https://stackoverflow.com/questions/19796383
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RussianThe problem is that the loader only supports one shared library with -z initfirst, and libpthread.so (which is used by almost everything) already has this set. Even if you use LD_PRELOAD to load a library, libpthread's constructors will be called first.
You can get around this by patching the loader to support multiple shared libraries with -z initfirst. Here is a patch for ld.so version 2.21 which preserves the binary ABI but makes a linked list out of initfirst libraries and calls them with LD_PRELOAD constructors first.
diff --git a/elf/dl-load.c b/elf/dl-load.c
index 6dd8550..ac3b079 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1387,7 +1387,27 @@ 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;
+ {
+#if 0
+ struct initfirst_list *first = malloc(sizeof(*first));
+ first->which = l;
+ first->next = GL(dl_initfirst);
+ GL(dl_initfirst) = first;
+#else
+ struct initfirst_list *node = malloc(sizeof(*node));
+ node->which = l;
+ node->next = NULL;
+ struct initfirst_list *it = GL(dl_initfirst);
+ if (!it)
+ GL(dl_initfirst) = node;
+ else
+ {
+ while (it->next)
+ it = it->next;
+ it->next = node;
+ }
+#endif
+ }
/* Finally the file information. */
l->l_dev = st.st_dev;
diff --git a/elf/dl-map-segments.h b/elf/dl-map-segments.h
index baaa813..bca961c 100644
--- a/elf/dl-map-segments.h
+++ b/elf/dl-map-segments.h
@@ -55,7 +55,11 @@ _dl_map_segments (struct link_map *l, int fd,
/* Remember which part of the address space this object uses. */
l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
c->prot,
+#if 0
MAP_COPY|MAP_FILE,
+#else
+ MAP_COPY|MAP_FILE|MAP_32BIT,
+#endif
fd, c->mapoff);
if (__glibc_unlikely ((void *) l->l_map_start == MAP_FAILED))
return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
diff --git a/elf/dl-support.c b/elf/dl-support.c
index 835dcb3..9ea0c05 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -148,7 +148,7 @@ struct r_search_path_elem *_dl_all_dirs;
struct r_search_path_elem *_dl_init_all_dirs;
/* The object to be initialized first. */
-struct link_map *_dl_initfirst;
+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 b421931..7bb7a69 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -318,7 +318,11 @@ struct rtld_global
EXTERN unsigned long long _dl_load_adds;
/* The object to be initialized first. */
- EXTERN struct link_map *_dl_initfirst;
+ /*EXTERN struct link_map *_dl_initfirst;*/
+ EXTERN struct initfirst_list {
+ struct link_map *which;
+ struct initfirst_list *next;
+ } *_dl_initfirst;
#if HP_SMALL_TIMING_AVAIL || defined HP_TIMING_PAD
/* Start time on CPU clock. */
I suppose you can try hacking libpthread to not use -z initfirst, but this seems like the simplest option. I have used it successfully to get a constructor called before anything else. You just have to make sure your LD_PRELOADed library does not use libc, because then libc's constructors will be called first, and in a multithreaded program libc depends on libpthread, so pthread's constructors will be called before that.
Here's an example. I compile a hello, world program with -pthread (otherwise there are no problems). I write a small library which is meant to be LD_PRELOADed and which does not depend on libc. With the default loader, you can't get your init function called first:
$ cat hello.c
#include <stdio.h>
int main() {
puts("Hello, world!");
return 0;
}
$ gcc hello.c -o hello -pthread
$ cat superearly.c
#include <unistd.h>
#include <sys/syscall.h>
long write(int fd, const void *buffer, size_t len) {
unsigned long ret;
__asm__ __volatile__("syscall" : "=a"(ret) : "a"(__NR_write), "D"(fd),
"S"(buffer), "d"(len));
return ret;
}
void hello(void) __attribute__((constructor));
void hello(void) {
write(STDOUT_FILENO, "Got in first!\n", 14);
}
$ gcc superearly.c -fPIC -shared -nostdlib -o libsuperearly.so -Wl,-z,initfirst
$ LD_DEBUG=libs LD_PRELOAD=./libsuperearly.so ./hello
19997: find library=libpthread.so.0 [0]; searching
19997: search cache=/etc/ld.so.cache
19997: trying file=/lib/x86_64-linux-gnu/libpthread.so.0
19997:
19997: find library=libc.so.6 [0]; searching
19997: search cache=/etc/ld.so.cache
19997: trying file=/lib/x86_64-linux-gnu/libc.so.6
19997:
19997:
19997: calling init: /lib/x86_64-linux-gnu/libpthread.so.0
19997:
19997:
19997: calling init: /lib/x86_64-linux-gnu/libc.so.6
19997:
19997:
19997: calling init: ./libsuperearly.so
19997:
Got in first!
19997:
19997: initialize program: ./hello
19997:
19997:
19997: transferring control: ./hello
19997:
Hello, world!
19997:
19997: calling fini: ./hello [0]
19997:
19997:
19997: calling fini: /lib/x86_64-linux-gnu/libpthread.so.0 [0]
19997:
$
However, with an ld.so which has been patched with my patch above:
$ LD_DEBUG=libs LD_PRELOAD=./libsuperearly.so ~/libc/lib/ld-2.21.so ./hello
19986: find library=libpthread.so.0 [0]; searching
19986: search cache=/home/user/libc/etc/ld.so.cache
19986: trying file=/home/user/libc/lib/libpthread.so.0
19986:
19986: find library=libc.so.6 [0]; searching
19986: search cache=/home/user/libc/etc/ld.so.cache
19986: trying file=/home/user/libc/lib/libc.so.6
19986:
19986:
19986: calling init: ./libsuperearly.so
19986:
Got in first!
19986:
19986: calling init: /home/user/libc/lib/libpthread.so.0
19986:
19986:
19986: calling init: /home/user/libc/lib/libc.so.6
19986:
19986:
19986: initialize program: ./hello
19986:
Hello, world!
19986:
19986: calling fini: ./hello [0]
19986:
19986:
19986: calling fini: /home/user/libc/lib/libpthread.so.0 [0]
19986:
$
