ldso: allow symbol resolution during dl_itera_phdr

When callback functions of `dl_iterate_phdr` required further jump slot
relocations this lead to a deadlock. Therefore, we allow the resolution
of further symbols from callback functions, but protect the ELF object
list during the iteration, which blocks any dynamic loading (e.g.,
dlopen/dlcose) of shared object by other threads while in program header
iteration.

fixes #4071

Author: ssumpf

repos/base/src/lib/ldso/exception.cc

@@ -38,7 +38,8 @@ extern "C" int dl_iterate_phdr(int (*callback) (Phdr_info *info, size_t size, vo
 	int err = 0;
 	Phdr_info info;
 
-	Mutex::Guard guard(mutex());
+	/* forbid object list manipulation during traversal */
+	Mutex::Guard guard(Linker::shared_object_mutex());
 
 	for (Object *e = obj_list_head();e; e = e->next_obj()) {
 
@@ -88,6 +89,9 @@ extern "C" unsigned long dl_unwind_find_exidx(unsigned long pc, int *pcount)
 	enum { EXIDX_ENTRY_SIZE = 8 };
 	*pcount = 0;
 
+	/* forbid object list manipulation during traversal */
+	Mutex::Guard guard(Linker::shared_object_mutex());
+
 	for (Object *e = obj_list_head(); e; e = e->next_obj()) {
 
 		/* address of first PT_LOAD header */

repos/base/src/lib/ldso/include/linker.h

@@ -129,6 +129,11 @@ namespace Linker {
 	 * Global ELF access mutex
 	 */
 	Mutex &mutex();
+
+	/**
+	 * Mutex that protects shared object list
+	 */
+	Mutex &shared_object_mutex();
 }
 
 

repos/base/src/lib/ldso/main.cc

@@ -69,6 +69,12 @@ Genode::Mutex &Linker::mutex()
 }
 
 
+Genode::Mutex &Linker::shared_object_mutex()
+{
+	static Mutex _mutex;
+	return _mutex;
+}
+
 /**************************************************************
  ** ELF object types (shared object, dynamic binaries, ldso  **
  **************************************************************/

repos/base/src/lib/ldso/shared_object.cc

@@ -25,17 +25,6 @@ static Linker::Root_object const &to_root(void *h)
 }
 
 
-/**
- * Needed during shared object creation and destruction, since global lists are
- * manipulated
- */
-static Genode::Mutex & shared_object_lock()
-{
-	static Genode::Mutex _lock;
-	return _lock;
-}
-
-
 static Linker::Object *find_obj(Genode::addr_t addr)
 {
 	for (Linker::Object *e = Linker::obj_list_head(); e; e = e->next_obj())
@@ -61,7 +50,7 @@ Genode::Shared_object::Shared_object(Env &env, Allocator &md_alloc,
 		log("LD: open '", file ? file : "binary", "'");
 
 	try {
-		Mutex::Guard guard(shared_object_lock());
+		Mutex::Guard guard(Linker::shared_object_mutex());
 
 		_handle = new (md_alloc)
 			Root_object(env, md_alloc, file ? file : binary_name(),
@@ -121,7 +110,7 @@ Genode::Shared_object::~Shared_object()
 	if (verbose_shared)
 		log("LD: close shared object");
 
-	Mutex::Guard guard(shared_object_lock());
+	Mutex::Guard guard(Linker::shared_object_mutex());
 	destroy(_md_alloc, &const_cast<Root_object &>(to_root(_handle)));
 }