Load libraries in breadth-first order
This patch fixes the problem with symbol search order
for dlsym(RTLD_DEFAULT/RTLD_NEXT, .) by loading libraries
and ld_preloads in correct order.
Bug: https://code.google.com/p/android/issues/detail?id=74255
Change-Id: If1ba5c807322409faa914e27ecb675e2c4541f0d
Attempt: 2
diff --git a/linker/linker.cpp b/linker/linker.cpp
index 2186b3d..793ffd5 100644
--- a/linker/linker.cpp
+++ b/linker/linker.cpp
@@ -44,6 +44,8 @@
#include "private/KernelArgumentBlock.h"
#include "private/ScopedPthreadMutexLocker.h"
#include "private/ScopedFd.h"
+#include "private/ScopeGuard.h"
+#include "private/UniquePtr.h"
#include "linker.h"
#include "linker_debug.h"
@@ -170,7 +172,6 @@
DISALLOW_ALLOCATION(void*, realloc, (void* u1 __unused, size_t u2 __unused));
DISALLOW_ALLOCATION(void*, calloc, (size_t u1 __unused, size_t u2 __unused));
-static char tmp_err_buf[768];
static char __linker_dl_err_buf[768];
char* linker_get_error_buffer() {
@@ -512,7 +513,7 @@
unsigned elf_hash = elfhash(name);
ElfW(Sym)* s = nullptr;
- if (si != nullptr && somain != nullptr) {
+ if (somain != nullptr) {
/*
* Local scope is executable scope. Just start looking into it right away
* for the shortcut.
@@ -657,22 +658,47 @@
}
};
+class LoadTask {
+ public:
+ struct deleter_t {
+ void operator()(LoadTask* t) {
+ TypeBasedAllocator<LoadTask>::free(t);
+ }
+ };
+
+ typedef UniquePtr<LoadTask, deleter_t> unique_ptr;
+
+ static deleter_t deleter;
+
+ static LoadTask* create(const char* name, soinfo* needed_by) {
+ LoadTask* ptr = TypeBasedAllocator<LoadTask>::alloc();
+ return new (ptr) LoadTask(name, needed_by);
+ }
+
+ const char* get_name() const {
+ return name_;
+ }
+
+ soinfo* get_needed_by() const {
+ return needed_by_;
+ }
+ private:
+ LoadTask(const char* name, soinfo* needed_by)
+ : name_(name), needed_by_(needed_by) {}
+
+ const char* name_;
+ soinfo* needed_by_;
+
+ DISALLOW_IMPLICIT_CONSTRUCTORS(LoadTask);
+};
+
template <typename T>
using linked_list_t = LinkedList<T, TypeBasedAllocator<LinkedListEntry<T>>>;
typedef linked_list_t<soinfo> SoinfoLinkedList;
+typedef linked_list_t<const char> StringLinkedList;
+typedef linked_list_t<LoadTask> LoadTaskList;
-static LinkerAllocator<LinkedListEntry<soinfo>> g_soinfo_list_allocator_rw;
-class SoinfoListAllocatorRW {
- public:
- static LinkedListEntry<soinfo>* alloc() {
- return g_soinfo_list_allocator_rw.alloc();
- }
-
- static void free(LinkedListEntry<soinfo>* ptr) {
- g_soinfo_list_allocator_rw.free(ptr);
- }
-};
// This is used by dlsym(3). It performs symbol lookup only within the
// specified soinfo object and its dependencies in breadth first order.
@@ -798,73 +824,80 @@
return fd;
}
-static soinfo* load_library(const char* name, int dlflags, const android_dlextinfo* extinfo) {
- int fd = -1;
- ScopedFd file_guard(-1);
-
- if (extinfo != nullptr && (extinfo->flags & ANDROID_DLEXT_USE_LIBRARY_FD) != 0) {
- fd = extinfo->library_fd;
- } else {
- // Open the file.
- fd = open_library(name);
- if (fd == -1) {
- DL_ERR("library \"%s\" not found", name);
- return nullptr;
- }
-
- file_guard.reset(fd);
+template<typename F>
+static void for_each_dt_needed(const soinfo* si, F action) {
+ for (ElfW(Dyn)* d = si->dynamic; d->d_tag != DT_NULL; ++d) {
+ if (d->d_tag == DT_NEEDED) {
+ action(si->strtab + d->d_un.d_val);
}
+ }
+}
- ElfReader elf_reader(name, fd);
+static soinfo* load_library(LoadTaskList& load_tasks, const char* name, int dlflags, const android_dlextinfo* extinfo) {
+ int fd = -1;
+ ScopedFd file_guard(-1);
- struct stat file_stat;
- if (TEMP_FAILURE_RETRY(fstat(fd, &file_stat)) != 0) {
- DL_ERR("unable to stat file for the library %s: %s", name, strerror(errno));
+ if (extinfo != nullptr && (extinfo->flags & ANDROID_DLEXT_USE_LIBRARY_FD) != 0) {
+ fd = extinfo->library_fd;
+ } else {
+ // Open the file.
+ fd = open_library(name);
+ if (fd == -1) {
+ DL_ERR("library \"%s\" not found", name);
return nullptr;
}
- // Check for symlink and other situations where
- // file can have different names.
- for (soinfo* si = solist; si != nullptr; si = si->next) {
- if (si->get_st_dev() != 0 &&
- si->get_st_ino() != 0 &&
- si->get_st_dev() == file_stat.st_dev &&
- si->get_st_ino() == file_stat.st_ino) {
- TRACE("library \"%s\" is already loaded under different name/path \"%s\" - will return existing soinfo", name, si->name);
- return si;
- }
+ file_guard.reset(fd);
+ }
+
+ struct stat file_stat;
+ if (TEMP_FAILURE_RETRY(fstat(fd, &file_stat)) != 0) {
+ DL_ERR("unable to stat file for the library %s: %s", name, strerror(errno));
+ return nullptr;
+ }
+
+ // Check for symlink and other situations where
+ // file can have different names.
+ for (soinfo* si = solist; si != nullptr; si = si->next) {
+ if (si->get_st_dev() != 0 &&
+ si->get_st_ino() != 0 &&
+ si->get_st_dev() == file_stat.st_dev &&
+ si->get_st_ino() == file_stat.st_ino) {
+ TRACE("library \"%s\" is already loaded under different name/path \"%s\" - will return existing soinfo", name, si->name);
+ return si;
}
+ }
- if ((dlflags & RTLD_NOLOAD) != 0) {
- return nullptr;
- }
+ if ((dlflags & RTLD_NOLOAD) != 0) {
+ return nullptr;
+ }
- // Read the ELF header and load the segments.
- if (!elf_reader.Load(extinfo)) {
- return nullptr;
- }
+ // Read the ELF header and load the segments.
+ ElfReader elf_reader(name, fd);
+ if (!elf_reader.Load(extinfo)) {
+ return nullptr;
+ }
- soinfo* si = soinfo_alloc(SEARCH_NAME(name), &file_stat);
- if (si == nullptr) {
- return nullptr;
- }
- si->base = elf_reader.load_start();
- si->size = elf_reader.load_size();
- si->load_bias = elf_reader.load_bias();
- si->phnum = elf_reader.phdr_count();
- si->phdr = elf_reader.loaded_phdr();
+ soinfo* si = soinfo_alloc(SEARCH_NAME(name), &file_stat);
+ if (si == nullptr) {
+ return nullptr;
+ }
+ si->base = elf_reader.load_start();
+ si->size = elf_reader.load_size();
+ si->load_bias = elf_reader.load_bias();
+ si->phnum = elf_reader.phdr_count();
+ si->phdr = elf_reader.loaded_phdr();
- // At this point we know that whatever is loaded @ base is a valid ELF
- // shared library whose segments are properly mapped in.
- TRACE("[ load_library base=%p size=%zu name='%s' ]",
- reinterpret_cast<void*>(si->base), si->size, si->name);
+ if (!si->PrelinkImage()) {
+ soinfo_free(si);
+ return nullptr;
+ }
- if (!si->LinkImage(extinfo)) {
- soinfo_free(si);
- return nullptr;
- }
+ for_each_dt_needed(si, [&] (const char* name) {
+ load_tasks.push_back(LoadTask::create(name, si));
+ });
- return si;
+ return si;
}
static soinfo *find_loaded_library_by_name(const char* name) {
@@ -877,33 +910,122 @@
return nullptr;
}
-static soinfo* find_library_internal(const char* name, int dlflags, const android_dlextinfo* extinfo) {
- if (name == nullptr) {
- return somain;
- }
+static soinfo* find_library_internal(LoadTaskList& load_tasks, const char* name, int dlflags, const android_dlextinfo* extinfo) {
soinfo* si = find_loaded_library_by_name(name);
// Library might still be loaded, the accurate detection
- // of this fact is done by load_library
+ // of this fact is done by load_library.
if (si == nullptr) {
TRACE("[ '%s' has not been found by name. Trying harder...]", name);
- si = load_library(name, dlflags, extinfo);
- }
-
- if (si != nullptr && (si->flags & FLAG_LINKED) == 0) {
- DL_ERR("recursive link to \"%s\"", si->name);
- return nullptr;
+ si = load_library(load_tasks, name, dlflags, extinfo);
}
return si;
}
-static soinfo* find_library(const char* name, int dlflags, const android_dlextinfo* extinfo) {
- soinfo* si = find_library_internal(name, dlflags, extinfo);
- if (si != nullptr) {
- si->ref_count++;
+static void soinfo_unload(soinfo* si);
+
+static bool is_recursive(soinfo* si, soinfo* parent) {
+ if (parent == nullptr) {
+ return false;
}
+
+ if (si == parent) {
+ DL_ERR("recursive link to \"%s\"", si->name);
+ return true;
+ }
+
+ return !parent->get_parents().visit([&](soinfo* grandparent) {
+ return !is_recursive(si, grandparent);
+ });
+}
+
+static bool find_libraries(const char* const library_names[], size_t library_names_size, soinfo* soinfos[],
+ soinfo* ld_preloads[], size_t ld_preloads_size, int dlflags, const android_dlextinfo* extinfo) {
+ // Step 0: prepare.
+ LoadTaskList load_tasks;
+ for (size_t i = 0; i < library_names_size; ++i) {
+ const char* name = library_names[i];
+ load_tasks.push_back(LoadTask::create(name, nullptr));
+ }
+
+ // Libraries added to this list in reverse order so that we can
+ // start linking from bottom-up - see step 2.
+ SoinfoLinkedList found_libs;
+ size_t soinfos_size = 0;
+
+ auto failure_guard = create_scope_guard([&]() {
+ // Housekeeping
+ load_tasks.for_each([] (LoadTask* t) {
+ LoadTask::deleter(t);
+ });
+
+ for (size_t i = 0; i<soinfos_size; ++i) {
+ soinfo_unload(soinfos[i]);
+ }
+ });
+
+ // Step 1: load and pre-link all DT_NEEDED libraries in breadth first order.
+ for (LoadTask::unique_ptr task(load_tasks.pop_front()); task.get() != nullptr; task.reset(load_tasks.pop_front())) {
+ soinfo* si = find_library_internal(load_tasks, task->get_name(), dlflags, extinfo);
+ if (si == nullptr) {
+ return false;
+ }
+
+ soinfo* needed_by = task->get_needed_by();
+
+ if (is_recursive(si, needed_by)) {
+ soinfo_free(si);
+ return false;
+ }
+
+ si->ref_count++;
+ if (needed_by != nullptr) {
+ needed_by->add_child(si);
+ }
+ found_libs.push_front(si);
+
+ // When ld_preloads is not null first
+ // ld_preloads_size libs are in fact ld_preloads.
+ if (ld_preloads != nullptr && soinfos_size < ld_preloads_size) {
+ ld_preloads[soinfos_size] = si;
+ }
+
+ if (soinfos_size<library_names_size) {
+ soinfos[soinfos_size++] = si;
+ }
+ }
+
+ // Step 2: link libraries.
+ soinfo* si;
+ while ((si = found_libs.pop_front()) != nullptr) {
+ if ((si->flags & FLAG_LINKED) == 0) {
+ if (!si->LinkImage(extinfo)) {
+ return false;
+ }
+ si->flags |= FLAG_LINKED;
+ }
+ }
+
+ // All is well - found_libs and load_tasks are empty at this point
+ // and all libs are successfully linked.
+ failure_guard.disable();
+ return true;
+}
+
+static soinfo* find_library(const char* name, int dlflags, const android_dlextinfo* extinfo) {
+ if (name == nullptr) {
+ somain->ref_count++;
+ return somain;
+ }
+
+ soinfo* si;
+
+ if (!find_libraries(&name, 1, &si, nullptr, 0, dlflags, extinfo)) {
+ return nullptr;
+ }
+
return si;
}
@@ -913,32 +1035,23 @@
si->CallDestructors();
if (si->has_min_version(0)) {
- // It is not safe to do si->get_children().for_each, because
- // during soinfo_free the child will concurrently modify the si->children
- // list, therefore we create a copy and use it to unload children.
- size_t children_count = si->get_children().size();
- soinfo* children[children_count];
- si->get_children().copy_to_array(children, children_count);
-
- for (size_t i = 0; i < children_count; ++i) {
- TRACE("%s needs to unload %s", si->name, children[i]->name);
- soinfo_unload(children[i]);
+ soinfo* child = nullptr;
+ while ((child = si->get_children().pop_front()) != nullptr) {
+ TRACE("%s needs to unload %s", si->name, child->name);
+ soinfo_unload(child);
}
} else {
- for (ElfW(Dyn)* d = si->dynamic; d->d_tag != DT_NULL; ++d) {
- if (d->d_tag == DT_NEEDED) {
- const char* library_name = si->strtab + d->d_un.d_val;
- TRACE("%s needs to unload %s", si->name, library_name);
- soinfo* needed = find_library(library_name, RTLD_NOLOAD, nullptr);
- if (needed != nullptr) {
- soinfo_unload(needed);
- } else {
- // Not found: for example if symlink was deleted between dlopen and dlclose
- // Since we cannot really handle errors at this point - print and continue.
- PRINT("warning: couldn't find %s needed by %s on unload.", library_name, si->name);
- }
+ for_each_dt_needed(si, [&] (const char* library_name) {
+ TRACE("deprecated (old format of soinfo): %s needs to unload %s", si->name, library_name);
+ soinfo* needed = find_library(library_name, RTLD_NOLOAD, nullptr);
+ if (needed != nullptr) {
+ soinfo_unload(needed);
+ } else {
+ // Not found: for example if symlink was deleted between dlopen and dlclose
+ // Since we cannot really handle errors at this point - print and continue.
+ PRINT("warning: couldn't find %s needed by %s on unload.", library_name, si->name);
}
- }
+ });
}
notify_gdb_of_unload(si);
@@ -1047,9 +1160,6 @@
#if defined(USE_RELA)
int soinfo::Relocate(ElfW(Rela)* rela, unsigned count) {
- ElfW(Sym)* s;
- soinfo* lsi;
-
for (size_t idx = 0; idx < count; ++idx, ++rela) {
unsigned type = ELFW(R_TYPE)(rela->r_info);
unsigned sym = ELFW(R_SYM)(rela->r_info);
@@ -1061,6 +1171,10 @@
if (type == 0) { // R_*_NONE
continue;
}
+
+ ElfW(Sym)* s = nullptr;
+ soinfo* lsi = nullptr;
+
if (sym != 0) {
sym_name = reinterpret_cast<const char*>(strtab + symtab[sym].st_name);
s = soinfo_do_lookup(this, sym_name, &lsi);
@@ -1119,8 +1233,6 @@
sym_addr = static_cast<ElfW(Addr)>(s->st_value + lsi->load_bias);
}
count_relocation(kRelocSymbol);
- } else {
- s = nullptr;
}
switch (type) {
@@ -1314,9 +1426,6 @@
#else // REL, not RELA.
int soinfo::Relocate(ElfW(Rel)* rel, unsigned count) {
- ElfW(Sym)* s;
- soinfo* lsi;
-
for (size_t idx = 0; idx < count; ++idx, ++rel) {
unsigned type = ELFW(R_TYPE)(rel->r_info);
// TODO: don't use unsigned for 'sym'. Use uint32_t or ElfW(Addr) instead.
@@ -1329,6 +1438,10 @@
if (type == 0) { // R_*_NONE
continue;
}
+
+ ElfW(Sym)* s = nullptr;
+ soinfo* lsi = nullptr;
+
if (sym != 0) {
sym_name = reinterpret_cast<const char*>(strtab + symtab[sym].st_name);
s = soinfo_do_lookup(this, sym_name, &lsi);
@@ -1390,8 +1503,6 @@
sym_addr = static_cast<ElfW(Addr)>(s->st_value + lsi->load_bias);
}
count_relocation(kRelocSymbol);
- } else {
- s = nullptr;
}
switch (type) {
@@ -1546,7 +1657,7 @@
for (size_t g = gotsym; g < symtabno; g++, sym++, got++) {
// This is an undefined reference... try to locate it.
const char* sym_name = si->strtab + sym->st_name;
- soinfo* lsi;
+ soinfo* lsi = nullptr;
ElfW(Sym)* s = soinfo_do_lookup(si, sym_name, &lsi);
if (s == nullptr) {
// We only allow an undefined symbol if this is a weak reference.
@@ -1643,6 +1754,9 @@
}
void soinfo::CallDestructors() {
+ if (!constructors_called) {
+ return;
+ }
TRACE("\"%s\": calling destructors", name);
// DT_FINI_ARRAY must be parsed in reverse order.
@@ -1728,7 +1842,7 @@
return false;
}
-// This is a return on get_children() in case
+// This is a return on get_children()/get_parents() if
// 'this->flags' does not have FLAG_NEW_SOINFO set.
static soinfo::soinfo_list_t g_empty_list;
@@ -1740,6 +1854,14 @@
return g_empty_list;
}
+soinfo::soinfo_list_t& soinfo::get_parents() {
+ if ((this->flags & FLAG_NEW_SOINFO) == 0) {
+ return g_empty_list;
+ }
+
+ return this->parents;
+}
+
/* Force any of the closed stdin, stdout and stderr to be associated with
/dev/null. */
static int nullify_closed_stdio() {
@@ -1801,20 +1923,18 @@
return return_value;
}
-bool soinfo::LinkImage(const android_dlextinfo* extinfo) {
- bool relocating_linker = (flags & FLAG_LINKER) != 0;
+bool soinfo::PrelinkImage() {
+ phdr_table_get_dynamic_section(phdr, phnum, load_bias, &dynamic);
- /* We can't debug anything until the linker is relocated */
+ /* We can't log anything until the linker is relocated */
+ bool relocating_linker = (flags & FLAG_LINKER) != 0;
if (!relocating_linker) {
INFO("[ linking %s ]", name);
DEBUG("si->base = %p si->flags = 0x%08x", reinterpret_cast<void*>(base), flags);
}
/* Extract dynamic section */
- size_t dynamic_count;
- ElfW(Word) dynamic_flags;
- phdr_table_get_dynamic_section(phdr, phnum, load_bias, &dynamic,
- &dynamic_count, &dynamic_flags);
+ ElfW(Word) dynamic_flags = phdr->p_flags;
if (dynamic == nullptr) {
if (!relocating_linker) {
DL_ERR("missing PT_DYNAMIC in \"%s\"", name);
@@ -1882,7 +2002,7 @@
// if the dynamic table is writable
// FIXME: not working currently for N64
// The flags for the LOAD and DYNAMIC program headers do not agree.
-// The LOAD section containng the dynamic table has been mapped as
+// The LOAD section containing the dynamic table has been mapped as
// read-only, but the DYNAMIC header claims it is writable.
#if !(defined(__mips__) && defined(__LP64__))
if ((dynamic_flags & PF_W) != 0) {
@@ -2028,38 +2148,10 @@
DL_ERR("empty/missing DT_SYMTAB in \"%s\"", name);
return false;
}
+ return true;
+}
- // If this is the main executable, then load all of the libraries from LD_PRELOAD now.
- if (flags & FLAG_EXE) {
- memset(g_ld_preloads, 0, sizeof(g_ld_preloads));
- size_t preload_count = 0;
- for (size_t i = 0; g_ld_preload_names[i] != nullptr; i++) {
- soinfo* lsi = find_library(g_ld_preload_names[i], 0, nullptr);
- if (lsi != nullptr) {
- g_ld_preloads[preload_count++] = lsi;
- } else {
- // As with glibc, failure to load an LD_PRELOAD library is just a warning.
- DL_WARN("could not load library \"%s\" from LD_PRELOAD for \"%s\"; caused by %s",
- g_ld_preload_names[i], name, linker_get_error_buffer());
- }
- }
- }
-
- for (ElfW(Dyn)* d = dynamic; d->d_tag != DT_NULL; ++d) {
- if (d->d_tag == DT_NEEDED) {
- const char* library_name = strtab + d->d_un.d_val;
- DEBUG("%s needs %s", name, library_name);
- soinfo* lsi = find_library(library_name, 0, nullptr);
- if (lsi == nullptr) {
- strlcpy(tmp_err_buf, linker_get_error_buffer(), sizeof(tmp_err_buf));
- DL_ERR("could not load library \"%s\" needed by \"%s\"; caused by %s",
- library_name, name, tmp_err_buf);
- return false;
- }
-
- add_child(lsi);
- }
- }
+bool soinfo::LinkImage(const android_dlextinfo* extinfo) {
#if !defined(__LP64__)
if (has_text_relocations) {
@@ -2121,7 +2213,6 @@
}
#endif
- flags |= FLAG_LINKED;
DEBUG("[ finished linking %s ]", name);
#if !defined(__LP64__)
@@ -2183,6 +2274,7 @@
si->size = phdr_table_get_load_size(si->phdr, si->phnum);
si->load_bias = get_elf_exec_load_bias(ehdr_vdso);
+ si->PrelinkImage();
si->LinkImage(nullptr);
#endif
}
@@ -2216,7 +2308,7 @@
ElfW(Ehdr)* elf_hdr = reinterpret_cast<ElfW(Ehdr)*>(linker_base);
ElfW(Phdr)* phdr = reinterpret_cast<ElfW(Phdr)*>(linker_base + elf_hdr->e_phoff);
phdr_table_get_dynamic_section(phdr, elf_hdr->e_phnum, linker_base,
- &linker_soinfo_for_gdb.dynamic, nullptr, nullptr);
+ &linker_soinfo_for_gdb.dynamic);
insert_soinfo_into_debug_map(&linker_soinfo_for_gdb);
}
@@ -2312,6 +2404,37 @@
somain = si;
+ si->PrelinkImage();
+
+ // Load ld_preloads and dependencies.
+ StringLinkedList needed_library_name_list;
+ size_t needed_libraries_count = 0;
+ size_t ld_preloads_count = 0;
+ while (g_ld_preload_names[ld_preloads_count] != nullptr) {
+ needed_library_name_list.push_back(g_ld_preload_names[ld_preloads_count++]);
+ ++needed_libraries_count;
+ }
+
+ for_each_dt_needed(si, [&](const char* name) {
+ needed_library_name_list.push_back(name);
+ ++needed_libraries_count;
+ });
+
+ const char* needed_library_names[needed_libraries_count];
+ soinfo* needed_library_si[needed_libraries_count];
+
+ memset(needed_library_names, 0, sizeof(needed_library_names));
+ needed_library_name_list.copy_to_array(needed_library_names, needed_libraries_count);
+
+ if (needed_libraries_count > 0 && !find_libraries(needed_library_names, needed_libraries_count, needed_library_si, g_ld_preloads, ld_preloads_count, 0, nullptr)) {
+ __libc_format_fd(2, "CANNOT LINK EXECUTABLE DEPENDENCIES: %s\n", linker_get_error_buffer());
+ exit(EXIT_FAILURE);
+ }
+
+ for (size_t i = 0; i<needed_libraries_count; ++i) {
+ si->add_child(needed_library_si[i]);
+ }
+
if (!si->LinkImage(nullptr)) {
__libc_format_fd(2, "CANNOT LINK EXECUTABLE: %s\n", linker_get_error_buffer());
exit(EXIT_FAILURE);
@@ -2321,11 +2444,7 @@
si->CallPreInitConstructors();
- for (size_t i = 0; g_ld_preloads[i] != nullptr; ++i) {
- g_ld_preloads[i]->CallConstructors();
- }
-
- /* After the LinkImage, the si->load_bias is initialized.
+ /* After the PrelinkImage, the si->load_bias is initialized.
* For so lib, the map->l_addr will be updated in notify_gdb_of_load.
* We need to update this value for so exe here. So Unwind_Backtrace
* for some arch like x86 could work correctly within so exe.
@@ -2440,7 +2559,7 @@
linker_so.phnum = elf_hdr->e_phnum;
linker_so.flags |= FLAG_LINKER;
- if (!linker_so.LinkImage(nullptr)) {
+ if (!(linker_so.PrelinkImage() && linker_so.LinkImage(nullptr))) {
// It would be nice to print an error message, but if the linker
// can't link itself, there's no guarantee that we'll be able to
// call write() (because it involves a GOT reference). We may as