The objective of this tutorial is to hook a library function.
In the previous tutorial, we saw how to swap symbol names in a shared library. We will now see the mechanism for hooking a function in a shared library.
The targeted library is the standard math library (libm.so), and we will insert a hook on the exp function so that \(\exp(x) = x + 1\). The source code of the sample that uses this function is provided in the following listing:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main(int argc, char **argv) {
if (argc != 2) {
printf("Usage: %s <a> \n", argv[0]);
exit(-1);
}
int a = atoi(argv[1]);
printf("exp(%d) = %f\n", a, exp(a));
return 0;
}
The hook function is as follows:
double hook(double x) {
return x + 1;
}
Compiled with gcc -Os -nostdlib -nodefaultlibs -fPIC -Wl,-shared hook.c -o hook.
To inject this hook into the library, we use the add() (segment) method:
Overloaded function.
add(self, arg: lief._lief.ELF.DynamicEntry, /) -> lief._lief.ELF.DynamicEntry
dynamic_entry
add(self, section: lief._lief.ELF.Section, loaded: bool = True, pos: lief._lief.ELF.Binary.SEC_INSERT_POS = SEC_INSERT_POS.AUTO) -> lief._lief.ELF.Section
Add the given
Sectionto the binary.If the section does not aim at being loaded in memory, the
loadedparameter has to be set toFalse(default:True)
add(self, segment: lief._lief.ELF.Segment, base: int = 0) -> lief._lief.ELF.Segment
Add a new Segment in the binary
add(self, note: lief._lief.ELF.Note) -> lief._lief.ELF.Note
Add a new Note in the binary
First, we find the code for our hook function and add it to the library:
import lief
libm = lief.parse("/usr/lib/libm.so.6")
hook = lief.parse("hook")
exp_symbol = libm.get_symbol("exp")
hook_symbol = hook.get_symbol("hook")
code_segment = hook.segment_from_virtual_address(hook_symbol.value)
segment_added = libm.add(code_segment)
Once the stub is injected, we must calculate the new address for the exp symbol and update it:
new_address = segment_added.virtual_address + hook_symbol.value - code_segment.virtual_address
exp_symbol.value = new_address
exp_symbol.type = lief.ELF.Symbol.TYPE.FUNC # it might have been GNU_IFUNC
Note that we must update the symbol type to a regular FUNC because, on many distributions, libm.so is built with automatic hardware detection and exposes symbols as GNU_IFUNC, which uses a different dynamic binding protocol compared to regular functions.
Finally, we write the patched library to a file in the current directory:
libm.write("libm.so.6")
To test the patched library:
$ ./do_math.bin 1
exp(1) = 2.718282
$ LD_LIBRARY_PATH=. ./do_math.bin 1
exp(1) = 2.000000