[PATCH] Support -z initfirst for multiple shared libraries

[d wk <dwksrc at gmail dot com>]

Hello libc developers,

In a project of mine, I needed to run some code before any constructors
from any system libraries (such as libc or libpthread). The linker/loader
-z initfirst feature is perfect for this, but it only supports one shared
library. Unfortunately libpthread also uses this feature (I assume the
feature exists because pthread needed it), so my project was incompatible
with libpthread.

So, I wrote a small patch which changes the single dl_initfirst variable
into a linked list. This patch does not change the size of any data
structures (it's ABI compatible), just turns dl_initfirst into a list. The
list is not freed (the allocator wouldn't free it anyway), and insertion
into the list is quadratic, but I expect there will never be more than
a handful of initfirst libraries!

This patch records initfirst libraries in load order, so LD_PRELOAD
libraries will have their constructors called before libpthread. If the
opposite behaviour is desired, the LD_PRELOAD'd library can always declare
a dependency on libpthread. Normally LD_PRELOAD constructors are run last,
which is very inconvenient when trying to inject new functionality, and
I expect anyone using -z initfirst with LD_PRELOAD to really want to run
first. The patch is written against latest glibc 2.23 (I also tested on
glibc 2.21, and it's not quite compatible with 2.17 since the other data
structures changed).

I was not the first person to run into this problem,
someone wanted the same thing on stack overflow two years
ago. You can see my answer there with a complete test case.
http://stackoverflow.com/questions/19796383/linux-ld-preload-z-initfirst/36143861

Hope you will accept this patch. Comments welcome. Thanks,

~ dwk.


----[ cut here ]----
Support -z initfirst for multiple shared libraries (run in load order).

This is particularly useful when combined with LD_PRELOAD, as it is then
possible to run constructors before any code in other libraries runs.
---
 elf/dl-init.c              |  9 ++++++++-
 elf/dl-load.c              | 19 ++++++++++++++++++-
 elf/dl-support.c           |  4 ++--
 sysdeps/generic/ldsodefs.h |  7 +++++--
 4 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/elf/dl-init.c b/elf/dl-init.c
index 818c3aa..da59d1f 100644
--- a/elf/dl-init.c
+++ b/elf/dl-init.c
@@ -84,7 +84,14 @@ _dl_init (struct link_map *main_map, int argc, char
**argv, char **env)

   if (__glibc_unlikely (GL(dl_initfirst) != NULL))
     {
-      call_init (GL(dl_initfirst), argc, argv, env);
+      struct initfirst_list *initfirst;
+      for(initfirst = GL(dl_initfirst); initfirst; initfirst = initfirst->next)
+        {
+          call_init (initfirst->which, argc, argv, env);
+        }
+
+      /* We do not try to free this list, as the memory will not be reclaimed
+         by the allocator unless there were no intervening malloc()'s.  */
       GL(dl_initfirst) = NULL;
     }

diff --git a/elf/dl-load.c b/elf/dl-load.c
index c0d6249..1efabbf 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1388,7 +1388,24 @@ cannot enable executable stack as shared object
requires");

   /* Remember whether this object must be initialized first.  */
   if (l->l_flags_1 & DF_1_INITFIRST)
-    GL(dl_initfirst) = l;
+    {
+      struct initfirst_list *new_node = malloc(sizeof(*node));
+      struct initfirst_list *it = GL(dl_initfirst);
+      new_node->which = l;
+      new_node->next = NULL;
+
+      /* We append to the end of the linked list. Whichever library was loaded
+         first has higher initfirst priority. This means that LD_PRELOAD
+         initfirst overrides initfirst in libraries linked normally.  */
+      if (!it)
+        GL(dl_initfirst) = new_node;
+      else
+        {
+          while (it->next)
+            it = it->next;
+          it->next = new_node;
+        }
+    }

   /* Finally the file information.  */
   l->l_file_id = id;
diff --git a/elf/dl-support.c b/elf/dl-support.c
index c30194c..d8b8acc 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -147,8 +147,8 @@ struct r_search_path_elem *_dl_all_dirs;
 /* All directories after startup.  */
 struct r_search_path_elem *_dl_init_all_dirs;

-/* The object to be initialized first.  */
-struct link_map *_dl_initfirst;
+/* The list of objects to be initialized first.  */
+struct initfirst_list *_dl_initfirst;

 /* Descriptor to write debug messages to.  */
 int _dl_debug_fd = STDERR_FILENO;
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index ddec0be..198c089 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -326,8 +326,11 @@ struct rtld_global
   /* Incremented whenever something may have been added to dl_loaded.  */
   EXTERN unsigned long long _dl_load_adds;

-  /* The object to be initialized first.  */
-  EXTERN struct link_map *_dl_initfirst;
+  /* The list of objects to be initialized first.  */
+  EXTERN struct initfirst_list {
+    struct link_map *which;
+    struct initfirst_list *next;
+  } *_dl_initfirst;

 #if HP_SMALL_TIMING_AVAIL
   /* Start time on CPU clock.  */
-- 
1.9.1