How to replace a dependency shared object of an ELF executable

gistfile1.txt

/*
 * Copyright (c) 2016 Dima Krasner
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/mman.h>
#include <elf.h>

int main(int argc, char *argv[])
{
	struct stat stbuf;
	unsigned char *data, *sectend;
	const Elf32_Ehdr *hdr;
	const Elf32_Shdr *sect, *dynamic, *dynstr;
	Elf32_Dyn *ent;
	char *strtab, *lib;
	Elf32_Half i;
	int fd, found;

	fd = open(argv[1], O_RDWR);
	if (fd < 0)
		return EXIT_FAILURE;

	if (fstat(fd, &stbuf) < 0) {
		close(fd);
		return EXIT_FAILURE;
	}

	data = mmap(NULL,
	            (size_t)stbuf.st_size,
	            PROT_READ | PROT_WRITE,
	            MAP_SHARED,
	            fd,
	            0);
	if (!data) {
		close(fd);
		return EXIT_FAILURE;
	}

	hdr = (const Elf32_Ehdr *)data;
	sect = (const Elf32_Shdr *)(data + hdr->e_shoff);
	/* this points to the section header string table - it contains the section
	 * names */
	strtab = (char *)(data + sect[hdr->e_shstrndx].sh_offset);

	/* iterate over all sections until we find the .dynstr and .dynamic
	 * sections */
	dynamic = dynstr = NULL;
	for (i = 0; i < hdr->e_shnum; ++i) {
		if (!dynstr && strcmp(strtab + sect[i].sh_name, ".dynstr") == 0)
			dynstr = &sect[i];

		if (!dynamic && sect[i].sh_type == SHT_DYNAMIC)
			dynamic = &sect[i	];

		if (dynamic && dynstr)
			goto found;
	}

	munmap(data, (size_t)stbuf.st_size);
	close(fd);
	return EXIT_FAILURE;

found:
	ent = (Elf32_Dyn *)(data + dynamic->sh_offset);
	sectend = (unsigned char *)(data + dynamic->sh_offset + dynamic->sh_size);
	found = 0;
	do {
		/* iterate over all entries in the .dynamic section - these are sonames
		 * of dependency shared objects */
		if (ent->d_tag == DT_NEEDED) {
			/* this points to the soname */
			lib = (char *)(data + dynstr->sh_offset + ent->d_un.d_val);

			if (strcmp(lib, argv[2]) == 0) {
				strcpy(lib, argv[3]);
				found = 1;
				break;
			}
		}

		++ent;
	} while ((unsigned char *)ent < sectend);

	munmap(data, (size_t)stbuf.st_size);
	close(fd);
	return found ? EXIT_SUCCESS : EXIT_FAILURE;
}