From mboxrd@z Thu Jan 1 00:00:00 1970 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 4E6BD82899; Sun, 1 Mar 2026 01:37:50 +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=1772329070; cv=none; b=ufpIb3DGOcXxseoQKHJOrT9ZcOA7cAgS93iSCWZT5tetvDsU0ZwrOK9jgosJiofSv1DJ21IxudZ6UCe1Tsl0f65w9w3HIY/QZle++1MQjv8G+eoK/oc+O0okFs3N3Qz7i+zcfE7ou7Nm5iti6wH9ZLfXpl7rdbwkS8+74sBi9TI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772329070; c=relaxed/simple; bh=co1AthSFIq0rfOMJLFzAav4ZdGSzHYRKBuEoMntLpsg=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=H1ub9+gEcsCbjIFHmxZ2G9eyGx2N8FM43EZN3oz2Mv9ydl0X5Cu6KjUW1VvFZDCWHzELr5JnD5H59jaKWzimCiBomjVAFjvOeytstl9QCTegyE2utWAnkTlKffeltyZlQv/jcTZxVYiDV5OAk0WQ2ndIxqRJxUzXUAesT6ubyFc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mTPrUCpa; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mTPrUCpa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4588AC19421; Sun, 1 Mar 2026 01:37:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772329069; bh=co1AthSFIq0rfOMJLFzAav4ZdGSzHYRKBuEoMntLpsg=; h=From:To:Cc:Subject:Date:From; b=mTPrUCpa1Qa6J0y6sgM+6jZhhKTDhBdc85O5ytnHVeFJvCVIBAXcoNTXeQbvP3Kof lK6tyk2h/k2Bm6pgaxkLWWV+lgv3yAXYsamm67GghKsfazHW3KbJRjc/uKu3uI8f++ RZk5FTInvg/zwYWcbG8YeaLyt6Hm7Nwuxv5FsaGoUVrcORos2UWS5kXVYKsSa6V64i apd2XipBFo/MQYYv6m942gq626V0PWGhqOJJ/JRMKYVaw8tjrNCUeV5JK3QhUpcKib EiiOViNXBruZ101JZfGXbKgdp+4/KvWVNYFn4ZCmJGMUAKM0sI+z/ON1SUwsntMlTS 7WkVuxRCBZoug== From: Sasha Levin To: stable@vger.kernel.org, me@ziyao.cc Cc: Nathan Chancellor , Thomas Bogendoerfer , linux-mips@vger.kernel.org, llvm@lists.linux.dev Subject: FAILED: Patch "MIPS: Work around LLVM bug when gp is used as global register variable" failed to apply to 6.6-stable tree Date: Sat, 28 Feb 2026 20:37:47 -0500 Message-ID: <20260301013748.1698055-1-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Patchwork-Hint: ignore X-stable: review Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.6-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . Thanks, Sasha ------------------ original commit in Linus's tree ------------------ >From 30bfc2d6a1132a89a5f1c3b96c59cf3e4d076ea3 Mon Sep 17 00:00:00 2001 From: Yao Zi Date: Thu, 5 Feb 2026 15:56:44 +0000 Subject: [PATCH] MIPS: Work around LLVM bug when gp is used as global register variable On MIPS, __current_thread_info is defined as global register variable locating in $gp, and is simply assigned with new address during kernel relocation. This however is broken with LLVM, which always restores $gp if it finds $gp is clobbered in any form, including when intentionally through a global register variable. This is against GCC's documentation[1], which requires a callee-saved register used as global register variable not to be restored if it's clobbered. As a result, $gp will continue to point to the unrelocated kernel after the epilog of relocate_kernel(), leading to an early crash in init_idle, [ 0.000000] CPU 0 Unable to handle kernel paging request at virtual address 0000000000000000, epc == ffffffff81afada8, ra == ffffffff81afad90 [ 0.000000] Oops[#1]: [ 0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper Tainted: G W 6.19.0-rc5-00262-gd3eeb99bbc99-dirty #188 VOLUNTARY [ 0.000000] Tainted: [W]=WARN [ 0.000000] Hardware name: loongson,loongson64v-4core-virtio [ 0.000000] $ 0 : 0000000000000000 0000000000000000 0000000000000001 0000000000000000 [ 0.000000] $ 4 : ffffffff80b80ec0 ffffffff80b53d48 0000000000000000 00000000000f4240 [ 0.000000] $ 8 : 0000000000000100 ffffffff81d82f80 ffffffff81d82f80 0000000000000001 [ 0.000000] $12 : 0000000000000000 ffffffff81776f58 00000000000005da 0000000000000002 [ 0.000000] $16 : ffffffff80b80e40 0000000000000000 ffffffff80b81614 9800000005dfbe80 [ 0.000000] $20 : 00000000540000e0 ffffffff81980000 0000000000000000 ffffffff80f81c80 [ 0.000000] $24 : 0000000000000a26 ffffffff8114fb90 [ 0.000000] $28 : ffffffff80b50000 ffffffff80b53d40 0000000000000000 ffffffff81afad90 [ 0.000000] Hi : 0000000000000000 [ 0.000000] Lo : 0000000000000000 [ 0.000000] epc : ffffffff81afada8 init_idle+0x130/0x270 [ 0.000000] ra : ffffffff81afad90 init_idle+0x118/0x270 [ 0.000000] Status: 540000e2 KX SX UX KERNEL EXL [ 0.000000] Cause : 00000008 (ExcCode 02) [ 0.000000] BadVA : 0000000000000000 [ 0.000000] PrId : 00006305 (ICT Loongson-3) [ 0.000000] Process swapper (pid: 0, threadinfo=(____ptrval____), task=(____ptrval____), tls=0000000000000000) [ 0.000000] Stack : 9800000005dfbf00 ffffffff8178e950 0000000000000000 0000000000000000 [ 0.000000] 0000000000000000 ffffffff81970000 000000000000003f ffffffff810a6528 [ 0.000000] 0000000000000001 9800000005dfbe80 9800000005dfbf00 ffffffff81980000 [ 0.000000] ffffffff810a6450 ffffffff81afb6c0 0000000000000000 ffffffff810a2258 [ 0.000000] ffffffff81d82ec8 ffffffff8198d010 ffffffff81b67e80 ffffffff8197dd98 [ 0.000000] ffffffff81d81c80 ffffffff81930000 0000000000000040 0000000000000000 [ 0.000000] 0000000000000000 0000000000000000 0000000000000000 0000000000000000 [ 0.000000] 0000000000000000 000000000000009e ffffffff9fc01000 0000000000000000 [ 0.000000] 0000000000000000 0000000000000000 0000000000000000 0000000000000000 [ 0.000000] 0000000000000000 ffffffff81ae86dc ffffffff81b3c741 0000000000000002 [ 0.000000] ... [ 0.000000] Call Trace: [ 0.000000] [] init_idle+0x130/0x270 [ 0.000000] [] sched_init+0x5c8/0x6c0 [ 0.000000] [] start_kernel+0x27c/0x7a8 This bug has been reported to LLVM[2] and affects version from (at least) 18 to 21. Let's work around this by using inline assembly to assign $gp before a fix is widely available. Cc: stable@vger.kernel.org Link: https://gcc.gnu.org/onlinedocs/gcc-15.2.0/gcc/Global-Register-Variables.html # [1] Link: https://github.com/llvm/llvm-project/issues/176546 # [2] Signed-off-by: Yao Zi Acked-by: Nathan Chancellor Signed-off-by: Thomas Bogendoerfer --- arch/mips/kernel/relocate.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c index 7f1c136ad8506..59833210542ff 100644 --- a/arch/mips/kernel/relocate.c +++ b/arch/mips/kernel/relocate.c @@ -420,7 +420,20 @@ void *__init relocate_kernel(void) goto out; /* The current thread is now within the relocated image */ +#ifndef CONFIG_CC_IS_CLANG __current_thread_info = RELOCATED(&init_thread_union); +#else + /* + * LLVM may wrongly restore $gp ($28) in epilog even if it's + * intentionally modified. Work around this by using inline + * assembly to assign $gp. $gp couldn't be listed as output or + * clobber, or LLVM will still restore its original value. + * See also LLVM upstream issue + * https://github.com/llvm/llvm-project/issues/176546 + */ + asm volatile("move $28, %0" : : + "r" (RELOCATED(&init_thread_union))); +#endif /* Return the new kernel's entry point */ kernel_entry = RELOCATED(start_kernel); -- 2.51.0