Date: Mon, 05 Jan 2026 10:44:06 +0000
Precedence: bulk
Mime-Version: 1.0
Message-ID: <20260105-bitops-find-helper-v2-1-ae70b4fc9ecc@google.com>
Subject: [PATCH v2] rust: bitops: fix missing _find_* functions on 32-bit ARM
From: Alice Ryhl <aliceryhl@google.com>
To: Yury Norov <yury.norov@gmail.com>, Burak Emir <bqe@google.com>, 
	Andreas Hindborg <a.hindborg@kernel.org>
Cc: Miguel Ojeda <ojeda@kernel.org>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	"=?utf-8?q?Bj=C3=B6rn_Roy_Baron?=" <bjorn3_gh@protonmail.com>, Benno Lossin <lossin@kernel.org>, 
	Trevor Gross <tmgross@umich.edu>, Danilo Krummrich <dakr@kernel.org>, rust-for-linux@vger.kernel.org, 
	linux-kernel@vger.kernel.org, stable@vger.kernel.org, 
	Alice Ryhl <aliceryhl@google.com>
Content-Type: text/plain; charset="utf-8"
DKIMCheck: Server passes DKIM test, 0 Spam score
SpamTally: Final spam score: -70

On 32-bit ARM, you may encounter linker errors such as this one:

	ld.lld: error: undefined symbol: _find_next_zero_bit
	>>> referenced by rust_binder_main.43196037ba7bcee1-cgu.0
	>>>               drivers/android/binder/rust_binder_main.o:(<rust_binder_main::process::Process>::insert_or_update_handle) in archive vmlinux.a
	>>> referenced by rust_binder_main.43196037ba7bcee1-cgu.0
	>>>               drivers/android/binder/rust_binder_main.o:(<rust_binder_main::process::Process>::insert_or_update_handle) in archive vmlinux.a

This error occurs because even though the functions are declared by
include/linux/find.h, the definition is #ifdef'd out on 32-bit ARM. This
is because arch/arm/include/asm/bitops.h contains:

	#define find_first_zero_bit(p,sz)	_find_first_zero_bit_le(p,sz)
	#define find_next_zero_bit(p,sz,off)	_find_next_zero_bit_le(p,sz,off)
	#define find_first_bit(p,sz)		_find_first_bit_le(p,sz)
	#define find_next_bit(p,sz,off)		_find_next_bit_le(p,sz,off)

And the underscore-prefixed function is conditional on #ifndef of the
non-underscore-prefixed name, but the declaration in find.h is *not*
conditional on that #ifndef.

To fix the linker error, we ensure that the symbols in question exist
when compiling Rust code. We do this by definining them in rust/helpers/
whenever the normal definition is #ifndef'd out.

Note that these helpers are somewhat unusual in that they do not have
the rust_helper_ prefix that most helpers have. Adding the rust_helper_
prefix does not compile, as 'bindings::_find_next_zero_bit()' will
result in a call to a symbol called _find_next_zero_bit as defined by
include/linux/find.h rather than a symbol with the rust_helper_ prefix.
This is because when a symbol is present in both include/ and
rust/helpers/, the one from include/ wins under the assumption that the
current configuration is one where that helper is unnecessary. This
heuristic fails for _find_next_zero_bit() because the header file always
declares it even if the symbol does not exist.

The functions still use the __rust_helper annotation. This lets the
wrapper function be inlined into Rust code even if full kernel LTO is
not used once the patch series for that feature lands.

Cc: stable@vger.kernel.org
Fixes: 6cf93a9ed39e ("rust: add bindings for bitops.h")
Reported-by: Andreas Hindborg <a.hindborg@kernel.org>
Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/561677301
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Remove rust_helper_ prefix from helpers.
- Improve commit message.
- The set of functions for which a helper is added is changed so that it
  matches arch/arm/include/asm/bitops.h
- Link to v1: https://lore.kernel.org/r/20251203-bitops-find-helper-v1-1-5193deb57766@google.com
---
 rust/helpers/bitops.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
index 5d0861d29d3f0d705a014ae4601685828405f33b..e79ef9e6d98f969e2a0a2a6f62d9fcec3ef0fd72 100644
--- a/rust/helpers/bitops.c
+++ b/rust/helpers/bitops.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include <linux/bitops.h>
+#include <linux/find.h>
 
 void rust_helper___set_bit(unsigned long nr, unsigned long *addr)
 {
@@ -21,3 +22,44 @@ void rust_helper_clear_bit(unsigned long nr, volatile unsigned long *addr)
 {
 	clear_bit(nr, addr);
 }
+
+/*
+ * The rust_helper_ prefix is intentionally omitted below so that the
+ * declarations in include/linux/find.h are compatible with these helpers.
+ *
+ * Note that the below #ifdefs mean that the helper is only created if C does
+ * not provide a definition.
+ */
+#ifdef find_first_zero_bit
+__rust_helper
+unsigned long _find_first_zero_bit(const unsigned long *p, unsigned long size)
+{
+	return find_first_zero_bit(p, size);
+}
+#endif /* find_first_zero_bit */
+
+#ifdef find_next_zero_bit
+__rust_helper
+unsigned long _find_next_zero_bit(const unsigned long *addr,
+				  unsigned long size, unsigned long offset)
+{
+	return find_next_zero_bit(addr, size, offset);
+}
+#endif /* find_next_zero_bit */
+
+#ifdef find_first_bit
+__rust_helper
+unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
+{
+	return find_first_bit(addr, size);
+}
+#endif /* find_first_bit */
+
+#ifdef find_next_bit
+__rust_helper
+unsigned long _find_next_bit(const unsigned long *addr, unsigned long size,
+			     unsigned long offset)
+{
+	return find_next_bit(addr, size, offset);
+}
+#endif /* find_next_bit */

---
base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20251203-bitops-find-helper-25ed1bbae700

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>



From - Mon Jan 05 10:55:59 2026
X-Mozilla-Status: 0001
X-Mozilla-Status2: 00000000
Return-Path: <stable+bounces-204641-hi=josie.lol@vger.kernel.org>
Delivered-To: hi@josie.lol
Received: from witcher.mxrouting.net
	by witcher.mxrouting.net with LMTP
	id UOccJLuYW2kASDsAYBR5ng
	(envelope-from <stable+bounces-204641-hi=josie.lol@vger.kernel.org>)
	for <hi@josie.lol>; Mon, 05 Jan 2026 10:55:55 +0000
Return-path: <stable+bounces-204641-hi=josie.lol@vger.kernel.org>
Envelope-to: hi@josie.lol
Delivery-date: Mon, 05 Jan 2026 10:55:55 +0000
Received: from sto.lore.kernel.org ([172.232.135.74])
	by witcher.mxrouting.net with esmtps  (TLS1.3) tls TLS_AES_256_GCM_SHA384
	(Exim 4.98)
	(envelope-from <stable+bounces-204641-hi=josie.lol@vger.kernel.org>)
	id 1vciFf-0000000HOYJ-0BXD
	for hi@josie.lol;
	Mon, 05 Jan 2026 10:55:55 +0000
Received: from smtp.subspace.kernel.org (conduit.subspace.kernel.org [100.90.174.1])
	by sto.lore.kernel.org (Postfix) with ESMTP id 5E7443006989
	for <hi@josie.lol>; Mon,  5 Jan 2026 10:55:52 +0000 (UTC)
Received: from localhost.localdomain (localhost.localdomain [127.0.0.1])
	by smtp.subspace.kernel.org (Postfix) with ESMTP id 1F48132ED59;
	Mon,  5 Jan 2026 10:55:35 +0000 (UTC)
Authentication-Results: smtp.subspace.kernel.org;
	dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="1r5ytgYd"
X-Original-To: stable@vger.kernel.org
Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201])
	(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
	(No client certificate requested)
	by smtp.subspace.kernel.org (Postfix) with ESMTPS id E187032ED4B
	for <stable@vger.kernel.org>; Mon,  5 Jan 2026 10:55:34 +0000 (UTC)
Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201
ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116;
	t=1767610535; cv=none; b=MlDm6+vBG9pqo7hNXOHwB7Iz5aah/ptkAVbK6hIFsYZw7R32ut74fvTei12vVkAQL1JkQYArpwn2M5urWQBL8hrZhETtcra7dniTFAI14rCZfhyQRz6UweA20ZBUMD9l2nlKibwmuMtYH8xWBNVN4GrFb2oddeRna5cNTcx0Oro=
ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org;
	s=arc-20240116; t=1767610535; c=relaxed/simple;
	bh=xLlnbopLMc1ETsTsQLjo1tVDdphoFXx4CkClEEzF2co=;
	h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=Nz+0iGWs3GDcY8eQvaeG6++aTFswErznEiF6tOpxmPlgQHXuMDkkc/qlstvSbuavGEaCD1m+hKXy0r3CrRm+eOBWpAiZva82dCWA3rxErS85N/uc632CoPgQWAIYlegFjINESJ6yUJCwYb0LXp4Rnb5ymjgYJwWS5JwInRxGys0=
ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=1r5ytgYd; arc=none smtp.client-ip=10.30.226.201
Received: by smtp.kernel.org (Postfix) with ESMTPSA id 17062C19423;
	Mon,  5 Jan 2026 10:55:33 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org;
	s=korg; t=1767610534;
	bh=xLlnbopLMc1ETsTsQLjo1tVDdphoFXx4CkClEEzF2co=;
	h=Subject:To:Cc:From:Date:From;
	b=1r5ytgYd/l8gXKXsa+PUGKYsaJJnhV1B9JKbZWk9NbuViOqUmY/RMyagO16wMc1J0
	 yuoWWDKJOuwBxE3fQe+9QLbCAwHx/+N0W4vxu5wfnNvV9NeuSDxh/lwbERaVUaX/UK
	 aMNjbv/OHv4pPFx4JWtYJ6r5rglFWBiRylvyDFJ4=
Subject: FAILED: patch "[PATCH] PCI: brcmstb: Fix disabling L0s capability" failed to apply to 5.15-stable tree
To: james.quinlan@broadcom.com,bhelgaas@google.com,florian.