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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 6BDDEEDE98F for ; Thu, 14 Sep 2023 08:19:14 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qghYK-0000Ne-MW; Thu, 14 Sep 2023 04:18:20 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qghYH-0000NP-HR for qemu-devel@nongnu.org; Thu, 14 Sep 2023 04:18:17 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qghYD-0000yI-I9 for qemu-devel@nongnu.org; Thu, 14 Sep 2023 04:18:17 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1694679492; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=KHZkvMYQUVYiftqEwDriFaOumZdYBnbGU8+Fb6CiNeA=; b=OrbcZVXV3eKNqhMtBYzbe+OrIKV2/0Y5/rUY7jjU4uYwxkhJqHGFPu6DaxTxyXS73MAHsu 2sCKVG7bSbzm6XOX60xFEQtyqVMxOAxKHgm3Ksac75wvTCUnAI3g/nEIk3QuJ/ylf2dF8e DykV+vVztZRxyn2aBG1NON+6j12PA7I= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-599-7-mSPFGfOg-Pc3ck9gsVwg-1; Thu, 14 Sep 2023 04:18:08 -0400 X-MC-Unique: 7-mSPFGfOg-Pc3ck9gsVwg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4A921856501; Thu, 14 Sep 2023 08:18:08 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.119]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BDD0410F1BE8; Thu, 14 Sep 2023 08:18:07 +0000 (UTC) Date: Thu, 14 Sep 2023 09:18:05 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Michael Tokarev Cc: qemu-devel@nongnu.org Subject: Re: [PATCH v3 3/3] linux-user/syscall.c: do_ppoll: eliminate large alloca Message-ID: References: <20230914074337.149897-1-mjt@tls.msk.ru> <20230914074337.149897-4-mjt@tls.msk.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230914074337.149897-4-mjt@tls.msk.ru> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Sep 14, 2023 at 10:43:37AM +0300, Michael Tokarev wrote: > do_ppoll() in linux-user/syscall.c uses alloca() to allocate > an array of struct pullfds on the stack. The only upper > boundary for number of entries for this array is so that > whole thing fits in INT_MAX. This is definitely too much > for stack allocation. > > Use heap allocation when large number of entries is requested > (currently 32, arbitrary), and continue to use alloca() for Typo ? The code uses 64 rather than 32. > smaller allocations, to optimize small operations for small > sizes. The code for this optimization is small, I see no > reason for dropping it. > > This eliminates last large user-controlled on-stack allocation > from syscall.c. > > Signed-off-by: Michael Tokarev > --- > linux-user/syscall.c | 15 +++++++++++++-- > 1 file changed, 13 insertions(+), 2 deletions(-) > > diff --git a/linux-user/syscall.c b/linux-user/syscall.c > index eabdf50abc..1dbe28eba4 100644 > --- a/linux-user/syscall.c > +++ b/linux-user/syscall.c > @@ -1489,7 +1489,7 @@ static abi_long do_ppoll(abi_long arg1, abi_long arg2, abi_long arg3, > { > struct target_pollfd *target_pfd = NULL; > unsigned int nfds = arg2; > - struct pollfd *pfd = NULL; > + struct pollfd *pfd = NULL, *heap_pfd = NULL; g_autofree struct pollfd *heap_pdf = NULL; > unsigned int i; > abi_long ret; > > @@ -1503,7 +1503,17 @@ static abi_long do_ppoll(abi_long arg1, abi_long arg2, abi_long arg3, > return -TARGET_EFAULT; > } > > - pfd = alloca(sizeof(struct pollfd) * nfds); > + /* arbitrary "small" number to limit stack usage */ > + if (nfds <= 64) { > + pfd = alloca(sizeof(struct pollfd) * nfds); > + } else { > + heap_pfd = g_try_new(struct pollfd, nfds); > + if (!heap_pfd) { > + ret = -TARGET_ENOMEM; > + goto out; > + } > + pfd = heap_pfd; > + } > for (i = 0; i < nfds; i++) { > pfd[i].fd = tswap32(target_pfd[i].fd); > pfd[i].events = tswap16(target_pfd[i].events); > @@ -1567,6 +1577,7 @@ static abi_long do_ppoll(abi_long arg1, abi_long arg2, abi_long arg3, > } > > out: > + g_free(heap_pfd); This can be dropped with g_autofree usage > unlock_user(target_pfd, arg1, sizeof(struct target_pollfd) * nfds); > return ret; > } > -- > 2.39.2 > > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|