We have two issues with dlopen  issued from static executables when
the global scope is accessed.
First, an attempt to add a dynamic shared module to the global scope
(i.e. with RTLD_GLOBAL requested) crashes like this:
Program received signal SIGSEGV, Segmentation fault.
add_to_global (new=0x49ef60) at dl-open.c:101
This is because at this point in static executables no global search list
data structure has been initialised and as result the _ns_main_searchlist
member of _dl_ns[LM_ID_BASE] (the initial namespace) is null.
Second, an attempt to access the global scope itself (i.e. by using a
null file name pointer) fails with an obscure error like:
/path/to/dir/: cannot read file data: Is a directory
(that can be retrieved with dlerror), where /path/to/dir/ is the first
directory searched according to the dynamic load engine's shared module
lookup policy. This is because the null file name pointer is internally
converted to an empty string, that is in turn looked for and then opened
in the directories examined. Of course that succeeds -- the directory
itself is opened, but then the engine gets confused by the outcome and
bails out. The use of an empty string works in dynamic executables,
because for them the dynamic loader has already prepared a link map
referring to a file whose name is empty -- which, by means of how that
code works, is the executable itself -- and that makes the loader abandon
any file look-ups and use the link map already present.
The two issues both boil down to the same bug -- no global scope present.
Patch offered: http://sourceware.org/ml/libc-alpha/2013-01/msg00635.html
Author: Maciej W. Rozycki <email@example.com>
Date: Fri Jun 28 16:20:26 2013 +0100
[BZ #15022] Correct global-scope dlopen issues in static executables.
This change creates a link map in static executables to serve as the
global search list for dlopen. It fixes a problem with the inability
to access the global symbol object and a crash on an attempt to map a
DSO into the global scope. Some code that has become dead after the
addition of this link map is removed too and test cases are provided.
2013-06-28 Maciej W. Rozycki <firstname.lastname@example.org>
* elf/dl-support.c (_dl_main_map): New variable.
(_dl_ns): Use it to initialize [LM_ID_BASE] element.
(_dl_nns, _dl_load_adds): Set to 1.
(_dl_initial_searchlist): Refer to _dl_main_map.
(_dl_non_dynamic_init): Initialize _dl_main_map.l_origin.
* elf/dl-load.c (_dl_dst_substitute) [!SHARED]: Remove fallback
call to _dl_get_origin.
* elf/dl-open.c (dl_open_worker): Remove !SHARED special casing
(_dl_open) [!SHARED]: Remove code to initialize GL(dl_nns).
* dlfcn/modstatic3.c: New file.
* dlfcn/tststatic3.c: New file.
* dlfcn/tststatic4.c: New file.
* dlfcn/Makefile (tests): Add tststatic3 and tststatic4.
(modules-names): Add modstatic3.
(tststatic3-ENV, tststatic4-ENV): New variables.
($(objpfx)tststatic3, $(objpfx)tststatic3.out): New dependencies.
($(objpfx)tststatic4, $(objpfx)tststatic4.out): Likewise.
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- Log -----------------------------------------------------------------
Author: Maciej W. Rozycki <email@example.com>
Date: Fri Jan 31 17:51:31 2014 +0000
[BZ #16046] Static dlopen correction fallout fixes.
Fixes to address issues from BZ #15022 resolution, as follows:
* TLS updates to csu/libc-tls.c -- we now have a proper main map, so
there's no longer a need to create a separate fake one to keep TLS
* random updates to elf/dl-close.c -- LM_ID_BASE is now a valid name
space ID for static executables as well, so assert that we don't
unload the main map. Similarly dl_nns isn't supposed to be 0 for
static executables anymore,
* actual BZ #16046 fix to elf/dl-iteratephdr.c -- the dl_iterate_phdr
special function for static executables isn't needed anymore, provided
that l_phdr and l_phnum members of the main map have been properly
initialized (done in _dl_non_dynamic_init in elf/dl-support.c now),
* ld.so.cache loader update to elf/dl-load.c --
GL(dl_ns)[LM_ID_BASE]._ns_loaded is now always initialized in static
executables so can become the fallback loader map to check for
DF_1_NODEFLIB, provided that the l_flags_1 member of the main map has
been properly initialized (done in elf/dl-support.c now); this also
ensures previous semantics elsewhere in elf/dl-load.c,
* matching updates to elf/dl-support.c -- to complement the two fixes
Summary of changes:
ChangeLog | 16 ++++++++++++++++
NEWS | 14 +++++++-------
csu/libc-tls.c | 27 ++++++++++++---------------
elf/dl-close.c | 10 +---------
elf/dl-iteratephdr.c | 30 ------------------------------
elf/dl-load.c | 14 ++++----------
elf/dl-support.c | 3 +++
7 files changed, 43 insertions(+), 71 deletions(-)