Binutils GAS file-less frontend

gas_rapl.c

/**
 * @file libgas.c
 * @author Stefano Moioli <[email protected]>
 * @brief use GAS from Binutils as a library, without temporary files
 * 		  this example implements a GAS assembler RAPL - Read Assemble Print Loop
 * @version 0.1
 * @date 2022-04-28
 * 
 * @copyright Copyright (c) Stefano Moioli 2022
 * 
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <windows.h>
#include <sys/stat.h>

static HMODULE gas;

static void *resolveSymbol(char *sym){
	return (void *)GetProcAddress(gas, sym);
}

#define GVAR(T, sym) T sym = (T)resolveSymbol(#sym)

#define GFUNC(ret_type, function, ...) \
	ret_type(*function)(__VA_ARGS__) = resolveSymbol(#function)

typedef FILE *(*fopen_fn)(const char *filename, const char *mode);
typedef void (*appender_fn)(int datum);

/**
 * @brief 
 * the hooked frag_append_1_char declared with --wrap in libgas
 */
static appender_fn wrapped_fappend;


static void *iovec_open (void *nbfd, void *open_closure){
	return open_closure;
}
static off_t iovec_read(
	void *nbfd, void *stream, void *buf,
	off_t nbytes, off_t offset
){
	unsigned char *mem = (unsigned char *)stream;
	memcpy(buf, &mem[offset], nbytes);
	return nbytes;
}

static void my_fappend(int datum){
	printf(">> %02X\n", datum);
}

int main(int argc, char *argv[]){
	setvbuf(stdout, NULL, _IONBF, 0);
	setvbuf(stderr, NULL, _IONBF, 0);

	gas = LoadLibraryA("gas-x86_64-unknown-linux.dll");
	if(gas == NULL){
		fprintf(stderr, "LoadLibraryA failed\n");
		return 1;
	}

	GFUNC(void, subsegs_begin);
	GFUNC(void, symbol_begin);
	GFUNC(void, frag_init);

	GFUNC(void, elf_begin);
	GFUNC(void, write_object_file);

	GFUNC(void, output_file_create, const char *name);
	GFUNC(void, output_file_close, const char *filename);

	GFUNC(void *, subseg_new, const char *segname, int subseg);
	GFUNC(void, bfd_set_section_flags, void *, unsigned);
	GFUNC(void, subseg_set, void *, int);

	GFUNC(void, md_begin);
	GFUNC(void, md_assemble, char *line);
	GFUNC(void, md_end);

	GFUNC(void *, bfd_openr_iovec,
		const char *filename, const char *target,
		void *(*open) (void *nbfd, void *open_closure),
		void *open_closure,
		off_t (*pread) (
			void *nbfd,
			void *stream, void *buf,
			off_t nbytes, off_t offset),
		int (*close) (void *nbfd, void *stream),
		int (*stat) (void *abfd, void *stream, struct stat *sb)
	);

	GFUNC(int, bfd_default_set_arch_mach, void *abfd, unsigned arch, unsigned long mach);
	GFUNC(void, bfd_set_format, void *abfd, int format);

	GFUNC(int, bfd_close, void *abfd);

	// address of the fopen function pointer
	GVAR(fopen_fn *, pfn_fopen);
	GVAR(appender_fn *, pfn_fappend);

	GVAR(void **, reg_section);
	GVAR(void **, expr_section);
	GVAR(void **, stdoutput);

	wrapped_fappend = *pfn_fappend;
	*pfn_fappend = &my_fappend;

	symbol_begin();
	subsegs_begin();

	#define MEM_SIZE 1024 * 1024
	unsigned char *mem = calloc(MEM_SIZE, 1);

	/**
	 * create a fake output buffer.
	 * it's actually in read mode, as signaled by the r in "openr"
	 * this will conveniently cause all write operations to be ignored
	 */
	*stdoutput = bfd_openr_iovec(
		"dummy", NULL,
		iovec_open, mem,
		iovec_read, NULL, NULL
	);

	// set object format
	bfd_set_format(*stdoutput, 1);
	bfd_default_set_arch_mach(*stdoutput,
		8, // bfd_arch_i386
		8 // bfd_mach_x86_64
	);

	/**
	 * create the .text section 
	 */
	void *textSeg = subseg_new(".text", 0);
	bfd_set_section_flags(textSeg, 1 | 2 | 4 | 0x10 |8);
	subseg_set(textSeg, 0);

	md_begin();

	char *line = NULL;
	/**
	 * IMPORTANT: md_assemble modifies the input line
	 * so we must always make a copy 
	 */
	#define ASM(x) do { \
		line = strdup(x); \
		md_assemble(line); \
		free(line); line = NULL; \
	} while(0);

	char buffer[128];
	while(!feof(stdin)){
		fgets(buffer, sizeof(buffer), stdin);
		
		char *p = strrchr(buffer, '\n');
		if(p) *p = '\0';

		ASM(buffer);
	}

	//ASM("xorl %ebx, %ebx");

	if(md_end != NULL){
		md_end();
	}

	free(mem);
	return 0;
}