Skip to content

Instantly share code, notes, and snippets.

@jrk

jrk/elf.cpp

Last active August 29, 2015 14:13
Show Gist options
  • Save jrk/d8f1dbf8cf61c9eb80c0 to your computer and use it in GitHub Desktop.
Save jrk/d8f1dbf8cf61c9eb80c0 to your computer and use it in GitHub Desktop.
Download ZIP
Saving an ELF object of LLVM JIT-compiled assembly for a function
Raw
elf.cpp
#include "elf.h"
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void saveELF(const char *filename, void *buf, size_t len) {
FILE *f = fopen(filename, "w");
struct elf64_hdr {
uint8_t ident[16]; /* ELF "magic number" */
uint16_t type;
uint16_t machine;
uint32_t version;
uint64_t entry; /* Entry point virtual address */
uint64_t phoff; /* Program header table file offset */
uint64_t shoff; /* Section header table file offset */
uint32_t flags;
uint16_t ehsize;
uint16_t phentsize;
uint16_t phnum;
uint16_t shentsize;
uint16_t shnum;
uint16_t shstrndx;
} header;
// N.B 17 chars long. Normal string algos will not work.
const char *strtab_contents = "\0.shstrtab\0.text\0";
memset(header.ident, 0, 16);
header.ident[0] = 0x7f;
header.ident[1] = 'E';
header.ident[2] = 'L';
header.ident[3] = 'F';
header.ident[4] = 2;
header.ident[5] = 1;
header.ident[6] = 1;
header.type = 1;
header.machine = 62;
header.version = 1;
header.entry = 0;
header.phoff = 0;
header.shoff = 64 + len + 17;
header.flags = 0;
header.ehsize = 64;
header.phentsize = 0;
header.phnum = 0;
header.shentsize = 64;
header.shnum = 3;
header.shstrndx = 1;
struct elf64_shdr {
uint32_t name; /* Section name, index in string tbl */
uint32_t type; /* Type of section */
uint64_t flags; /* Miscellaneous section attributes */
uint64_t addr; /* Section virtual addr at execution */
uint64_t offset; /* Section file offset */
uint64_t size; /* Size of section in bytes */
uint32_t link; /* Index of another section */
uint32_t info; /* Additional section information */
uint64_t addralign; /* Section alignment */
uint64_t entsize; /* Entry size if section holds table */
} zero, strtab, code;
memset(&zero, 0, sizeof(zero));
strtab.name = 1;
strtab.type = 3;
strtab.flags = 0;
strtab.addr = 0;
strtab.offset = sizeof(header) + len;
strtab.size = 17;
strtab.link = 0;
strtab.info = 0;
strtab.addralign = 1;
strtab.entsize = 0;
code.name = 11;
code.type = 1;
code.flags = 6;
code.addr = 0;
code.offset = sizeof(header);
code.size = len;
code.link = 0;
code.info = 0;
code.addralign = 4;
code.entsize = 0;
fwrite(&header, 1, 64, f);
fwrite(buf, 1, len, f);
fwrite(strtab_contents, 1, 17, f);
fwrite(&zero, 1, 64, f);
fwrite(&strtab, 1, 64, f);
fwrite(&code, 1, 64, f);
fclose(f);
}
Raw
elf.h
#ifndef ELF_H
#define ELF_H
#include <stdlib.h>
void saveELF(const char *filename, void *buf, size_t len);
#endif
Raw
usage.cpp
void *ptr = ee->getPointerToFunction(myJITEdFunction);
printf("dumping machine code to file...\n");
saveELF("generated.o", ptr, 8192); // this length is arbitrary, just guaranteed to be at least as big as our code pages
printf("Done dumping machine code to file\n");
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment