experimental dynamic linker!

some notes:
- library search path is hard coded
- x86_64 code is untested and may not work
- dlopen/dlsym is not yet implemented
- relocations in read-only memory won't work

arch/i386/reloc.h

+#include <string.h>
+#include <elf.h>
+
+#define IS_COPY(x) ((x)==R_386_COPY)
+
+static inline void do_single_reloc(size_t *reloc_addr, int type, size_t sym_val, size_t sym_size, unsigned char *base_addr, size_t addend)
+{
+	switch(type) {
+	case R_386_32:
+		*reloc_addr += sym_val;
+		break;
+	case R_386_PC32:
+		*reloc_addr += sym_val - (size_t)reloc_addr;
+		break;
+	case R_386_GLOB_DAT:
+	case R_386_JMP_SLOT:
+		*reloc_addr = sym_val;
+		break;
+	case R_386_RELATIVE:
+		*reloc_addr += (size_t)base_addr;
+		break;
+	case R_386_COPY:
+		memcpy(reloc_addr, (void *)sym_val, sym_size);
+		break;
+	}
+}

arch/x86_64/reloc.h

+#include <stdint.h>
+#include <string.h>
+#include <elf.h>
+
+#define IS_COPY(x) ((x)==R_X86_64_COPY)
+
+static inline void do_single_reloc(size_t *reloc_addr, int type, size_t sym_val, size_t sym_size, unsigned char *base_addr, size_t addend)
+{
+	switch(type) {
+	case R_X86_64_GLOB_DAT:
+	case R_X86_64_JUMP_SLOT:
+	case R_X86_64_64:
+		*reloc_addr = sym_val + addend;
+		break;
+	case R_X86_64_32:
+		*(uint32_t *)reloc_addr = sym_val + addend;
+		break;
+	case R_X86_64_PC32:
+		*reloc_addr = sym_val + addend - (size_t)reloc_addr + (size_t)base_addr;
+		break;
+	case R_X86_64_RELATIVE:
+		*reloc_addr = (size_t)base_addr + addend;
+		break;
+	case R_X86_64_COPY:
+		memcpy(reloc_addr, (void *)sym_val, sym_size);
+		break;
+	}
+}

src/ldso/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;
+	size_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;
+
+#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, size_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 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 {
+		static const char path[] = "/lib/\0/usr/local/lib/\0/usr/lib/\0";
+		const char *s;
+		char buf[NAME_MAX+32];
+		if (strlen(name) > NAME_MAX || strchr(name, '/')) return 0;
+		for (s=path; *s; s+=strlen(s)+1) {
+			strcpy(buf, s);
+			strcat(buf, name);
+			if ((fd = open(buf, O_RDONLY))>=0) break;
+		}
+	}
+	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);
+	}
+}
+
+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++);
+	auxv = (void *)(argv+i+1);
+
+	decode_vec(auxv, aux, AUX_CNT);
+
+	/* 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);
+
+	errno = 0;
+	return (void *)aux[AT_ENTRY];
+}

src/ldso/i386/start.s

+.text
+.global _start
+_start:
+	xorl %ebp,%ebp
+	popl %ecx
+	movl %esp,%eax
+	andl $-16,%esp
+	movl %eax,%esi
+	movl %ecx,%edi
+	pushl %ebp
+	call 1f
+1:	addl $_GLOBAL_OFFSET_TABLE_,(%esp)
+	pushl %eax
+	pushl %ecx
+	call __dynlink
+	movl %esi,%esp
+	pushl %edi
+	xorl %edx,%edx
+	jmp *%eax

src/ldso/x86_64/start.s

+.text
+.global _start
+_start:
+	mov (%rsp),%rdi
+	lea 8(%rsp),%rsi
+	lea _GLOBAL_OFFSET_TABLE_(%rip),%rdx
+	call __dynlink
+	jmp *%rax