libdw: Fix eu_search_tree TOCTOU bugs

libdw: Fix eu_search_tree TOCTOU bugs

eu_tfind is used to facilitate lazy loading throughout libdw.
If a result is not found via eu_tfind, work is done to load
the result and cache it in an eu_search_tree.

Some calls to eu_tfind allow for TOCTOU bugs.  Multiple threads
might race to call eu_tfind on some result that hasn't yet been
cached.  Multiple threads may then attempt to load the result
which might cause an unnecessary amount of memory to be allocated.
Additionally this memory may not get released when the associated
libdw data structure is freed.

Fix this by adding additional locking to ensure that only one
thread performs lazy loading.

One approach used in this patch is to preserve calls to eu_tfind
without additional locking, but when the result isn't found then
a lock is then used to synchronize access to the lazy loading code.
An extra eu_tfind call has been added at the start of these critical
section to synchronize verification that lazy loading should proceed.

Another approach used is to simply synchronize entire calls to
functions where lazy loading via eu_tfind might occur (__libdw_find_fde
and __libdw_intern_expression).  In this case, new _nolock variants of
the eu_t* functions are used to avoid unnecessary double locking.

lib/
	* eu-search.c: Add eu_tsearch_nolock, eu_tfind_nolock and
	eu_tdelete_nolock functions.
	* eu-search.h: Ditto.

libdw/
	* cfi.h (struct Dwarf_CFI_s): Declare new mutex.
	* dwarf_begin_elf.c (valid_p): Initialize all locks for fake CUs.
	* dwarf_cfi_addrframe.c (dwarf_cfi_addrframe): Place lock around
	__libdw_find_fde.
	* dwarf_end.c (cu_free): Deallocate all locks unconditionally,
	whether or not the CU is fake.
	* dwarf_frame_cfa.c (dwarf_frame_cfa): Place lock around
	__libdw_intern_expression.
	* dwarf_frame_register.c (dwarf_frame_register): Ditto.
	* dwarf_getcfi.c (dwarf_getcfi): Initialize cfi lock.
	* dwarf_getlocation.c (is_constant_offset): Synchronize access
	to lazy loading section.
	(getlocation): Place lock around __libdw_intern_expression.
	* dwarf_getmacros.c (cache_op_table): Synchronize access to lazy
	loading section.
	* frame-cache.c (__libdw_destroy_frame_cache): Free Dwarf_CFI
	mutex.
	* libdwP.h (struct Dwarf): Update macro_lock comment.
	(struct Dwarf_CU): Declare new mutex.
	libdw_findcu.c (__libdw_intern_next_unit): Initialize
	intern_lock.
	(__libdw_findcu): Adjust locking so that the first eu_tfind
	can be done without extra lock overhead.

Signed-off-by: Aaron Merey <amerey@redhat.com>