From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760703AbdJQWB5 (ORCPT ); Tue, 17 Oct 2017 18:01:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:35076 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754772AbdJQWBz (ORCPT ); Tue, 17 Oct 2017 18:01:55 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0809121879 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=luto@kernel.org X-Google-Smtp-Source: AOwi7QAbSpl0gzCzdywZyv70E5q09GMbn97X2zq57l2VSnyVEzaC7Kn5kbzttkNMUJl5zsy5oLWVL4HKMsEZOR52vAI= MIME-Version: 1.0 In-Reply-To: <20171017192231.72C327B4@viggo.jf.intel.com> References: <20171017192231.72C327B4@viggo.jf.intel.com> From: Andy Lutomirski Date: Tue, 17 Oct 2017 15:01:34 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for sys_modify_ldt() To: Dave Hansen Cc: "linux-kernel@vger.kernel.org" , X86 ML , Andrew Lutomirski , Brian Gerst Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 17, 2017 at 12:22 PM, Dave Hansen wrote: > > We do not have tracepoints for sys_modify_ldt() because we define > it directly instead of using the normal SYSCALL_DEFINEx() macros. > > However, there is a reason sys_modify_ldt() does not use the macros: > it has an 'int' return type instead of 'unsigned long'. This is > a bug, but it's a bug cemented in the ABI. > > What does this mean? If we return -EINVAL from a function that > returns 'int', we have 0x00000000ffffffea in %rax. But, if we > return -EINVAL from a function returning 'unsigned long', we end > up with 0xffffffffffffffea in %rax, which is wrong. > > To work around this and maintain the 'int' behavior while using > the SYSCALL_DEFINEx() macros, so we add a cast to 'unsigned int' > in both implementations of sys_modify_ldt(). Reviewed-by: Andy Lutomirski > > Cc: x86@kernel.org > Cc: Andy Lutomirski > Cc: Brian Gerst > > --- > > b/arch/x86/include/asm/syscalls.h | 2 +- > b/arch/x86/kernel/ldt.c | 16 +++++++++++++--- > b/arch/x86/um/ldt.c | 6 ++++-- > 3 files changed, 18 insertions(+), 6 deletions(-) > > diff -puN arch/x86/include/asm/syscalls.h~x86-syscall-macros-modify_ldt arch/x86/include/asm/syscalls.h > --- a/arch/x86/include/asm/syscalls.h~x86-syscall-macros-modify_ldt 2017-10-17 10:30:53.032718307 -0700 > +++ b/arch/x86/include/asm/syscalls.h 2017-10-17 10:30:53.039718307 -0700 > @@ -21,7 +21,7 @@ asmlinkage long sys_ioperm(unsigned long > asmlinkage long sys_iopl(unsigned int); > > /* kernel/ldt.c */ > -asmlinkage int sys_modify_ldt(int, void __user *, unsigned long); > +asmlinkage long sys_modify_ldt(int, void __user *, unsigned long); > > /* kernel/signal.c */ > asmlinkage long sys_rt_sigreturn(void); > diff -puN arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt arch/x86/kernel/ldt.c > --- a/arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt 2017-10-17 10:30:53.034718307 -0700 > +++ b/arch/x86/kernel/ldt.c 2017-10-17 12:02:51.137704545 -0700 > @@ -12,6 +12,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -294,8 +295,8 @@ out: > return error; > } > > -asmlinkage int sys_modify_ldt(int func, void __user *ptr, > - unsigned long bytecount) > +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr , > + unsigned long , bytecount) > { > int ret = -ENOSYS; > > @@ -313,5 +314,14 @@ asmlinkage int sys_modify_ldt(int func, > ret = write_ldt(ptr, bytecount, 0); > break; > } > - return ret; > + /* > + * The SYSCALL_DEFINE() macros give us an 'unsigned long' > + * return type, but tht ABI for sys_modify_ldt() expects > + * 'int'. This cast gives us an int-sized value in %rax > + * for the return code. The 'unsigned' is necessary so > + * the compiler does not try to sign-extend the negative > + * return codes into the high half of the register when > + * taking the value from int->long. > + */ > + return (unsigned int)ret; > } > diff -puN arch/x86/um/ldt.c~x86-syscall-macros-modify_ldt arch/x86/um/ldt.c > --- a/arch/x86/um/ldt.c~x86-syscall-macros-modify_ldt 2017-10-17 10:30:53.036718307 -0700 > +++ b/arch/x86/um/ldt.c 2017-10-17 10:58:58.711714103 -0700 > @@ -369,7 +369,9 @@ void free_ldt(struct mm_context *mm) > mm->arch.ldt.entry_count = 0; > } > > -int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount) > +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr , > + unsigned long , bytecount) > { > - return do_modify_ldt_skas(func, ptr, bytecount); > + /* See non-um modify_ldt() for why we do this cast */ > + return (unsigned int)do_modify_ldt_skas(func, ptr, bytecount); > } > _