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

diff --git a/arch/i386/reloc.h b/arch/i386/reloc.h
new file mode 100644 (file)
index 0000000..02b5fa0
--- /dev/null
+++ b/arch/i386/reloc.h
@@ -0,0 +1,26 @@
+#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;
+       }
+}

diff --git a/arch/x86_64/reloc.h b/arch/x86_64/reloc.h
new file mode 100644 (file)
index 0000000..2db6115
--- /dev/null
+++ b/arch/x86_64/reloc.h
@@ -0,0 +1,28 @@
+#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;
+       }
+}

diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
new file mode 100644 (file)
index 0000000..b444425
--- /dev/null
+++ b/src/ldso/dynlink.c
@@ -0,0 +1,385 @@
+#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];
+}

diff --git a/src/ldso/i386/start.s b/src/ldso/i386/start.s
new file mode 100644 (file)
index 0000000..47c2748
--- /dev/null
+++ b/src/ldso/i386/start.s
@@ -0,0 +1,19 @@
+.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

diff --git a/src/ldso/x86_64/start.s b/src/ldso/x86_64/start.s
new file mode 100644 (file)
index 0000000..efb2745
--- /dev/null
+++ b/src/ldso/x86_64/start.s
@@ -0,0 +1,8 @@
+.text
+.global _start
+_start:
+       mov (%rsp),%rdi
+       lea 8(%rsp),%rsi
+       lea _GLOBAL_OFFSET_TABLE_(%rip),%rdx
+       call __dynlink
+       jmp *%rax