function:

I’m sure thousands of people will find their way here, so, here’s a quicky. To bypass this issue (which is an issue in Adobe Flash), you may run the below “fix” brought forth in comment #38

1. Cut and paste this into a prompt:

---Cut below---
cat > $HOME/Downloads/linusmemcpy.c <<EOF
#include <sys/types.h>

void *memcpy(void *dst, const void *src, size_t size)
{
void *orig = dst;
asm volatile("rep ; movsq"
:"=D" (dst), "=S" (src)
:"0" (dst), "1" (src), "c" (size >> 3)
:"memory");
asm volatile("rep ; movsb"
:"=D" (dst), "=S" (src)
:"0" (dst), "1" (src), "c" (size & 7)
:"memory");
return orig;
}
EOF
cd $HOME/Downloads
gcc -O2 -c linusmemcpy.c
ld -G linusmemcpy.o -o linusmemcpy.so
---Stop cutting here---

2. Shutdown any running copies of your webbrowser.

3. Until a Adobe has fixed their Flash player, start your webbrowser as below:

For Firefox users:

LD_PRELOAD=$HOME/Downloads/linusmemcpy.so /usr/bin/firefox &

For Google Chrome users:

LD_PRELOAD=$HOME/Downloads/linusmemcpy.so /opt/google/chrome/google-chrome &