dynlink.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdint.h>
#include <elf.h>
#include <sys/mman.h>
#include <limits.h>
#include <stdint.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>
#include <limits.h>
#include <elf.h>

#include "reloc.h"

#if ULONG_MAX == 0xffffffff
typedef Elf32_Ehdr Ehdr;
typedef Elf32_Phdr Phdr;
typedef Elf32_Sym Sym;
#define R_TYPE(x) ((x)&255)
#define R_SYM(x) ((x)>>8)
#else
typedef Elf64_Ehdr Ehdr;
typedef Elf64_Phdr Phdr;
typedef Elf64_Sym Sym;
#define R_TYPE(x) ((x)&0xffffffff)
#define R_SYM(x) ((x)>>32)
#endif

struct dso
{
	struct dso *next, *prev;
	int refcnt;
	size_t *dynv;
	Sym *syms;
	uint32_t *hashtab;
	char *strings;
	unsigned char *base;
	unsigned char *map;
	size_t map_len;
	dev_t dev;
	ino_t ino;
	int global;
	int relocated;
	char name[];
};

static struct dso *head, *tail, *libc;
static char *env_path, *sys_path;

#define AUX_CNT 15
#define DYN_CNT 34

static void decode_vec(size_t *v, size_t *a, size_t cnt)
{
	memset(a, 0, cnt*sizeof(size_t));
	for (; v[0]; v+=2) if (v[0]<cnt) {
		a[0] |= 1ULL<<v[0];
		a[v[0]] = v[1];
	}
}

static uint32_t hash(const char *s)
{
	uint_fast32_t h = 0;
	while (*s) {
		h = 16*h + *s++;
		h ^= h>>24 & 0xf0;
	}
	return h & 0xfffffff;
}

static Sym *lookup(const char *s, uint32_t h, Sym *syms, uint32_t *hashtab, char *strings)
{
	size_t i;
	for (i=hashtab[2+h%hashtab[0]]; i; i=hashtab[2+hashtab[0]+i]) {
		if (!strcmp(s, strings+syms[i].st_name))
			return syms+i;
	}
	return 0;
}

#define OK_TYPES (1<<STT_NOTYPE | 1<<STT_OBJECT | 1<<STT_FUNC | 1<<STT_COMMON)

static void *find_sym(struct dso *dso, const char *s, int need_def)
{
	uint32_t h = hash(s);
	for (; dso; dso=dso->next) {
		Sym *sym = lookup(s, h, dso->syms, dso->hashtab, dso->strings);
		if (sym && (!need_def || sym->st_shndx) && sym->st_value
		 && (1<<(sym->st_info&0xf) & OK_TYPES))
			return dso->base + sym->st_value;
	}
	return 0;
}

static void do_relocs(unsigned char *base, size_t *rel, size_t rel_size, size_t stride, Sym *syms, char *strings, struct dso *dso)
{
	Sym *sym;
	const char *name;
	size_t sym_val, sym_size;
	size_t *reloc_addr;
	void *ctx;
	int type;
	int sym_index;

	for (; rel_size; rel+=stride, rel_size-=stride*sizeof(size_t)) {
		reloc_addr = (void *)(base + rel[0]);
		type = R_TYPE(rel[1]);
		sym_index = R_SYM(rel[1]);
		if (sym_index) {
			sym = syms + sym_index;
			name = strings + sym->st_name;
			ctx = IS_COPY(type) ? dso->next : dso;
			sym_val = (size_t)find_sym(ctx, name, 1);
			sym_size = sym->st_size;
		}
		do_single_reloc(reloc_addr, type, sym_val, sym_size, base, rel[2]);
	}
}

static void *map_library(int fd, size_t *lenp, unsigned char **basep, size_t *dynp)
{
	size_t buf[896/sizeof(size_t)];
	size_t phsize;
	size_t addr_min=SIZE_MAX, addr_max=0, map_len;
	size_t this_min, this_max;
	off_t off_start;
	Ehdr *eh;
	Phdr *ph;
	unsigned prot;
	unsigned char *map, *base;
	size_t dyn;
	size_t i;

	ssize_t l = read(fd, buf, sizeof buf);
	if (l<sizeof *eh) return 0;
	eh = (void *)buf;
	phsize = eh->e_phentsize * eh->e_phnum;
	if (phsize + sizeof *eh > l) return 0;
	if (eh->e_phoff + phsize > l) {
		l = pread(fd, buf+sizeof *eh, phsize, eh->e_phoff);
		if (l != phsize) return 0;
		eh->e_phoff = sizeof *eh;
	}
	ph = (void *)((char *)buf + eh->e_phoff);
	for (i=eh->e_phnum; i; i--, ph=(void *)((char *)ph+eh->e_phentsize)) {
		if (ph->p_type == PT_DYNAMIC)
			dyn = ph->p_vaddr;
		if (ph->p_type != PT_LOAD) continue;
		if (ph->p_vaddr < addr_min) {
			addr_min = ph->p_vaddr;
			off_start = ph->p_offset;
			prot = (((ph->p_flags&PF_R) ? PROT_READ : 0) |
				((ph->p_flags&PF_W) ? PROT_WRITE: 0) |
				((ph->p_flags&PF_X) ? PROT_EXEC : 0));
		}
		if (ph->p_vaddr+ph->p_memsz > addr_max) {
			addr_max = ph->p_vaddr+ph->p_memsz;
		}
	}
	if (!dyn) return 0;
	addr_max += PAGE_SIZE-1;
	addr_max &= -PAGE_SIZE;
	addr_min &= -PAGE_SIZE;
	off_start &= -PAGE_SIZE;
	map_len = addr_max - addr_min + off_start;
	/* The first time, we map too much, possibly even more than
	 * the length of the file. This is okay because we will not
	 * use the invalid part; we just need to reserve the right
	 * amount of virtual address space to map over later. */
	map = mmap(0, map_len, prot, MAP_PRIVATE, fd, off_start);
	if (map==MAP_FAILED) return 0;
	base = map - addr_min;
	ph = (void *)((char *)buf + eh->e_phoff);
	for (i=eh->e_phnum; i; i--, ph=(void *)((char *)ph+eh->e_phentsize)) {
		if (ph->p_type != PT_LOAD) continue;
		/* Reuse the existing mapping for the lowest-address LOAD */
		if ((ph->p_vaddr & -PAGE_SIZE) == addr_min) continue;
		this_min = ph->p_vaddr & -PAGE_SIZE;
		this_max = ph->p_vaddr+ph->p_memsz+PAGE_SIZE-1 & -PAGE_SIZE;
		off_start = ph->p_offset & -PAGE_SIZE;
		prot = (((ph->p_flags&PF_R) ? PROT_READ : 0) |
			((ph->p_flags&PF_W) ? PROT_WRITE: 0) |
			((ph->p_flags&PF_X) ? PROT_EXEC : 0));
		if (mmap(base+this_min, this_max-this_min, prot, MAP_PRIVATE|MAP_FIXED, fd, off_start) == MAP_FAILED) {
			munmap(map, map_len);
			return 0;
		}
		if (ph->p_memsz > ph->p_filesz) {
			size_t brk = (size_t)base+ph->p_vaddr+ph->p_filesz;
			size_t pgbrk = brk+PAGE_SIZE-1 & -PAGE_SIZE;
			memset((void *)brk, 0, pgbrk-brk & PAGE_SIZE-1);
			if (pgbrk-(size_t)base < this_max && mmap((void *)pgbrk, (size_t)base+this_max-pgbrk, prot, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) == MAP_FAILED) {
				munmap(map, map_len);
				return 0;
			}
		}
	}
	*lenp = map_len;
	*basep = base;
	*dynp = dyn;
	return map;
}

static int path_open(const char *name, const char *search)
{
	char buf[2*NAME_MAX+2];
	const char *s, *z;
	int l, fd;
	for (s=search; *s; s+=l+!!z) {
		z = strchr(s, ':');
		l = z ? z-s : strlen(s);
		snprintf(buf, sizeof buf, "%.*s/%s", l, s, name);
		if ((fd = open(buf, O_RDONLY))>=0) return fd;
	}
	return -1;
}

static struct dso *load_library(const char *name)
{
	unsigned char *base, *map;
	size_t dyno, map_len;
	struct dso *p;
	size_t dyn[DYN_CNT] = {0};
	int fd;
	struct stat st;

	/* Catch and block attempts to reload the implementation itself */
	if (name[0]=='l' && name[1]=='i' && name[2]=='b') {
		static const char *rp, reserved[] =
			"c\0pthread\0rt\0m\0dl\0util\0xnet\0";
		char *z = strchr(name, '.');
		if (z) {
			size_t l = z-name;
			for (rp=reserved; *rp && memcmp(name+3, rp, l-3); rp+=strlen(rp)+1);
			if (*rp) {
				if (!libc->prev) {
					tail->next = libc;
					libc->prev = tail;
					tail = libc;
				}
				return libc;
			}
		}
	}
	/* Search for the name to see if it's already loaded */
	for (p=head->next; p; p=p->next) {
		if (!strcmp(p->name, name)) {
			p->refcnt++;
			return p;
		}
	}
	if (name[0] == '/') {
		fd = open(name, O_RDONLY);
	} else {
		if (strlen(name) > NAME_MAX || strchr(name, '/')) return 0;
		fd = -1;
		if (env_path) fd = path_open(name, env_path);
		if (fd < 0) {
			if (!sys_path) {
				FILE *f = fopen(ETC_LDSO_PATH, "r");
				if (f) {
					if (getline(&sys_path, (size_t[1]){0}, f) > 0)
						sys_path[strlen(sys_path)-1]=0;
					fclose(f);
				}
			}
			if (sys_path) fd = path_open(name, sys_path);
			else fd = path_open(name, "/lib:/usr/local/lib:/usr/lib");
		}
	}
	if (fd < 0) return 0;
	if (fstat(fd, &st) < 0) {
		close(fd);
		return 0;
	}
	for (p=head->next; p; p=p->next) {
		if (p->dev == st.st_dev && p->ino == st.st_ino) {
			close(fd);
			p->refcnt++;
			return p;
		}
	}
	map = map_library(fd, &map_len, &base, &dyno);
	close(fd);
	if (!map) return 0;
	p = calloc(1, sizeof *p + strlen(name) + 1);
	if (!p) {
		munmap(map, map_len);
		return 0;
	}

	p->map = map;
	p->map_len = map_len;
	p->base = base;
	p->dynv = (void *)(base + dyno);
	decode_vec(p->dynv, dyn, DYN_CNT);

	p->syms = (void *)(base + dyn[DT_SYMTAB]);
	p->hashtab = (void *)(base + dyn[DT_HASH]);
	p->strings = (void *)(base + dyn[DT_STRTAB]);
	p->dev = st.st_dev;
	p->ino = st.st_ino;
	p->global = 1;
	p->refcnt = 1;
	strcpy(p->name, name);

	tail->next = p;
	p->prev = tail;
	tail = p;

	return p;
}

static void load_deps(struct dso *p)
{
	size_t i;
	for (; p; p=p->next) {
		for (i=0; p->dynv[i]; i+=2) {
			if (p->dynv[i] != DT_NEEDED) continue;
			load_library(p->strings + p->dynv[i+1]);
		}
	}
}

static void reloc_all(struct dso *p)
{
	size_t dyn[DYN_CNT] = {0};
	for (; p; p=p->next) {
		if (p->relocated) continue;
		decode_vec(p->dynv, dyn, DYN_CNT);
		do_relocs(p->base, (void *)(p->base+dyn[DT_JMPREL]), dyn[DT_PLTRELSZ],
			2+(dyn[DT_PLTREL]==DT_RELA), p->syms, p->strings, p);
		do_relocs(p->base, (void *)(p->base+dyn[DT_REL]), dyn[DT_RELSZ],
			2, p->syms, p->strings, p);
		do_relocs(p->base, (void *)(p->base+dyn[DT_RELA]), dyn[DT_RELASZ],
			3, p->syms, p->strings, p);
		p->relocated = 1;
	}
}

static void free_all(struct dso *p)
{
	struct dso *n;
	while (p) {
		n = p->next;
		if (p->map) free(p);
		p = n;
	}
}

void *__dynlink(int argc, char **argv, size_t *got)
{
	size_t *auxv, aux[AUX_CNT] = {0};
	size_t app_dyn[DYN_CNT] = {0};
	size_t lib_dyn[DYN_CNT] = {0};
	size_t i;
	Phdr *phdr;
	struct dso lib, app;

	/* Find aux vector just past environ[] */
	for (i=argc+1; argv[i]; i++)
		if (!memcmp(argv[i], "LD_LIBRARY_PATH=", 16))
			env_path = argv[i]+16;
	auxv = (void *)(argv+i+1);

	decode_vec(auxv, aux, AUX_CNT);

	/* Only trust user/env if kernel says we're not suid/sgid */
	if ((aux[0]&0x7800)!=0x7800 || aux[AT_UID]!=aux[AT_EUID]
	  || aux[AT_GID]!=aux[AT_EGID]) {
		env_path = 0;
	}

	/* Relocate ldso's DYNAMIC pointer and load vector */
	decode_vec((void *)(got[0] += aux[AT_BASE]), lib_dyn, DYN_CNT);

	/* Find the program image's DYNAMIC section and decode it */
	phdr = (void *)aux[AT_PHDR];
	for (i=aux[AT_PHNUM]; i--; phdr=(void *)((char *)phdr + aux[AT_PHENT])) {
		if (phdr->p_type == PT_DYNAMIC) {
			decode_vec((void *)phdr->p_vaddr, app_dyn, DYN_CNT);
			break;
		}
	}

	app = (struct dso){
		.base = 0,
		.strings = (void *)(app_dyn[DT_STRTAB]),
		.hashtab = (void *)(app_dyn[DT_HASH]),
		.syms = (void *)(app_dyn[DT_SYMTAB]),
		.dynv = (void *)(phdr->p_vaddr),
		.next = &lib
	};

	lib = (struct dso){
		.base = (void *)aux[AT_BASE],
		.strings = (void *)(aux[AT_BASE]+lib_dyn[DT_STRTAB]),
		.hashtab = (void *)(aux[AT_BASE]+lib_dyn[DT_HASH]),
		.syms = (void *)(aux[AT_BASE]+lib_dyn[DT_SYMTAB]),
		.dynv = (void *)(got[0]),
		.relocated = 1
	};

	/* Relocate the dynamic linker/libc */
	do_relocs((void *)aux[AT_BASE], (void *)(aux[AT_BASE]+lib_dyn[DT_REL]),
		lib_dyn[DT_RELSZ], 2, lib.syms, lib.strings, &app);
	do_relocs((void *)aux[AT_BASE], (void *)(aux[AT_BASE]+lib_dyn[DT_RELA]),
		lib_dyn[DT_RELASZ], 3, lib.syms, lib.strings, &app);

	/* At this point the standard library is fully functional */

	head = tail = &app;
	libc = &lib;
	app.next = 0;
	load_deps(head);

	reloc_all(head);

	free_all(head);
	free(sys_path);

	errno = 0;
	return (void *)aux[AT_ENTRY];
}