From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45434) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fijru-0004pQ-3U for qemu-devel@nongnu.org; Thu, 26 Jul 2018 13:16:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fijrr-0005Ro-0R for qemu-devel@nongnu.org; Thu, 26 Jul 2018 13:16:02 -0400 Received: from mail-pg1-x541.google.com ([2607:f8b0:4864:20::541]:40316) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1fijrq-0005RX-MQ for qemu-devel@nongnu.org; Thu, 26 Jul 2018 13:15:58 -0400 Received: by mail-pg1-x541.google.com with SMTP id x5-v6so1551418pgp.7 for ; Thu, 26 Jul 2018 10:15:58 -0700 (PDT) References: <153258768962.6738.11319866502689416568.stgit@dhcp-9-109-246-16> From: Richard Henderson Message-ID: <4671a0f0-e399-6187-3205-994f663fa260@linaro.org> Date: Thu, 26 Jul 2018 10:15:53 -0700 MIME-Version: 1.0 In-Reply-To: <153258768962.6738.11319866502689416568.stgit@dhcp-9-109-246-16> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] linux-user: ppc64: don't use volatile register during safe_syscall List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Shivaprasad G Bhat , dgibson@redhat.com, riku.voipio@iki.fi, laurent@vivier.eu Cc: qemu-devel@nongnu.org On 07/25/2018 11:48 PM, Shivaprasad G Bhat wrote: > r11 is a volatile register on PPC as per calling conventions. > The safe_syscall code uses it to check if the signal_pending > is set during the safe_syscall. When a syscall is interrupted > on return from signal handling, the r11 might be corrupted > before we retry the syscall leading to a crash. The registers > r0-r13 are not to be used here as they have > volatile/designated/reserved usages. Change the code to use > r14 which is non-volatile and is appropriate for local use in > safe_syscall. > > Signed-off-by: Shivaprasad G Bhat > --- > Steps to reproduce: > On PPC host, issue `qemu-ppc64le /usr/bin/cc -E -` > Attempt Ctrl-C, the issue is reproduced. > > Reference: > https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG > > linux-user/host/ppc64/safe-syscall.inc.S | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/linux-user/host/ppc64/safe-syscall.inc.S b/linux-user/host/ppc64/safe-syscall.inc.S > index d30050a67c..b0cbbe6a69 100644 > --- a/linux-user/host/ppc64/safe-syscall.inc.S > +++ b/linux-user/host/ppc64/safe-syscall.inc.S > @@ -49,7 +49,7 @@ safe_syscall_base: > * and returns the result in r3 > * Shuffle everything around appropriately. > */ > - mr 11, 3 /* signal_pending */ > + mr 14, 3 /* signal_pending */ I do see that I was incorrect in assuming that r11 would be unmodified. But you can't simply write to a call-saved register -- you must preserve its value for the caller. Saving the value requires that you find some space on, or create, a stack frame. Note that there are two different conventions for _CALL_AIX and _CALL_ELF==2. r~