Re: gcc-14 Wflex-array-member-not-at-end may-be-ub in struct pthread

[Paul Eggert <eggert@cs.ucla.edu>]

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On 2023-07-05 06:46, Cristian Rodríguez via Libc-alpha wrote:
>    char end_padding[]; --> This is incorrect, since struct rseq contains a
> flexible array it must be the last member..

? struct rseq doesn't contain a flexible array on the master branch, so 
there's no error here.

Perhaps you tried to nest 'struct pthread' inside another struct? If so, 
the C standard doesn't allow that but the attached untested patch should 
be a trivial fix.

Here's the wording in the C23 draft standard section 6.7.2.1 paragraph 
3, which is what GCC is trying to enforce here:

"the last member of a structure with more than one named member may have 
incomplete array type; such a structure (and any union containing, 
possibly recursively, a member that is such a structure) shall not be a 
member of a structure or an element of an array."

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From b4eb0714b3bf6e078314af6bbe5331d8f111b671 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 5 Jul 2023 08:50:26 -0700
Subject: [PATCH] =?UTF-8?q?make=20=E2=80=98struct=20pthread=E2=80=99=20a?=
 =?UTF-8?q?=20complete=20type?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* nptl/descr.h (struct pthread): Remove end_padding member, which
made this type incomplete.
(PTHREAD_STRUCT_END_PADDING): Stop using end_padding.
---
 nptl/descr.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/nptl/descr.h b/nptl/descr.h
index 746a4b9e4a..d06abd6ad9 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -408,11 +408,11 @@ struct pthread
   /* rseq area registered with the kernel.  */
   struct rseq rseq_area;
 
-  /* This member must be last.  */
-  char end_padding[];
-
+  /* Amount of end padding, if any, in this structure.
+     This definition relies on rseq_area being last.  */
 #define PTHREAD_STRUCT_END_PADDING \
-  (sizeof (struct pthread) - offsetof (struct pthread, end_padding))
+  (sizeof (struct pthread) - offsetof (struct pthread, rseq_area) \
+   + sizeof (struct rseq))
 } __attribute ((aligned (TCB_ALIGNMENT)));
 
 static inline bool
-- 
2.39.2