Re: [Tinycc-devel] Patching symbols after tcc_relocate

[grischka]

Henry Weller wrote:
> This requires significant change to the operation of the executable for all
> functions whether they are over-ridden or not which is an overhead I am not
> prepared to accept. What I want is to be able to have a REPL for an executable
> which is as efficient as possible.  If this is not going to be possible within
> libtcc then I will live with ELF-Hook but given that it is possible for DLs I
> see no reason why it is not possible from libtcc after relocation.

Everything is possible.  This is software after all.

Say your original function is:
    int add(int a, int b) { return a + b; }

Your replacement (same prototype) is:
    int sub(int a, int b) { return a - b; }

Have a tool to make it happen:
    void replace_function(TCCState *s, const char *name, void *new)
    {
        char *old = tcc_get_symbol(s, name);
        set_pages_executable(old, 5); //from tccrun.c
    #ifdef ___i386__
        *old = 0xe9;
        *(unsigned*)(old + 1) = (char *)new - old - 5;
    #endif
    }

Put all that into libtcc_test.c:
     /* run the code */
     func(32);
+    printf("-- Replacing -- \n");
+    replace_function(s, "add", sub);
+    func(32);

     /* delete the state */
     tcc_delete(s);

Run:

    Hello World!
    fib(32) = 2178309
    add(32, 64) = 96
    -- Replacing --
    Hello World!
    fib(32) = 2178309
    add(32, 64) = -32  !!! voilà

--- grischka