c_cpp 根据标头中的信息计算磁盘上ELF文件的大小

Posted 2021-05-14

/*

Compile with:

gcc elfsize.c -o elfsize

Example:

ls -l						126584

Calculation using the values also reported by readelf -h:

Start of section headers	e_shoff		124728
Size of section headers		e_shentsize	64
Number of section headers	e_shnum		29

e_shoff + ( e_shentsize * e_shnum ) =		126584

*/

#include <elf.h>
#include <byteswap.h>
#include <stdio.h>
#include <stdint.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>

typedef Elf32_Nhdr Elf_Nhdr;

static char *fname;
static Elf64_Ehdr ehdr;
static Elf64_Phdr *phdr;

#if __BYTE_ORDER == __LITTLE_ENDIAN
#define ELFDATANATIVE ELFDATA2LSB
#elif __BYTE_ORDER == __BIG_ENDIAN
#define ELFDATANATIVE ELFDATA2MSB
#else
#error "Unknown machine endian"
#endif

static uint16_t file16_to_cpu(uint16_t val)
{
	if (ehdr.e_ident[EI_DATA] != ELFDATANATIVE)
		val = bswap_16(val);
	return val;
}

static uint32_t file32_to_cpu(uint32_t val)
{
	if (ehdr.e_ident[EI_DATA] != ELFDATANATIVE)
		val = bswap_32(val);
	return val;
}

static uint64_t file64_to_cpu(uint64_t val)
{
	if (ehdr.e_ident[EI_DATA] != ELFDATANATIVE)
		val = bswap_64(val);
	return val;
}

static long unsigned int read_elf32(int fd)
{
	Elf32_Ehdr ehdr32;
	ssize_t ret, i;

	ret = pread(fd, &ehdr32, sizeof(ehdr32), 0);
	if (ret < 0 || (size_t)ret != sizeof(ehdr)) {
		fprintf(stderr, "Read of ELF header from %s failed: %s\n",
			fname, strerror(errno));
		exit(10);
	}

	ehdr.e_shoff		= file32_to_cpu(ehdr32.e_shoff);
	ehdr.e_shentsize	= file16_to_cpu(ehdr32.e_shentsize);
	ehdr.e_shnum		= file16_to_cpu(ehdr32.e_shnum);

	return(ehdr.e_shoff + (ehdr.e_shentsize * ehdr.e_shnum));
}

static long unsigned int read_elf64(int fd)
{
	Elf64_Ehdr ehdr64;
	ssize_t ret, i;

	ret = pread(fd, &ehdr64, sizeof(ehdr64), 0);
	if (ret < 0 || (size_t)ret != sizeof(ehdr)) {
		fprintf(stderr, "Read of ELF header from %s failed: %s\n",
			fname, strerror(errno));
		exit(10);
	}

	ehdr.e_shoff		= file64_to_cpu(ehdr64.e_shoff);
	ehdr.e_shentsize	= file16_to_cpu(ehdr64.e_shentsize);
	ehdr.e_shnum		= file16_to_cpu(ehdr64.e_shnum);

	return(ehdr.e_shoff + (ehdr.e_shentsize * ehdr.e_shnum));
}

long unsigned int get_elf_size(char *fname)
/* TODO, FIXME: This assumes that the section header table (SHT) is
the last part of the ELF. This is usually the case but
it could also be that the last section is the last part
of the ELF. This should be checked for.
*/
{
	ssize_t ret;
	int fd;
	long unsigned int size = 0;

	fd = open(fname, O_RDONLY);
	if (fd < 0) {
		fprintf(stderr, "Cannot open %s: %s\n",
			fname, strerror(errno));
		return(1);
	}
	ret = pread(fd, ehdr.e_ident, EI_NIDENT, 0);
	if (ret != EI_NIDENT) {
		fprintf(stderr, "Read of e_ident from %s failed: %s\n",
			fname, strerror(errno));
		return(1);
	}
	if ((ehdr.e_ident[EI_DATA] != ELFDATA2LSB) &&
	    (ehdr.e_ident[EI_DATA] != ELFDATA2MSB))
	{
		fprintf(stderr, "Unkown ELF data order %u\n",
			ehdr.e_ident[EI_DATA]);
		return(1);
	}
	if(ehdr.e_ident[EI_CLASS] == ELFCLASS32) {
		size = read_elf32(fd);
	} else if(ehdr.e_ident[EI_CLASS] == ELFCLASS64) {
		size = read_elf64(fd);
	} else {
		fprintf(stderr, "Unknown ELF class %u\n", ehdr.e_ident[EI_CLASS]);
		return(1);
	}

	close(fd);
	return size;
}

int main(int argc, char **argv)
{
	ssize_t ret;
	int fd;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s <ELF>\n", argv[0]);
		return 1;
	}
	fname = argv[1];

	long unsigned int size = get_elf_size(fname);
	fprintf(stderr, "Estimated ELF size on disk: %lu bytes \n", size);
	return 0;
}