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 63EA8C6FD1D for ; Tue, 4 Apr 2023 19:44:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235273AbjDDToD (ORCPT ); Tue, 4 Apr 2023 15:44:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39572 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235905AbjDDTn7 (ORCPT ); Tue, 4 Apr 2023 15:43:59 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0FD8E4EE9 for ; Tue, 4 Apr 2023 12:43:57 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 5569D63062 for ; Tue, 4 Apr 2023 19:43:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A0228C433EF; Tue, 4 Apr 2023 19:43:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1680637436; bh=1Pj9wervIvuhxnbud9M7uEnumbv+qewHT7HtKpfIDE0=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=u3JeDRjED8NfLSy7JgtnzRWXs8mnHtoxwLXgKI8TrOSi/KA99r+V3s3xMiq5k29wc mM2t1mpsctRZyI25Rxtg1sRpN6GavOMHoh071IbNwPezCuQS7D8+tQi8YKcwGiKHMy 3Z27iTHQOYUlsTYsSxers71SWSjBt2yrcMul/pNo= Date: Tue, 4 Apr 2023 12:43:55 -0700 From: Andrew Morton To: Josh Triplett Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] prctl: Add PR_GET_AUXV to copy auxv to userspace Message-Id: <20230404124355.3efa7a201d2aa9896dc6353e@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 4 Apr 2023 21:31:48 +0900 Josh Triplett wrote: > If a library wants to get information from auxv (for instance, > AT_HWCAP/AT_HWCAP2), it has a few options, none of them perfectly > reliable or ideal: > > - Be main or the pre-main startup code, and grub through the stack above > main. Doesn't work for a library. > - Call libc getauxval. Not ideal for libraries that are trying to be > libc-independent and/or don't otherwise require anything from other > libraries. > - Open and read /proc/self/auxv. Doesn't work for libraries that may run > in arbitrarily constrained environments that may not have /proc > mounted (e.g. libraries that might be used by an init program or a > container setup tool). > - Assume you're on the main thread and still on the original stack, and > try to walk the stack upwards, hoping to find auxv. Extremely bad > idea. > - Ask the caller to pass auxv in for you. Not ideal for a user-friendly > library, and then your caller may have the same problem. How does glibc's getauxval() do its thing? Why can't glibc-independent code do the same thing? > Add a prctl that copies current->mm->saved_auxv to a userspace buffer. > > ... > > --- a/include/uapi/linux/prctl.h > +++ b/include/uapi/linux/prctl.h > @@ -290,4 +290,6 @@ struct prctl_mm_map { > #define PR_SET_VMA 0x53564d41 > # define PR_SET_VMA_ANON_NAME 0 > > +#define PR_GET_AUXV 0x41555856 How was this constant arrived at? > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2377,6 +2377,16 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3, > PR_MDWE_REFUSE_EXEC_GAIN : 0; > } > > +static int prctl_get_auxv(void __user *addr, unsigned long len) > +{ > + struct mm_struct *mm = current->mm; > + unsigned long size = min_t(unsigned long, sizeof(mm->saved_auxv), len); > + > + if (size && copy_to_user(addr, mm->saved_auxv, size)) > + return -EFAULT; > + return sizeof(mm->saved_auxv); > +} The type choices are unpleasing. Maybe make `len' a size_t and make the function return a size_t? That way prctl_get_auxv() will be much nicer, but the caller less so. > SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > unsigned long, arg4, unsigned long, arg5) > { > @@ -2661,6 +2671,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > case PR_SET_VMA: > error = prctl_set_vma(arg2, arg3, arg4, arg5); > break; > + case PR_GET_AUXV: > + if (arg4 || arg5) > + return -EINVAL; > + error = prctl_get_auxv((void __user *)arg2, arg3); > + break; > default: > error = -EINVAL; > break;