git: 23427c8e1fed - main - libc: allow __cxa_atexit handlers to be added during __cxa_finalize

The branch main has been updated by kevans:

URL: https://cgit.FreeBSD.org/src/commit/?id=23427c8e1fedb9fc68ad0bd27a59c7ffd2b3008c

commit 23427c8e1fedb9fc68ad0bd27a59c7ffd2b3008c
Author:     Aurélien Croc de Suray <freebsd@ap2c.com>
AuthorDate: 2025-04-05 00:47:53 +0000
Commit:     Kyle Evans <kevans@FreeBSD.org>
CommitDate: 2025-04-05 00:47:53 +0000

    libc: allow __cxa_atexit handlers to be added during __cxa_finalize
    
    science/dlib-cpp reveals an interesting scenario that works fine on
    other platforms but not on FreeBSD; notably, it ends up creating a new
    global object from some destructor which is called during
    __cxa_finalize.  This breaks when libdlib is dlopen()ed and then
    subsequently dlclose()ed, as we never end up invoking the created
    object's dtor until program exit when the shlib is already unmapped.
    
    Fix it by noting when we're in the middle of __cxa_finalize for a dso,
    and then restarting the search if __cxa_atexit() was called in the
    middle somewhere.
    
    We wait until we've processed the initial set before starting over and
    processing the newly added handlers as if it were a complete set of
    handlers added during runtime.  The alternative is calling them as
    they're added to maintain a LIFO in terms of total ordering, but in
    theory a constructor could add another global object that also needs to
    be destroyed, and that object needs to be destroyed after the one that
    constructed it to avoid creating unexpected lifetime issues.
    
    This manifests in the pdlib PHP extension for dlib crashing, see [0].
    
    [0] https://github.com/goodspb/pdlib/issues/39
    
    PR:             285870
    Reviewed by:    kevans (also supplied commit message)
    MFC after:      1 week
---
 lib/libc/stdlib/atexit.c | 61 ++++++++++++++++++++++++++++--------------------
 1 file changed, 36 insertions(+), 25 deletions(-)

diff --git a/lib/libc/stdlib/atexit.c b/lib/libc/stdlib/atexit.c
index e5aa66c51f38..6e4a12f9e530 100644
--- a/lib/libc/stdlib/atexit.c
+++ b/lib/libc/stdlib/atexit.c
@@ -35,6 +35,7 @@
 #include "namespace.h"
 #include <errno.h>
 #include <link.h>
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -56,6 +57,8 @@ _Block_copy(void*);
 #define	ATEXIT_FN_CXA	2
 
 static pthread_mutex_t atexit_mutex = PTHREAD_MUTEX_INITIALIZER;
+static void *current_finalize_dso = NULL;
+static bool call_finalize_again = false;
 
 #define _MUTEX_LOCK(x)		if (__isthreaded) _pthread_mutex_lock(x)
 #define _MUTEX_UNLOCK(x)	if (__isthreaded) _pthread_mutex_unlock(x)
@@ -115,6 +118,9 @@ atexit_register(struct atexit_fn *fptr)
 		__atexit = p;
 	}
 	p->fns[p->ind++] = *fptr;
+	if (current_finalize_dso != NULL &&
+	    current_finalize_dso == fptr->fn_dso)
+		call_finalize_again = true;
 	_MUTEX_UNLOCK(&atexit_mutex);
 	return 0;
 }
@@ -208,33 +214,38 @@ __cxa_finalize(void *dso)
 	}
 
 	_MUTEX_LOCK(&atexit_mutex);
-	for (p = __atexit; p; p = p->next) {
-		for (n = p->ind; --n >= 0;) {
-			if (p->fns[n].fn_type == ATEXIT_FN_EMPTY)
-				continue; /* already been called */
-			fn = p->fns[n];
-			if (dso != NULL && dso != fn.fn_dso) {
-				/* wrong DSO ? */
-				if (!has_phdr || global_exit ||
-				    !__elf_phdr_match_addr(&phdr_info,
-				    fn.fn_ptr.cxa_func))
-					continue;
+	current_finalize_dso = dso;
+	do {
+		call_finalize_again = false;
+		for (p = __atexit; p; p = p->next) {
+			for (n = p->ind; --n >= 0;) {
+				if (p->fns[n].fn_type == ATEXIT_FN_EMPTY)
+					continue; /* already been called */
+				fn = p->fns[n];
+				if (dso != NULL && dso != fn.fn_dso) {
+					/* wrong DSO ? */
+					if (!has_phdr || global_exit ||
+					    !__elf_phdr_match_addr(&phdr_info,
+					    fn.fn_ptr.cxa_func))
+						continue;
+				}
+				/*
+				  Mark entry to indicate that this particular
+				  handler has already been called.
+				*/
+				p->fns[n].fn_type = ATEXIT_FN_EMPTY;
+				_MUTEX_UNLOCK(&atexit_mutex);
+
+				/* Call the function of correct type. */
+				if (fn.fn_type == ATEXIT_FN_CXA)
+					fn.fn_ptr.cxa_func(fn.fn_arg);
+				else if (fn.fn_type == ATEXIT_FN_STD)
+					fn.fn_ptr.std_func();
+				_MUTEX_LOCK(&atexit_mutex);
 			}
-			/*
-			  Mark entry to indicate that this particular handler
-			  has already been called.
-			*/
-			p->fns[n].fn_type = ATEXIT_FN_EMPTY;
-		        _MUTEX_UNLOCK(&atexit_mutex);
-		
-			/* Call the function of correct type. */
-			if (fn.fn_type == ATEXIT_FN_CXA)
-				fn.fn_ptr.cxa_func(fn.fn_arg);
-			else if (fn.fn_type == ATEXIT_FN_STD)
-				fn.fn_ptr.std_func();
-			_MUTEX_LOCK(&atexit_mutex);
 		}
-	}
+	} while (call_finalize_again);
+	current_finalize_dso = NULL;
 	_MUTEX_UNLOCK(&atexit_mutex);
 	if (dso == NULL)
 		_MUTEX_DESTROY(&atexit_mutex);