GLIBC 2.35 (and probably earlier) patch to use PLT `pthread_mutex_*` functions in `pthread_cond_*`

[goldsteinn]

If you use this patch + custom glibc (suggest using patchelf so you can stick with the zero-recompile strategy) all code should be interposable without the COND_VAR define. The only thing lost (at least in 2.35+ is that pthread_mutex_destroy may succeed while there are still users of a lock in a pthread_cond_var. This realistically isn't an issue in most correct code.

Also the README doesn't really discuss this but to use LD_PRELOAD interposition your locks initialization must be compatible with PTHREAD_MUTEX_INITIALIZER.

From 6b63d57cd66c224d80ca9a131fbc0a7e3f9dbd37 Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Thu, 12 May 2022 23:27:35 -0500
Subject: [PATCH v1] Make pthread_cond_var lock usage go through PLT for
 interposition

---
 nptl/Makefile                  |  1 -
 nptl/pthread_cond_wait.c       |  6 +++---
 nptl/pthread_mutex_cond_lock.c | 23 -----------------------
 nptl/pthread_mutex_lock.c      |  7 ++++++-
 4 files changed, 9 insertions(+), 28 deletions(-)
 delete mode 100644 nptl/pthread_mutex_cond_lock.c

diff --git a/nptl/Makefile b/nptl/Makefile
index b585663974..5c779e77eb 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -132,7 +132,6 @@ routines = \
   pthread_keys \
   pthread_kill \
   pthread_kill_other_threads \
-  pthread_mutex_cond_lock \
   pthread_mutex_conf \
   pthread_mutex_consistent \
   pthread_mutex_destroy \
diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
index 20c348a503..9c1aa9e1d0 100644
--- a/nptl/pthread_cond_wait.c
+++ b/nptl/pthread_cond_wait.c
@@ -186,7 +186,7 @@ __condvar_cleanup_waiting (void *arg)

   /* XXX If locking the mutex fails, should we just stop execution?  This
      might be better than silently ignoring the error.  */
-  __pthread_mutex_cond_lock (cbuffer->mutex);
+  pthread_mutex_lock (cbuffer->mutex);
 }

 /* This condvar implementation guarantees that all calls to signal and
@@ -416,7 +416,7 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
      mutex after registering as waiter.
      If releasing the mutex fails, we just cancel our registration as a
      waiter and confirm that we have woken up.  */
-  err = __pthread_mutex_unlock_usercnt (mutex, 0);
+  err = pthread_mutex_unlock (mutex);
   if (__glibc_unlikely (err != 0))
     {
       __condvar_cancel_waiting (cond, seq, g, private);
@@ -604,7 +604,7 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,

   /* Woken up; now re-acquire the mutex.  If this doesn't fail, return RESULT,
      which is set to ETIMEDOUT if a timeout occured, or zero otherwise.  */
-  err = __pthread_mutex_cond_lock (mutex);
+  err = pthread_mutex_lock (mutex);
   /* XXX Abort on errors that are disallowed by POSIX?  */
   return (err != 0) ? err : result;
 }
diff --git a/nptl/pthread_mutex_cond_lock.c b/nptl/pthread_mutex_cond_lock.c
deleted file mode 100644
index f3af514305..0000000000
--- a/nptl/pthread_mutex_cond_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-#include <pthreadP.h>
-
-#define LLL_MUTEX_LOCK(mutex) \
-  lll_cond_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex))
-#define LLL_MUTEX_LOCK_OPTIMIZED(mutex) LLL_MUTEX_LOCK (mutex)
-
-/* Not actually elided so far. Needed? */
-#define LLL_MUTEX_LOCK_ELISION(mutex)  \
-  ({ lll_cond_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex)); 0; })
-
-#define LLL_MUTEX_TRYLOCK(mutex) \
-  lll_cond_trylock ((mutex)->__data.__lock)
-#define LLL_MUTEX_TRYLOCK_ELISION(mutex) LLL_MUTEX_TRYLOCK(mutex)
-
-/* We need to assume that there are other threads blocked on the futex.
-   See __pthread_mutex_lock_full for further details.  */
-#define LLL_ROBUST_MUTEX_LOCK_MODIFIER FUTEX_WAITERS
-#define PTHREAD_MUTEX_LOCK  __pthread_mutex_cond_lock
-#define __pthread_mutex_lock_full __pthread_mutex_cond_lock_full
-#define NO_INCR
-#define PTHREAD_MUTEX_VERSIONS 0
-
-#include <nptl/pthread_mutex_lock.c>
diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index d2e652d151..b4adfa2937 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -625,7 +625,12 @@ versioned_symbol (libpthread, ___pthread_mutex_lock, pthread_mutex_lock,
 compat_symbol (libpthread, ___pthread_mutex_lock, __pthread_mutex_lock,
           GLIBC_2_0);
 # endif
-#endif /* PTHREAD_MUTEX_VERSIONS */
+#else
+libc_hidden_ver (PTHREAD_MUTEX_LOCK, __pthread_mutex_lock)
+# ifndef SHARED
+strong_alias (___pthread_mutex_lock, __pthread_mutex_lock)
+# endif
+#endif

 #ifdef NO_INCR
-- 
2.34.1