RE: [Tinycc-devel] libtcc not working properly on x86-64 targets

[Michael Kuklinski]

There appears to be a problem in how it configures the jump table in add_jump_table… after fixing the unsigned long to a uplong, that appears to be where the truncation occurs. However, there is no native jmp operation for 64-bit addresses which is problematic… everything is supposed to be handled as 32-bit offsets. There appear to be a variety of problems there and in the functions leading up to it where things are passed as 32-bit variables instead of 64-bit, which causes truncation before then.

 

 

From: address@hidden [mailto:address@hidden On Behalf Of Michael Kuklinski
Sent: Monday, December 21, 2009 3:52 PM
To: address@hidden
Subject: RE: [Tinycc-devel] libtcc not working properly on x86-64 targets

 

I'm going to try to restate my problem since I poorly presented it originally.

Using both the Master and the Mob revisions, I have the same problem in x86-64 mode. If I attempt to pass a pointer to a function using tcc_add_symbol, the pointer somewhere down the line becomes truncated to 32-bit.

I have tested this two ways (this is C++, btw):

#include <libtcc.h>

#include <stdio.h>

#define TESTMODE 0

static const char *s_program =          \

        "                                               \

        int main ()                                     \

        {                                               \

                print(\"Hello, World!\");       \

                return 0;                               \

        }                                               \

        ";

#if TESTMODE == 1

static void *s_funcptr = LL0x1122334455667788;

#else // TESTMODE == 0

void print (const char *str)

{

        printf(str);

}

static void *s_funcptr = (void*)print;

#endif // TESTMODE

int main ()

{

        TCCState *compile_state = tcc_new();

        tcc_add_symbol(compile_state, “print”, s_funcptr);

        tcc_set_output_type(compile_state, TCC_OUTPUT_MEMORY);

        tcc_compile_string(compile_state, s_program);

        tcc_run(compile_state, 0, 0);

        return 0;

}

If TESTMODE is 0, then it returns an unhandled exception – the reason? The function print is at address 0x000000013F8D100A, and the system is truncating it to 0x000000003F8D100A... illegal access exception. The same occurs when TESTMODE is 1 – I receive an illegal access exception in addressing 0x0000000055667788. The upper 32 bits are being truncated by some mechanism within the compiler.

I did find one bug:

static int put_elf_sym(

Section *s,

      unsigned long value,

unsigned long size,

      int info, int other, int shndx, const char *name);

The above is located at line 191 in libtcc.c (on the mob release), and the value parameter should be type uplong, which on 64-bit builds is defined as unsigned long long. However, fixing this does not solve the problem.

I am too unfamiliar with the compiler internals to figure out where this is failing, so any assistance would be grand.

--Michael