From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932681AbeCIXQE (ORCPT ); Fri, 9 Mar 2018 18:16:04 -0500 Received: from smtp.gentoo.org ([140.211.166.183]:47622 "EHLO smtp.gentoo.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932276AbeCIXQB (ORCPT ); Fri, 9 Mar 2018 18:16:01 -0500 From: Sergei Trofimovich To: Tony Luck Cc: Sergei Trofimovich , Fenghua Yu , linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2, simpler] ia64: fix ptrace(PTRACE_GETREGS) (unbreaks strace, gdb) Date: Fri, 9 Mar 2018 23:15:55 +0000 Message-Id: <20180309231555.16925-1-slyfox@gentoo.org> X-Mailer: git-send-email 2.16.2 In-Reply-To: <20180211185134.76ccfd45@sf> References: <20180211185134.76ccfd45@sf> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The strace breakage looks like that: ./strace: get_regs: get_regs_error: Input/output error It happens because ia64 needs to load unwind tables to read certain registers in 'PTRACE_GETREGS'. Unwind tables fail to load at kernel startup due to GCC quirk on the following code (logged as PR 84184): extern char __end_unwind[]; const struct unw_table_entry *end = (struct unw_table_entry *)table_end; table->end = segment_base + end[-1].end_offset; GCC does not generate correct code for this single memory reference after constant propagation. Two triggers are required for bad code generation: - '__end_unwind' has alignment lower (char), than 'struct unw_table_entry' (8). - symbol offset is negative. This commit workarounds it by disabling inline on init_unwind_table(). This way we avoid const-propagation of '__end_unwind' and pass address via register. Tested in ski (emulator) and on rx2600, rx3600 (real hardware). In case of rx2600 it unbreaks booting. This patch is a lighter version of patch https://lkml.org/lkml/2018/2/2/914 CC: Tony Luck CC: Fenghua Yu CC: linux-ia64@vger.kernel.org CC: linux-kernel@vger.kernel.org Bug: https://github.com/strace/strace/issues/33 Bug: https://gcc.gnu.org/PR84184 Reported-by: Émeric Maschino Tested-by: stanton_arch@mail.com Signed-off-by: Sergei Trofimovich --- arch/ia64/kernel/unwind.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/arch/ia64/kernel/unwind.c b/arch/ia64/kernel/unwind.c index e04efa088902..a18190bc99a9 100644 --- a/arch/ia64/kernel/unwind.c +++ b/arch/ia64/kernel/unwind.c @@ -2078,7 +2078,14 @@ unw_init_from_blocked_task (struct unw_frame_info *info, struct task_struct *t) } EXPORT_SYMBOL(unw_init_from_blocked_task); -static void +/* + * We use 'noinline' to evade GCC bug https://gcc.gnu.org/PR84184 + * where gcc code generator emits incorrect code when '__end_unwind' + * is const-propagated to 'end[-1].end_offset' and gcc generates + * incorrect code. The prigger there is negative offset relative + * to externally-defined symbol. + */ +noinline static void init_unwind_table (struct unw_table *table, const char *name, unsigned long segment_base, unsigned long gp, const void *table_start, const void *table_end) { -- 2.16.2