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Revamp the symbol lookup algorithm to cope better with objects

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loaded separately by dlopen that have global symbols with identical
names.  Viewing each dlopened object as a DAG which is linked by its
DT_NEEDED entries in the dynamic table, the search order is as
follows:

  * If the referencing object was linked with -Bsymbolic, search it
    internally.
  * Search all dlopened DAGs containing the referencing object.
  * Search all objects loaded at program start up.
  * Search all objects which were dlopened() using the RTLD_GLOBAL
    flag (which is now supported too).

The search terminates as soon as a strong definition is found.
Lacking that, the first weak definition is used.

These rules match those of Solaris, as best I could determine them
from its vague manual pages and the results of experiments I performed.

PR:		misc/12438
This commit is contained in:
John Polstra 1999-08-30 01:48:19 +00:00
parent 7326e0b620
commit 926ea445fe
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=50608
4 changed files with 249 additions and 86 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
libexec/rtld-elf/alpha/reloc.c
View File

@ -60,8 +60,7 @@ extern Elf_Dyn _DYNAMIC;
/* Relocate a non-PLT object with addend. */
static int
reloc_non_plt_obj(Obj_Entry *obj_rtld, const Obj_Entry *obj,
const Elf_Rela *rela)
reloc_non_plt_obj(Obj_Entry *obj_rtld, Obj_Entry *obj, const Elf_Rela *rela)
{
Elf_Addr *where = (Elf_Addr *) (obj->relocbase + rela->r_offset);

38
libexec/rtld-elf/map_object.c
View File

@ -30,6 +30,7 @@
#include <errno.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
@ -226,7 +227,7 @@ map_object(int fd, const char *path)
}
}
obj = CNEW(Obj_Entry);
obj = obj_new();
obj->mapbase = mapbase;
obj->mapsize = mapsize;
obj->textsize = round_page(segs[0]->p_vaddr + segs[0]->p_memsz) -
@ -246,6 +247,41 @@ map_object(int fd, const char *path)
return obj;
}
void
obj_free(Obj_Entry *obj)
{
Objlist_Entry *elm;
free(obj->path);
while (obj->needed != NULL) {
Needed_Entry *needed = obj->needed;
obj->needed = needed->next;
free(needed);
}
while (!STAILQ_EMPTY(&obj->dldags)) {
elm = STAILQ_FIRST(&obj->dldags);
STAILQ_REMOVE_HEAD(&obj->dldags, link);
free(elm);
}
while (!STAILQ_EMPTY(&obj->dagmembers)) {
elm = STAILQ_FIRST(&obj->dagmembers);
STAILQ_REMOVE_HEAD(&obj->dagmembers, link);
free(elm);
}
free(obj);
}
Obj_Entry *
obj_new(void)
{
Obj_Entry *obj;
obj = CNEW(Obj_Entry);
STAILQ_INIT(&obj->dldags);
STAILQ_INIT(&obj->dagmembers);
return obj;
}
/*
* Given a set of ELF protection flags, return the corresponding protection
* flags for MMAP.

277
libexec/rtld-elf/rtld.c
View File

@ -60,6 +60,10 @@
#define msg(s) (write(1, s, strlen(s)))
#define trace() msg("trace: " __XSTRING(__LINE__) "\n");
#ifndef _PATH_RTLD
#define _PATH_RTLD "/usr/libexec/ld-elf.so.1"
#endif
#define END_SYM "_end"
/* Types. */
@ -77,6 +81,8 @@ static Obj_Entry *digest_phdr(const Elf_Phdr *, int, caddr_t, const char *);
static Obj_Entry *dlcheck(void *);
static char *find_library(const char *, const Obj_Entry *);
static const char *gethints(void);
static void init_dag(Obj_Entry *);
static void init_dag1(Obj_Entry *root, Obj_Entry *obj);
static void init_rtld(caddr_t);
static bool is_exported(const Elf_Sym *);
static void linkmap_add(Obj_Entry *);
@ -85,13 +91,18 @@ static int load_needed_objects(Obj_Entry *);
static int load_preload_objects(void);
static Obj_Entry *load_object(char *);
static Obj_Entry *obj_from_addr(const void *);
static void objlist_add(Objlist *, Obj_Entry *);
static Objlist_Entry *objlist_find(Objlist *, const Obj_Entry *);
static void objlist_remove(Objlist *, Obj_Entry *);
static int relocate_objects(Obj_Entry *, bool);
static void rtld_exit(void);
static char *search_library_path(const char *, const char *);
static void set_program_var(const char *, const void *);
static const Elf_Sym *symlook_list(const char *, unsigned long,
Objlist *, const Obj_Entry **, bool in_plt);
static void trace_loaded_objects(Obj_Entry *obj);
static void unload_object(Obj_Entry *, bool do_fini_funcs);
static void unref_object_dag(Obj_Entry *);
static void unref_dag(Obj_Entry *);
void r_debug_state(void);
void xprintf(const char *, ...);
@ -108,12 +119,16 @@ static char *ld_library_path; /* Environment variable for search path */
static char *ld_preload; /* Environment variable for libraries to
load first */
static char *ld_tracing; /* Called from ldd to print libs */
static Obj_Entry **main_tail; /* Value of obj_tail after loading main and
its needed shared libraries */
static Obj_Entry *obj_list; /* Head of linked list of shared objects */
static Obj_Entry **obj_tail; /* Link field of last object in list */
static Obj_Entry *obj_main; /* The main program shared object */
static Obj_Entry obj_rtld; /* The dynamic linker shared object */
static unsigned long curmark; /* Current mark value */
static Objlist list_global = /* Objects dlopened with RTLD_GLOBAL */
STAILQ_HEAD_INITIALIZER(list_global);
static Objlist list_main = /* Objects loaded at program startup */
STAILQ_HEAD_INITIALIZER(list_main);
static Elf_Sym sym_zero; /* For resolving undefined weak refs. */
@ -171,6 +186,7 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
Elf_Auxinfo *aux;
Elf_Auxinfo *auxp;
const char *argv0;
Obj_Entry *obj;
/*
* On entry, the dynamic linker itself has not been relocated yet.
@ -273,7 +289,9 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
dbg("loading needed objects");
if (load_needed_objects(obj_main) == -1)
die();
main_tail = obj_tail;
for (obj = obj_list; obj != NULL; obj = obj->next)
objlist_add(&list_main, obj);
if (ld_tracing) { /* We're done */
trace_loaded_objects(obj_main);
@ -307,7 +325,7 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
}
Elf_Addr
_rtld_bind(const Obj_Entry *obj, Elf_Word reloff)
_rtld_bind(Obj_Entry *obj, Elf_Word reloff)
{
const Elf_Rel *rel;
const Elf_Sym *def;
@ -548,11 +566,12 @@ digest_dynamic(Obj_Entry *obj)
static Obj_Entry *
digest_phdr(const Elf_Phdr *phdr, int phnum, caddr_t entry, const char *path)
{
Obj_Entry *obj = CNEW(Obj_Entry);
Obj_Entry *obj;
const Elf_Phdr *phlimit = phdr + phnum;
const Elf_Phdr *ph;
int nsegs = 0;
obj = obj_new();
for (ph = phdr; ph < phlimit; ph++) {
switch (ph->p_type) {
@ -681,7 +700,7 @@ find_library(const char *name, const Obj_Entry *refobj)
* defining object via the reference parameter DEFOBJ_OUT.
*/
const Elf_Sym *
find_symdef(unsigned long symnum, const Obj_Entry *refobj,
find_symdef(unsigned long symnum, Obj_Entry *refobj,
const Obj_Entry **defobj_out, bool in_plt)
{
const Elf_Sym *ref;
@ -689,6 +708,7 @@ find_symdef(unsigned long symnum, const Obj_Entry *refobj,
const Elf_Sym *symp;
const Obj_Entry *obj;
const Obj_Entry *defobj;
const Objlist_Entry *elm;
const char *name;
unsigned long hash;
@ -697,79 +717,77 @@ find_symdef(unsigned long symnum, const Obj_Entry *refobj,
hash = elf_hash(name);
def = NULL;
defobj = NULL;
curmark++;
if (refobj->symbolic) { /* Look first in the referencing object */
if ((symp = symlook_obj(name, hash, refobj, in_plt)) != NULL) {
symp = symlook_obj(name, hash, refobj, in_plt);
refobj->mark = curmark;
if (symp != NULL) {
def = symp;
defobj = refobj;
if (ELF_ST_BIND(def->st_info) != STB_WEAK) {
*defobj_out = defobj;
return def;
}
}
}
/* Search all dlopened DAGs containing the referencing object. */
STAILQ_FOREACH(elm, &refobj->dldags, link) {
if (def != NULL && ELF_ST_BIND(def->st_info) != STB_WEAK)
break;
symp = symlook_list(name, hash, &elm->obj->dagmembers, &obj, in_plt);
if (symp != NULL &&
(def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) {
def = symp;
defobj = obj;
}
}
/* Search all objects loaded at program start up. */
if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) {
symp = symlook_list(name, hash, &list_main, &obj, in_plt);
if (symp != NULL &&
(def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) {
def = symp;
defobj = obj;
}
}
/* Search all RTLD_GLOBAL objects. */
if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) {
symp = symlook_list(name, hash, &list_global, &obj, in_plt);
if (symp != NULL &&
(def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) {
def = symp;
defobj = obj;
}
}
/*
* Look in all loaded objects. Skip the referencing object,
* if we have already searched it. We remember the first weak
* definition we encounter, but we keep searching for a strong
* definition. If we find a strong definition we stop searching,
* because there won't be anything better than that.
* Search the dynamic linker itself, and possibly resolve the
* symbol from there. This is how the application links to
* dynamic linker services such as dlopen. Only the values listed
* in the "exports" array can be resolved from the dynamic linker.
*/
for (obj = obj_list; obj != NULL; obj = obj->next) {
if (obj == refobj && refobj->symbolic)
continue;
if ((symp = symlook_obj(name, hash, obj, in_plt)) != NULL) {
if (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK) {
def = symp;
defobj = obj;
if (ELF_ST_BIND(def->st_info) != STB_WEAK) {
*defobj_out = defobj;
return def;
}
}
}
}
/*
* We still don't have a strong definition. Search the dynamic
* linker itself, and possibly resolve the symbol from there.
* This is how the application links to dynamic linker services
* such as dlopen. Only the values listed in the "exports" array
* can be resolved from the dynamic linker.
*/
if ((symp = symlook_obj(name, hash, &obj_rtld, in_plt)) != NULL &&
is_exported(symp)) {
if (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK) {
if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) {
symp = symlook_obj(name, hash, &obj_rtld, in_plt);
if (symp != NULL && is_exported(symp)) {
def = symp;
defobj = &obj_rtld;
if (ELF_ST_BIND(def->st_info) != STB_WEAK) {
*defobj_out = defobj;
return def;
}
}
}
/*
* We didn't find a strong definition. Accept a weak definition
* if we found one.
*/
if (def != NULL) {
*defobj_out = defobj;
return def;
}
/*
* We found no definition. If the reference is weak, treat the
* If we found no definition and the reference is weak, treat the
* symbol as having the value zero.
*/
if (ELF_ST_BIND(ref->st_info) == STB_WEAK) {
*defobj_out = obj_main;
return &sym_zero;
if (def == NULL && ELF_ST_BIND(ref->st_info) == STB_WEAK) {
def = &sym_zero;
defobj = obj_main;
}
_rtld_error("%s: Undefined symbol \"%s\"", refobj->path, name);
return NULL;
if (def != NULL)
*defobj_out = defobj;
else
_rtld_error("%s: Undefined symbol \"%s\"", refobj->path, name);
return def;
}
/*
@ -811,6 +829,28 @@ gethints(void)
return hints[0] != '\0' ? hints : NULL;
}
static void
init_dag(Obj_Entry *root)
{
curmark++;
init_dag1(root, root);
}
static void
init_dag1(Obj_Entry *root, Obj_Entry *obj)
{
const Needed_Entry *needed;
if (obj->mark == curmark)
return;
obj->mark = curmark;
objlist_add(&obj->dldags, root);
objlist_add(&root->dagmembers, obj);
for (needed = obj->needed; needed != NULL; needed = needed->next)
if (needed->obj != NULL)
init_dag1(root, needed->obj);
}
/*
* Initialize the dynamic linker. The argument is the address at which
* the dynamic linker has been mapped into memory. The primary task of
@ -826,7 +866,7 @@ init_rtld(caddr_t mapbase)
* aren't yet initialized sufficiently to do that. Below we will
* replace the static version with a dynamically-allocated copy.
*/
obj_rtld.path = "/usr/libexec/ld-elf.so.1";
obj_rtld.path = _PATH_RTLD;
obj_rtld.rtld = true;
obj_rtld.mapbase = mapbase;
#ifdef PIC
@ -1001,6 +1041,38 @@ obj_from_addr(const void *addr)
return NULL;
}
static void
objlist_add(Objlist *list, Obj_Entry *obj)
{
Objlist_Entry *elm;
elm = NEW(Objlist_Entry);
elm->obj = obj;
STAILQ_INSERT_TAIL(list, elm, link);
}
static Objlist_Entry *
objlist_find(Objlist *list, const Obj_Entry *obj)
{
Objlist_Entry *elm;
STAILQ_FOREACH(elm, list, link)
if (elm->obj == obj)
return elm;
return NULL;
}
static void
objlist_remove(Objlist *list, Obj_Entry *obj)
{
Objlist_Entry *elm;
if ((elm = objlist_find(list, obj)) != NULL) {
STAILQ_REMOVE(list, elm, Struct_Objlist_Entry, link);
free(elm);
}
}
/*
* Relocate newly-loaded shared objects. The argument is a pointer to
* the Obj_Entry for the first such object. All objects from the first
@ -1154,11 +1226,14 @@ dlopen(const char *name, int mode)
if (obj) {
obj->dl_refcount++;
if (mode & RTLD_GLOBAL && objlist_find(&list_global, obj) == NULL)
objlist_add(&list_global, obj);
mode &= RTLD_MODEMASK;
if (*old_obj_tail != NULL) { /* We loaded something new. */
assert(*old_obj_tail == obj);
if (load_needed_objects(obj) == -1 ||
relocate_objects(obj, mode == RTLD_NOW) == -1) {
(init_dag(obj), relocate_objects(obj, mode == RTLD_NOW)) == -1) {
unload_object(obj, false);
obj->dl_refcount--;
obj = NULL;
@ -1178,9 +1253,11 @@ dlsym(void *handle, const char *name)
const Obj_Entry *obj;
unsigned long hash;
const Elf_Sym *def;
const Obj_Entry *defobj;
hash = elf_hash(name);
def = NULL;
defobj = NULL;
if (handle == NULL || handle == RTLD_NEXT) {
void *retaddr;
@ -1190,12 +1267,16 @@ dlsym(void *handle, const char *name)
_rtld_error("Cannot determine caller's shared object");
return NULL;
}
if (handle == NULL) /* Just the caller's shared object. */
if (handle == NULL) { /* Just the caller's shared object. */
def = symlook_obj(name, hash, obj, true);
else { /* All the shared objects after the caller's */
while ((obj = obj->next) != NULL)
if ((def = symlook_obj(name, hash, obj, true)) != NULL)
defobj = obj;
} else { /* All the shared objects after the caller's */
while ((obj = obj->next) != NULL) {
if ((def = symlook_obj(name, hash, obj, true)) != NULL) {
defobj = obj;
break;
}
}
}
} else {
if ((obj = dlcheck(handle)) == NULL)
@ -1203,20 +1284,20 @@ dlsym(void *handle, const char *name)
if (obj->mainprog) {
/* Search main program and all libraries loaded by it. */
for ( ; obj != *main_tail; obj = obj->next)
if ((def = symlook_obj(name, hash, obj, true)) != NULL)
break;
curmark++;
def = symlook_list(name, hash, &list_main, &defobj, true);
} else {
/*
* XXX - This isn't correct. The search should include the whole
* DAG rooted at the given object.
*/
def = symlook_obj(name, hash, obj, true);
defobj = obj;
}
}
if (def != NULL)
return obj->relocbase + def->st_value;
return defobj->relocbase + def->st_value;
_rtld_error("Undefined symbol \"%s\"", name);
return NULL;
@ -1359,6 +1440,35 @@ set_program_var(const char *name, const void *value)
}
}
static const Elf_Sym *
symlook_list(const char *name, unsigned long hash, Objlist *objlist,
const Obj_Entry **defobj_out, bool in_plt)
{
const Elf_Sym *symp;
const Elf_Sym *def;
const Obj_Entry *defobj;
const Objlist_Entry *elm;
def = NULL;
defobj = NULL;
STAILQ_FOREACH(elm, objlist, link) {
if (elm->obj->mark == curmark)
continue;
elm->obj->mark = curmark;
if ((symp = symlook_obj(name, hash, elm->obj, in_plt)) != NULL) {
if (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK) {
def = symp;
defobj = elm->obj;
if (ELF_ST_BIND(def->st_info) != STB_WEAK)
break;
}
}
}
if (def != NULL)
*defobj_out = defobj;
return def;
}
/*
* Search the symbol table of a single shared object for a symbol of
* the given name. Returns a pointer to the symbol, or NULL if no
@ -1484,13 +1594,17 @@ trace_loaded_objects(Obj_Entry *obj)
}
}
/*
* Note, this is called only for objects loaded by dlopen().
*/
static void
unload_object(Obj_Entry *root, bool do_fini_funcs)
{
unref_object_dag(root);
unref_dag(root);
if (root->refcount == 0) { /* We are finished with some objects. */
Obj_Entry *obj;
Obj_Entry **linkp;
Objlist_Entry *elm;
/* Finalize objects that are about to be unmapped. */
if (do_fini_funcs)
@ -1498,21 +1612,22 @@ unload_object(Obj_Entry *root, bool do_fini_funcs)
if (obj->refcount == 0 && obj->fini != NULL)
(*obj->fini)();
/* Remove the DAG from all objects' DAG lists. */
STAILQ_FOREACH(elm, &root->dagmembers , link)
objlist_remove(&elm->obj->dldags, root);
/* Remove the DAG from the RTLD_GLOBAL list. */
objlist_remove(&list_global, root);
/* Unmap all objects that are no longer referenced. */
linkp = &obj_list->next;
while ((obj = *linkp) != NULL) {
if (obj->refcount == 0) {
dbg("unloading \"%s\"", obj->path);
munmap(obj->mapbase, obj->mapsize);
free(obj->path);
while (obj->needed != NULL) {
Needed_Entry *needed = obj->needed;
obj->needed = needed->next;
free(needed);
}
linkmap_delete(obj);
*linkp = obj->next;
free(obj);
obj_free(obj);
} else
linkp = &obj->next;
}
@ -1521,7 +1636,7 @@ unload_object(Obj_Entry *root, bool do_fini_funcs)
}
static void
unref_object_dag(Obj_Entry *root)
unref_dag(Obj_Entry *root)
{
const Needed_Entry *needed;
@ -1530,7 +1645,7 @@ unref_object_dag(Obj_Entry *root)
if (root->refcount == 0)
for (needed = root->needed; needed != NULL; needed = needed->next)
if (needed->obj != NULL)
unref_object_dag(needed->obj);
unref_dag(needed->obj);
}
/*

17
libexec/rtld-elf/rtld.h
View File

@ -29,6 +29,7 @@
#define RTLD_H 1
#include <sys/types.h>
#include <sys/queue.h>
#include <link.h>
#include <elf.h>
@ -50,6 +51,13 @@ typedef unsigned char bool;
struct Struct_Obj_Entry;
typedef struct Struct_Objlist_Entry {
STAILQ_ENTRY(Struct_Objlist_Entry) link;
struct Struct_Obj_Entry *obj;
} Objlist_Entry;
typedef STAILQ_HEAD(Struct_Objlist, Struct_Objlist_Entry) Objlist;
typedef struct Struct_Needed_Entry {
struct Struct_Needed_Entry *next;
struct Struct_Obj_Entry *obj;
@ -71,7 +79,10 @@ typedef struct Struct_Obj_Entry {
Elf_Word version; /* Version number of struct format */
struct Struct_Obj_Entry *next;
Objlist dldags; /* Object belongs to these dlopened DAGs (%) */
Objlist dagmembers; /* DAG has these members (%) */
char *path; /* Pathname of underlying file (%) */
unsigned long mark; /* Set to "curmark" to avoid repeat visits */
int refcount;
int dl_refcount; /* Number of times loaded by dlopen */
@ -135,9 +146,11 @@ extern Elf_Addr _GLOBAL_OFFSET_TABLE_[];
*/
int do_copy_relocations(Obj_Entry *);
unsigned long elf_hash(const char *);
const Elf_Sym *find_symdef(unsigned long, const Obj_Entry *,
const Obj_Entry **, bool);
const Elf_Sym *find_symdef(unsigned long, Obj_Entry *, const Obj_Entry **,
bool);
void init_pltgot(Obj_Entry *);
void obj_free(Obj_Entry *);
Obj_Entry *obj_new(void);
int reloc_non_plt(Obj_Entry *, Obj_Entry *);
int reloc_plt(Obj_Entry *, bool);
void _rtld_bind_start(void);