Extra changes needed for backport:

* rename field "rtld_catch" to "rtld_catch_f" to avoid conflict
  between "struct rtld-catch" and rtld_catch macro

* move rtld_catch into one of the unused padding fields to preserve
  ABI

* Validate that the padding fields used don't overlap other fields.

diff -rup a/elf/dl-catch.c b/elf/dl-catch.c
--- a/elf/dl-catch.c	2024-09-04 16:30:02.086402568 -0400
+++ b/elf/dl-catch.c	2024-09-04 16:55:01.933440181 -0400
@@ -59,7 +59,7 @@ get_catch (void)
     return rtld_catch_notls;
   else
 #endif
-    return THREAD_GETMEM (THREAD_SELF, rtld_catch);
+    return THREAD_GETMEM (THREAD_SELF, rtld_catch_f);
 }
 
 static void
@@ -70,7 +70,7 @@ set_catch (struct rtld_catch *catch)
     rtld_catch_notls = catch;
   else
 #endif
-    THREAD_SETMEM (THREAD_SELF, rtld_catch, catch);
+    THREAD_SETMEM (THREAD_SELF, rtld_catch_f, catch);
 }
 
 /* Lossage while resolving the program's own symbols is always fatal.  */
diff -rup a/nptl/descr.h b/nptl/descr.h
--- a/nptl/descr.h	2024-08-29 11:29:16.801811033 -0400
+++ b/nptl/descr.h	2024-08-29 11:48:56.547644398 -0400
@@ -164,6 +164,12 @@ struct pthread
     void *__padding[24];
   };
 
+#ifdef __x86_64__
+#define rtld_catch_f header.__padding[7]
+#else
+#define rtld_catch_f __padding[23]
+#endif
+
   /* This descriptor's link on the GL (dl_stack_used) or
      GL (dl_stack_user) list.  */
   list_t list;
@@ -396,9 +402,6 @@ struct pthread
      masked.)  */
   sigset_t sigmask;
 
-  /* Used by the exception handling implementation in the dynamic loader.  */
-  struct rtld_catch *rtld_catch;
-
   /* Indicates whether is a C11 thread created by thrd_creat.  */
   bool c11;
 
@@ -432,6 +435,12 @@ struct pthread
    + sizeof ((struct pthread) {}.rseq_area))
 } __attribute ((aligned (TCB_ALIGNMENT)));
 
+#ifdef __x86_64__
+_Static_assert (sizeof ((*(struct pthread *)0).header) > sizeof ((*(struct pthread *)0).__padding), "rtld_catch");
+#else
+_Static_assert (sizeof ((*(struct pthread *)0).header) < sizeof ((*(struct pthread *)0).__padding), "rtld_catch");
+#endif
+
 static inline bool
 cancel_enabled_and_canceled (int value)
 {
diff -rup a/sysdeps/mach/hurd/i386/tls.h b/sysdeps/mach/hurd/i386/tls.h
--- a/sysdeps/mach/hurd/i386/tls.h	2024-08-29 11:29:16.810811382 -0400
+++ b/sysdeps/mach/hurd/i386/tls.h	2024-08-29 11:35:45.262899113 -0400
@@ -50,7 +50,7 @@ typedef struct
   struct hurd_sigstate *_hurd_sigstate;
 
   /* Used by the exception handling implementation in the dynamic loader.  */
-  struct rtld_catch *rtld_catch;
+  struct rtld_catch *rtld_catch_f;
 } tcbhead_t;
 #endif