From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C090CC0015E for ; Sun, 16 Jul 2023 20:15:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231622AbjGPUPh (ORCPT ); Sun, 16 Jul 2023 16:15:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41458 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231610AbjGPUPh (ORCPT ); Sun, 16 Jul 2023 16:15:37 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DAE8BC0 for ; Sun, 16 Jul 2023 13:15:35 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 79D8E60E65 for ; Sun, 16 Jul 2023 20:15:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 84C24C433C8; Sun, 16 Jul 2023 20:15:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1689538534; bh=6Ae246nRNoemsnLlxqTfyQ8invwDXRrMsLSKlh5XlHA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=zujvyGDgO/3DcgPdqpr3iQ3NFKDH8knw0VkNj34nT7bGxBDmh4y7ds8v5ZjqakN7B 5zNf1CDO8Gxf6P0QzTqvBNA52FfJV2htio1PfXBJhpL2eA2wGuFHT4zdo7/nXu8f7m pT8kFxFwJlkMifbor3kHsFTuuI2UcglGhYSHK6RE= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Aditya Gupta , Michael Ellerman , Sasha Levin , Nicholas Piggin Subject: [PATCH 6.4 485/800] powerpc: update ppc_save_regs to save current r1 in pt_regs Date: Sun, 16 Jul 2023 21:45:38 +0200 Message-ID: <20230716195000.344968704@linuxfoundation.org> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20230716194949.099592437@linuxfoundation.org> References: <20230716194949.099592437@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Aditya Gupta [ Upstream commit b684c09f09e7a6af3794d4233ef785819e72db79 ] ppc_save_regs() skips one stack frame while saving the CPU register states. Instead of saving current R1, it pulls the previous stack frame pointer. When vmcores caused by direct panic call (such as `echo c > /proc/sysrq-trigger`), are debugged with gdb, gdb fails to show the backtrace correctly. On further analysis, it was found that it was because of mismatch between r1 and NIP. GDB uses NIP to get current function symbol and uses corresponding debug info of that function to unwind previous frames, but due to the mismatching r1 and NIP, the unwinding does not work, and it fails to unwind to the 2nd frame and hence does not show the backtrace. GDB backtrace with vmcore of kernel without this patch: --------- (gdb) bt #0 0xc0000000002a53e8 in crash_setup_regs (oldregs=, newregs=0xc000000004f8f8d8) at ./arch/powerpc/include/asm/kexec.h:69 #1 __crash_kexec (regs=) at kernel/kexec_core.c:974 #2 0x0000000000000063 in ?? () #3 0xc000000003579320 in ?? () --------- Further analysis revealed that the mismatch occurred because "ppc_save_regs" was saving the previous stack's SP instead of the current r1. This patch fixes this by storing current r1 in the saved pt_regs. GDB backtrace with vmcore of patched kernel: -------- (gdb) bt #0 0xc0000000002a53e8 in crash_setup_regs (oldregs=0x0, newregs=0xc00000000670b8d8) at ./arch/powerpc/include/asm/kexec.h:69 #1 __crash_kexec (regs=regs@entry=0x0) at kernel/kexec_core.c:974 #2 0xc000000000168918 in panic (fmt=fmt@entry=0xc000000001654a60 "sysrq triggered crash\n") at kernel/panic.c:358 #3 0xc000000000b735f8 in sysrq_handle_crash (key=) at drivers/tty/sysrq.c:155 #4 0xc000000000b742cc in __handle_sysrq (key=key@entry=99, check_mask=check_mask@entry=false) at drivers/tty/sysrq.c:602 #5 0xc000000000b7506c in write_sysrq_trigger (file=, buf=, count=2, ppos=) at drivers/tty/sysrq.c:1163 #6 0xc00000000069a7bc in pde_write (ppos=, count=, buf=, file=, pde=0xc00000000362cb40) at fs/proc/inode.c:340 #7 proc_reg_write (file=, buf=, count=, ppos=) at fs/proc/inode.c:352 #8 0xc0000000005b3bbc in vfs_write (file=file@entry=0xc000000006aa6b00, buf=buf@entry=0x61f498b4f60 , count=count@entry=2, pos=pos@entry=0xc00000000670bda0) at fs/read_write.c:582 #9 0xc0000000005b4264 in ksys_write (fd=, buf=0x61f498b4f60 , count=2) at fs/read_write.c:637 #10 0xc00000000002ea2c in system_call_exception (regs=0xc00000000670be80, r0=) at arch/powerpc/kernel/syscall.c:171 #11 0xc00000000000c270 in system_call_vectored_common () at arch/powerpc/kernel/interrupt_64.S:192 -------- Nick adds: So this now saves regs as though it was an interrupt taken in the caller, at the instruction after the call to ppc_save_regs, whereas previously the NIP was there, but R1 came from the caller's caller and that mismatch is what causes gdb's dwarf unwinder to go haywire. Signed-off-by: Aditya Gupta Fixes: d16a58f8854b1 ("powerpc: Improve ppc_save_regs()") Reivewed-by: Nicholas Piggin Signed-off-by: Michael Ellerman Link: https://msgid.link/20230615091047.90433-1-adityag@linux.ibm.com Signed-off-by: Sasha Levin --- arch/powerpc/kernel/ppc_save_regs.S | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/arch/powerpc/kernel/ppc_save_regs.S b/arch/powerpc/kernel/ppc_save_regs.S index 49813f9824681..a9b9c32d0c1ff 100644 --- a/arch/powerpc/kernel/ppc_save_regs.S +++ b/arch/powerpc/kernel/ppc_save_regs.S @@ -31,10 +31,10 @@ _GLOBAL(ppc_save_regs) lbz r0,PACAIRQSOFTMASK(r13) PPC_STL r0,SOFTE(r3) #endif - /* go up one stack frame for SP */ - PPC_LL r4,0(r1) - PPC_STL r4,GPR1(r3) + /* store current SP */ + PPC_STL r1,GPR1(r3) /* get caller's LR */ + PPC_LL r4,0(r1) PPC_LL r0,LRSAVE(r4) PPC_STL r0,_LINK(r3) mflr r0 -- 2.39.2