openwrt/tools/sstrip/src/sstrip.c

/* http://www.muppetlabs.com/~breadbox/software/elfkickers.html */

/* sstrip: Copyright (C) 1999-2001 by Brian Raiter, under the GNU
 * General Public License. No warranty. See COPYING for details.
 *
 * Aug 23, 2004 Hacked by Manuel Novoa III <mjn3@codepoet.org> to
 * handle targets of different endianness and/or elf class, making
 * it more useful in a cross-devel environment.
 */

/* ============== original README ===================
 *
 * sstrip is a small utility that removes the contents at the end of an
 * ELF file that are not part of the program's memory image.
 * 
 * Most ELF executables are built with both a program header table and a
 * section header table. However, only the former is required in order
 * for the OS to load, link and execute a program. sstrip attempts to
 * extract the ELF header, the program header table, and its contents,
 * leaving everything else in the bit bucket. It can only remove parts of
 * the file that occur at the end, after the parts to be saved. However,
 * this almost always includes the section header table, and occasionally
 * a few random sections that are not used when running a program.
 * 
 * It should be noted that the GNU bfd library is (understandably)
 * dependent on the section header table as an index to the file's
 * contents. Thus, an executable file that has no section header table
 * cannot be used with gdb, objdump, or any other program based upon the
 * bfd library, at all. In fact, the program will not even recognize the
 * file as a valid executable. (This limitation is noted in the source
 * code comments for bfd, and is marked "FIXME", so this may change at
 * some future date. However, I would imagine that it is a pretty
 * low-priority item, as executables without a section header table are
 * rare in the extreme.) This probably also explains why strip doesn't
 * offer the option to do this.
 * 
 * Shared library files may also have their section header table removed.
 * Such a library will still function; however, it will no longer be
 * possible for a compiler to link a new program against it.
 * 
 * As an added bonus, sstrip also tries to removes trailing zero bytes
 * from the end of the file. (This normally cannot be done with an
 * executable that has a section header table.)
 * 
 * sstrip is a very simplistic program. It depends upon the common
 * practice of putting the parts of the file that contribute to the
 * memory image at the front, and the remaining material at the end. This
 * permits it to discard the latter material without affecting file
 * offsets and memory addresses in what remains. Of course, the ELF
 * standard permits files to be organized in almost any order, so if a
 * pathological linker decided to put its section headers at the top,
 * sstrip would be useless on such executables.
 */

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

#ifndef TRUE
#define	TRUE		1
#define	FALSE		0
#endif

/* The name of the program.
 */
static char const	*progname;

/* The name of the current file.
 */
static char const	*filename;


/* A simple error-handling function. FALSE is always returned for the
 * convenience of the caller.
 */
static int err(char const *errmsg)
{
	fprintf(stderr, "%s: %s: %s\n", progname, filename, errmsg);
	return FALSE;
}

/* A flag to signal the need for endian reversal.
 */
static int do_reverse_endian;

/* Get a value from the elf header, compensating for endianness.
 */
#define EGET(X) \
	(__extension__ ({ \
		uint64_t __res; \
		if (!do_reverse_endian) { \
			__res = (X); \
		} else if (sizeof(X) == 1) { \
			__res = (X); \
		} else if (sizeof(X) == 2) { \
			__res = bswap_16((X)); \
		} else if (sizeof(X) == 4) { \
			__res = bswap_32((X)); \
		} else if (sizeof(X) == 8) { \
			__res = bswap_64((X)); \
		} else { \
			fprintf(stderr, "%s: %s: EGET failed for size %zu\n", \
					progname, filename, sizeof(X)); \
			exit(EXIT_FAILURE); \
		} \
		__res; \
	}))

/* Set a value 'Y' in the elf header to 'X', compensating for endianness.
 */
#define ESET(Y,X) \
		do if (!do_reverse_endian) { \
			Y = (X); \
		} else if (sizeof(Y) == 1) { \
			Y = (X); \
		} else if (sizeof(Y) == 2) { \
			Y = bswap_16((uint16_t)(X)); \
		} else if (sizeof(Y) == 4) { \
			Y = bswap_32((uint32_t)(X)); \
		} else if (sizeof(Y) == 8) { \
			Y = bswap_64((uint64_t)(X)); \
		} else { \
			fprintf(stderr, "%s: %s: ESET failed for size %zu\n", \
					progname, filename, sizeof(Y)); \
			exit(EXIT_FAILURE); \
		} while (0)


/* A macro for I/O errors: The given error message is used only when
 * errno is not set.
 */
#define	ferr(msg)	(err(errno ? strerror(errno) : (msg)))



#define HEADER_FUNCTIONS(CLASS) \
 \
/* readelfheader() reads the ELF header into our global variable, and \
 * checks to make sure that this is in fact a file that we should be \
 * munging. \
 */ \
static int readelfheader ## CLASS (int fd, Elf ## CLASS ## _Ehdr *ehdr) \
{ \
	 if (read(fd, ((char *)ehdr)+EI_NIDENT, sizeof(*ehdr) - EI_NIDENT) \
		!= sizeof(*ehdr) - EI_NIDENT) \
		return ferr("missing or incomplete ELF header."); \
 \
	/* Verify the sizes of the ELF header and the program segment \
	 * header table entries. \
	 */ \
	if (EGET(ehdr->e_ehsize) != sizeof(Elf ## CLASS ## _Ehdr)) \
		return err("unrecognized ELF header size."); \
	if (EGET(ehdr->e_phentsize) != sizeof(Elf ## CLASS ## _Phdr)) \
		return err("unrecognized program segment header size."); \
 \
	/* Finally, check the file type. \
	 */ \
	if (EGET(ehdr->e_type) != ET_EXEC && EGET(ehdr->e_type) != ET_DYN) \
		return err("not an executable or shared-object library."); \
 \
	return TRUE; \
} \
 \
/* readphdrtable() loads the program segment header table into memory. \
 */ \
static int readphdrtable ## CLASS (int fd, Elf ## CLASS ## _Ehdr const *ehdr, \
								   Elf ## CLASS ## _Phdr **phdrs) \
{ \
	size_t	size; \
 \
	if (!EGET(ehdr->e_phoff) || !EGET(ehdr->e_phnum) \
)		return err("ELF file has no program header table."); \
 \
	size = EGET(ehdr->e_phnum) * sizeof **phdrs; \
	if (!(*phdrs = malloc(size))) \
		return err("Out of memory!"); \
 \
	errno = 0; \
	if (read(fd, *phdrs, size) != (ssize_t)size) \
		return ferr("missing or incomplete program segment header table."); \
 \
	return TRUE; \
} \
 \
/* getmemorysize() determines the offset of the last byte of the file \
 * that is referenced by an entry in the program segment header table. \
 * (Anything in the file after that point is not used when the program \
 * is executing, and thus can be safely discarded.) \
 */ \
static int getmemorysize ## CLASS (Elf ## CLASS ## _Ehdr const *ehdr, \
								   Elf ## CLASS ## _Phdr const *phdrs, \
						 unsigned long *newsize) \
{ \
	Elf ## CLASS ## _Phdr const   *phdr; \
	unsigned long	size, n; \
	size_t			i; \
 \
	/* Start by setting the size to include the ELF header and the \
	 * complete program segment header table. \
	 */ \
	size = EGET(ehdr->e_phoff) + EGET(ehdr->e_phnum) * sizeof *phdrs; \
	if (size < sizeof *ehdr) \
		size = sizeof *ehdr; \
 \
	/* Then keep extending the size to include whatever data the \
	 * program segment header table references. \
	 */ \
	for (i = 0, phdr = phdrs ; i < EGET(ehdr->e_phnum) ; ++i, ++phdr) { \
		if (EGET(phdr->p_type) != PT_NULL) { \
			n = EGET(phdr->p_offset) + EGET(phdr->p_filesz); \
			if (n > size) \
				size = n; \
		} \
	} \
 \
	*newsize = size; \
	return TRUE; \
} \
 \
/* modifyheaders() removes references to the section header table if \
 * it was stripped, and reduces program header table entries that \
 * included truncated bytes at the end of the file. \
 */ \
static int modifyheaders ## CLASS (Elf ## CLASS ## _Ehdr *ehdr, \
								   Elf ## CLASS ## _Phdr *phdrs, \
								   unsigned long newsize) \
{ \
	Elf ## CLASS ## _Phdr *phdr; \
	size_t		i; \
 \
	/* If the section header table is gone, then remove all references \
	 * to it in the ELF header. \
	 */ \
	if (EGET(ehdr->e_shoff) >= newsize) { \
		ESET(ehdr->e_shoff,0); \
		ESET(ehdr->e_shnum,0); \
		ESET(ehdr->e_shentsize,0); \
		ESET(ehdr->e_shstrndx,0); \
	} \
 \
	/* The program adjusts the file size of any segment that was \
	 * truncated. The case of a segment being completely stripped out \
	 * is handled separately. \
	 */ \
	for (i = 0, phdr = phdrs ; i < EGET(ehdr->e_phnum) ; ++i, ++phdr) { \
		if (EGET(phdr->p_offset) >= newsize) { \
			ESET(phdr->p_offset,newsize); \
			ESET(phdr->p_filesz,0); \
		} else if (EGET(phdr->p_offset) + EGET(phdr->p_filesz) > newsize) { \
			ESET(phdr->p_filesz, newsize - EGET(phdr->p_offset)); \
		} \
	} \
 \
	return TRUE; \
} \
 \
/* commitchanges() writes the new headers back to the original file \
 * and sets the file to its new size. \
 */ \
static int commitchanges ## CLASS (int fd, Elf ## CLASS ## _Ehdr const *ehdr, \
								   Elf ## CLASS ## _Phdr *phdrs, \
								   unsigned long newsize) \
{ \
	size_t	n; \
 \
	/* Save the changes to the ELF header, if any. \
	 */ \
	if (lseek(fd, 0, SEEK_SET)) \
		return ferr("could not rewind file"); \
	errno = 0; \
	if (write(fd, ehdr, sizeof *ehdr) != sizeof *ehdr) \
		return err("could not modify file"); \
 \
	/* Save the changes to the program segment header table, if any. \
	 */ \
	if (lseek(fd, EGET(ehdr->e_phoff), SEEK_SET) == (off_t)-1) { \
		err("could not seek in file."); \
		goto warning; \
	} \
	n = EGET(ehdr->e_phnum) * sizeof *phdrs; \
	if (write(fd, phdrs, n) != (ssize_t)n) { \
		err("could not write to file"); \
		goto warning; \
	} \
 \
	/* Eleventh-hour sanity check: don't truncate before the end of \
	 * the program segment header table. \
	 */ \
	if (newsize < EGET(ehdr->e_phoff) + n) \
		newsize = EGET(ehdr->e_phoff) + n; \
 \
	/* Chop off the end of the file. \
	 */ \
	if (ftruncate(fd, newsize)) { \
		err("could not resize file"); \
		goto warning; \
	} \
 \
	return TRUE; \
 \
 warning: \
	return err("ELF file may have been corrupted!"); \
}


/* First elements of Elf32_Ehdr and Elf64_Ehdr are common.
 */
static int readelfheaderident(int fd, Elf32_Ehdr *ehdr)
{
	errno = 0;
	if (read(fd, ehdr, EI_NIDENT) != EI_NIDENT)
		return ferr("missing or incomplete ELF header.");

	/* Check the ELF signature.
	 */
	if (!(ehdr->e_ident[EI_MAG0] == ELFMAG0 &&
		  ehdr->e_ident[EI_MAG1] == ELFMAG1 &&
		  ehdr->e_ident[EI_MAG2] == ELFMAG2 &&
		  ehdr->e_ident[EI_MAG3] == ELFMAG3))
	{
		err("missing ELF signature.");
		return -1;
	}

	/* Compare the file's class and endianness with the program's.
	 */
#if __BYTE_ORDER == __LITTLE_ENDIAN
	if (ehdr->e_ident[EI_DATA] == ELFDATA2LSB) {
		do_reverse_endian = 0;
	} else if (ehdr->e_ident[EI_DATA] == ELFDATA2MSB) {
/* 		fprintf(stderr, "ELF file has different endianness.\n"); */
		do_reverse_endian = 1;
	}
#elif __BYTE_ORDER == __BIG_ENDIAN
	if (ehdr->e_ident[EI_DATA] == ELFDATA2LSB) {
/* 		fprintf(stderr, "ELF file has different endianness.\n"); */
		do_reverse_endian = 1;
	} else if (ehdr->e_ident[EI_DATA] == ELFDATA2MSB) {
		do_reverse_endian = 0;
	}
#else
#error unkown endianness
#endif
	else {
		err("Unsupported endianness");
		return -1;
	}

	/* Check the target architecture.
	 */
/*	 if (EGET(ehdr->e_machine) != ELF_ARCH) { */
/* 		/\* return err("ELF file created for different architecture."); *\/ */
/* 		fprintf(stderr, "ELF file created for different architecture.\n"); */
/* 	} */
	return ehdr->e_ident[EI_CLASS];
}


HEADER_FUNCTIONS(32)

HEADER_FUNCTIONS(64)

/* truncatezeros() examines the bytes at the end of the file's
 * size-to-be, and reduces the size to exclude any trailing zero
 * bytes.
 */
static int truncatezeros(int fd, unsigned long *newsize)
{
	unsigned char	contents[1024];
	unsigned long	size, n;

	size = *newsize;
	do {
		n = sizeof contents;
		if (n > size)
			n = size;
		if (lseek(fd, size - n, SEEK_SET) == (off_t)-1)
			return ferr("cannot seek in file.");
		if (read(fd, contents, n) != (ssize_t)n)
			return ferr("cannot read file contents");
		while (n && !contents[--n])
			--size;
	} while (size && !n);

	/* Sanity check.
	 */
	if (!size)
		return err("ELF file is completely blank!");

	*newsize = size;
	return TRUE;
}

/* main() loops over the cmdline arguments, leaving all the real work
 * to the other functions.
 */
int main(int argc, char *argv[])
{
	int				fd;
	union {
		Elf32_Ehdr	ehdr32;
		Elf64_Ehdr	ehdr64;
	} e;
	union {
		Elf32_Phdr	*phdrs32;
		Elf64_Phdr	*phdrs64;
	} p;
	unsigned long	newsize;
	char			**arg;
	int				failures = 0;

	if (argc < 2 || argv[1][0] == '-') {
		printf("Usage: sstrip FILE...\n"
			   "sstrip discards all nonessential bytes from an executable.\n\n"
			   "Version 2.0-X Copyright (C) 2000,2001 Brian Raiter.\n"
			   "Cross-devel hacks Copyright (C) 2004 Manuel Novoa III.\n"
			   "This program is free software, licensed under the GNU\n"
			   "General Public License. There is absolutely no warranty.\n");
		return EXIT_SUCCESS;
	}

	progname = argv[0];

	for (arg = argv + 1 ; *arg != NULL ; ++arg) {
		filename = *arg;

		fd = open(*arg, O_RDWR);
		if (fd < 0) {
			ferr("can't open");
			++failures;
			continue;
		}

		switch (readelfheaderident(fd, &e.ehdr32)) {
			case ELFCLASS32:
				if (!(readelfheader32(fd, &e.ehdr32)					&&
					  readphdrtable32(fd, &e.ehdr32, &p.phdrs32)		&&
					  getmemorysize32(&e.ehdr32, p.phdrs32, &newsize)	&&
					  truncatezeros(fd, &newsize)						&&
					  modifyheaders32(&e.ehdr32, p.phdrs32, newsize)	&&
					  commitchanges32(fd, &e.ehdr32, p.phdrs32, newsize)))
					++failures;
				break;
			case ELFCLASS64:
				if (!(readelfheader64(fd, &e.ehdr64)					&&
					  readphdrtable64(fd, &e.ehdr64, &p.phdrs64)		&&
					  getmemorysize64(&e.ehdr64, p.phdrs64, &newsize)	&&
					  truncatezeros(fd, &newsize)						&&
					  modifyheaders64(&e.ehdr64, p.phdrs64, newsize)	&&
					  commitchanges64(fd, &e.ehdr64, p.phdrs64, newsize)))
					++failures;
				break;
			default:
				++failures;
				break;
		}
		close(fd);
	}

	return failures ? EXIT_FAILURE : EXIT_SUCCESS;
}