Add a new function dllockinit() for registering thread locking

functions to be used by the dynamic linker.  This can be called by
threads packages at start-up time.  I will add the call to libc_r
soon.

Also add a default locking method that is used up until dllockinit()
is called.  The default method works by blocking SIGVTALRM, SIGPROF,
and SIGALRM in critical sections.  It is based on the observation
that most user-space threads packages implement thread preemption
with one of these signals (usually SIGVTALRM).

The dynamic linker has never been reentrant, but it became less
reentrant in revision 1.34 of "src/libexec/rtld-elf/rtld.c".
Starting with that revision, multiple threads each doing lazy
binding could interfere with each other.  The usual symptom was
that a symbol was falsely reported as undefined at start-up time.
It was rare but not unseen.  This commit fixes it.

1 files changed, 118 insertions, 16 deletions

diff --git a/libexec/rtld-elf/rtld.c b/libexec/rtld-elf/rtld.c
index 8ba21b4..5bcd390 100644
--- a/libexec/rtld-elf/rtld.c
+++ b/libexec/rtld-elf/rtld.c
@@ -52,21 +52,23 @@
 #include "debug.h"
 #include "rtld.h"
 
-/*
- * Debugging support.
- */
-
-#define assert(cond)	((cond) ? (void) 0 :\
-    (msg("oops: " __XSTRING(__LINE__) "\n"), abort()))
-#define msg(s)		(write(1, s, strlen(s)))
-#define trace()		msg("trace: " __XSTRING(__LINE__) "\n");
-
 #define END_SYM		"_end"
 #define PATH_RTLD	"/usr/libexec/ld-elf.so.1"
 
 /* Types. */
 typedef void (*func_ptr_type)();
 
+typedef struct Struct_LockInfo {
+    void *context;		/* Client context for creating locks */
+    void *thelock;		/* The one big lock */
+    /* Methods */
+    void (*rlock_acquire)(void *lock);
+    void (*wlock_acquire)(void *lock);
+    void (*lock_release)(void *lock);
+    void (*lock_destroy)(void *lock);
+    void (*context_destroy)(void *context);
+} LockInfo;
+
 /*
  * Function declarations.
  */
@@ -88,6 +90,7 @@ static void linkmap_delete(Obj_Entry *);
 static int load_needed_objects(Obj_Entry *);
 static int load_preload_objects(void);
 static Obj_Entry *load_object(char *);
+static void lock_nop(void *);
 static Obj_Entry *obj_from_addr(const void *);
 static void objlist_add(Objlist *, Obj_Entry *);
 static Objlist_Entry *objlist_find(Objlist *, const Obj_Entry *);
@@ -128,6 +131,8 @@ static Objlist list_global =	/* Objects dlopened with RTLD_GLOBAL */
 static Objlist list_main =	/* Objects loaded at program startup */
   STAILQ_HEAD_INITIALIZER(list_main);
 
+static LockInfo lockinfo;
+
 static Elf_Sym sym_zero;	/* For resolving undefined weak refs. */
 
 #define GDB_STATE(s)	r_debug.r_state = s; r_debug_state();
@@ -147,6 +152,7 @@ static func_ptr_type exports[] = {
     (func_ptr_type) &dlopen,
     (func_ptr_type) &dlsym,
     (func_ptr_type) &dladdr,
+    (func_ptr_type) &dllockinit,
     NULL
 };
 
@@ -157,6 +163,24 @@ static func_ptr_type exports[] = {
 char *__progname;
 char **environ;
 
+static __inline void
+rlock_acquire(void)
+{
+    lockinfo.rlock_acquire(lockinfo.thelock);
+}
+
+static __inline void
+wlock_acquire(void)
+{
+    lockinfo.wlock_acquire(lockinfo.thelock);
+}
+
+static __inline void
+lock_release(void)
+{
+    lockinfo.lock_release(lockinfo.thelock);
+}
+
 /*
  * Main entry point for dynamic linking.  The first argument is the
  * stack pointer.  The stack is expected to be laid out as described
@@ -322,6 +346,9 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
     set_program_var("__progname", argv[0] != NULL ? basename(argv[0]) : "");
     set_program_var("environ", env);
 
+    dbg("initializing default locks");
+    dllockinit(NULL, NULL, NULL, NULL, NULL, NULL, NULL);
+
     r_debug_state();		/* say hello to gdb! */
 
     dbg("calling _init functions");
@@ -344,6 +371,7 @@ _rtld_bind(Obj_Entry *obj, Elf_Word reloff)
     Elf_Addr *where;
     Elf_Addr target;
 
+    wlock_acquire();
     if (obj->pltrel)
 	rel = (const Elf_Rel *) ((caddr_t) obj->pltrel + reloff);
     else
@@ -361,6 +389,7 @@ _rtld_bind(Obj_Entry *obj, Elf_Word reloff)
       (void *)target, basename(defobj->path));
 
     reloc_jmpslot(where, target);
+    lock_release();
     return target;
 }
 
@@ -1058,6 +1087,11 @@ load_object(char *path)
     return obj;
 }
 
+static void
+lock_nop(void *lock)
+{
+}
+
 static Obj_Entry *
 obj_from_addr(const void *addr)
 {
@@ -1223,16 +1257,21 @@ search_library_path(const char *name, const char *path)
 int
 dlclose(void *handle)
 {
-    Obj_Entry *root = dlcheck(handle);
+    Obj_Entry *root;
 
-    if (root == NULL)
+    wlock_acquire();
+    root = dlcheck(handle);
+    if (root == NULL) {
+	lock_release();
 	return -1;
+    }
 
     GDB_STATE(RT_DELETE);
     unload_object(root, true);
     root->dl_refcount--;
     GDB_STATE(RT_CONSISTENT);
 
+    lock_release();
     return 0;
 }
 
@@ -1244,14 +1283,67 @@ dlerror(void)
     return msg;
 }
 
+void
+dllockinit(void *context,
+	   void *(*lock_create)(void *context),
+           void (*rlock_acquire)(void *lock),
+           void (*wlock_acquire)(void *lock),
+           void (*lock_release)(void *lock),
+           void (*lock_destroy)(void *lock),
+	   void (*context_destroy)(void *context))
+{
+    /* NULL arguments mean reset to the built-in locks. */
+    if (lock_create == NULL) {
+	context = NULL;
+	lock_create = lockdflt_create;
+	rlock_acquire = wlock_acquire = lockdflt_acquire;
+	lock_release = lockdflt_release;
+	lock_destroy = lockdflt_destroy;
+	context_destroy = NULL;
+    }
+
+    /* Temporarily set locking methods to no-ops. */
+    lockinfo.rlock_acquire = lock_nop;
+    lockinfo.wlock_acquire = lock_nop;
+    lockinfo.lock_release = lock_nop;
+
+    /* Release any existing locks and context. */
+    if (lockinfo.lock_destroy != NULL)
+	lockinfo.lock_destroy(lockinfo.thelock);
+    if (lockinfo.context_destroy != NULL)
+	lockinfo.context_destroy(lockinfo.context);
+
+    /*
+     * Allocate the locks we will need and call all the new locking
+     * methods, to accomplish any needed lazy binding for the methods
+     * themselves.
+     */
+    lockinfo.thelock = lock_create(lockinfo.context);
+    rlock_acquire(lockinfo.thelock);
+    lock_release(lockinfo.thelock);
+    wlock_acquire(lockinfo.thelock);
+    lock_release(lockinfo.thelock);
+
+    /* Record the new method information. */
+    lockinfo.context = context;
+    lockinfo.rlock_acquire = rlock_acquire;
+    lockinfo.wlock_acquire = wlock_acquire;
+    lockinfo.lock_release = lock_release;
+    lockinfo.lock_destroy = lock_destroy;
+    lockinfo.context_destroy = context_destroy;
+}
+
 void *
 dlopen(const char *name, int mode)
 {
-    Obj_Entry **old_obj_tail = obj_tail;
-    Obj_Entry *obj = NULL;
+    Obj_Entry **old_obj_tail;
+    Obj_Entry *obj;
 
+    wlock_acquire();
     GDB_STATE(RT_ADD);
 
+    old_obj_tail = obj_tail;
+    obj = NULL;
     if (name == NULL) {
 	obj = obj_main;
 	obj->refcount++;
@@ -1280,7 +1372,7 @@ dlopen(const char *name, int mode)
     }
 
     GDB_STATE(RT_CONSISTENT);
-
+    lock_release();
     return obj;
 }
 
@@ -1296,12 +1388,14 @@ dlsym(void *handle, const char *name)
     def = NULL;
     defobj = NULL;
 
+    wlock_acquire();
     if (handle == NULL || handle == RTLD_NEXT) {
 	void *retaddr;
 
 	retaddr = __builtin_return_address(0);	/* __GNUC__ only */
 	if ((obj = obj_from_addr(retaddr)) == NULL) {
 	    _rtld_error("Cannot determine caller's shared object");
+	    lock_release();
 	    return NULL;
 	}
 	if (handle == NULL) {	/* Just the caller's shared object. */
@@ -1316,8 +1410,10 @@ dlsym(void *handle, const char *name)
 	    }
 	}
     } else {
-	if ((obj = dlcheck(handle)) == NULL)
+	if ((obj = dlcheck(handle)) == NULL) {
+	    lock_release();
 	    return NULL;
+	}
 
 	if (obj->mainprog) {
 	    /* Search main program and all libraries loaded by it. */
@@ -1333,10 +1429,13 @@ dlsym(void *handle, const char *name)
 	}
     }
 
-    if (def != NULL)
+    if (def != NULL) {
+	lock_release();
 	return defobj->relocbase + def->st_value;
+    }
 
     _rtld_error("Undefined symbol \"%s\"", name);
+    lock_release();
     return NULL;
 }
 
@@ -1348,9 +1447,11 @@ dladdr(const void *addr, Dl_info *info)
     void *symbol_addr;
     unsigned long symoffset;
     
+    wlock_acquire();
     obj = obj_from_addr(addr);
     if (obj == NULL) {
         _rtld_error("No shared object contains address");
+	lock_release();
         return 0;
     }
     info->dli_fname = obj->path;
@@ -1389,6 +1490,7 @@ dladdr(const void *addr, Dl_info *info)
         if (info->dli_saddr == addr)
             break;
     }
+    lock_release();
     return 1;
 }