[{"id":39660,"web_url":"https://patchwork.libcamera.org/comment/39660/","msgid":"<20260709194716.GA1052680@killaraus.ideasonboard.com>","date":"2026-07-09T19:47:16","subject":"Re: [PATCH] DmaBufAllocator: Open heap device files O_RDONLY","submitter":{"id":2,"url":"https://patchwork.libcamera.org/api/people/2/","name":"Laurent Pinchart","email":"laurent.pinchart@ideasonboard.com"},"content":"On Thu, Jul 09, 2026 at 11:30:45AM -0700, T.J. Mercier wrote:\n> The write access mode is not required to issue ioctls and allocate\n> dma-bufs from heaps. Applications should be opening them as O_RDONLY.\n\nIs that explictly documented somewhere, or just the current behaviour ?\nI suppose it won't change as that would introduce regressions.\n\n> Permission errors can be encountered while attempting to open() files in\n> /dev/dma_heap/* on systems where write permissions are not available. So\n> apply the principle of least privilege and remove the write access mode.\n\nI'm curious, what systems have you encountered read-only dma-buf heap\ndevice nodes on ?\n\n> Signed-off-by: T.J. Mercier \n> ---\n> src/libcamera/dma_buf_allocator.cpp | 2 +-\n> 1 file changed, 1 insertion(+), 1 deletion(-)\n> \n> diff --git a/src/libcamera/dma_buf_allocator.cpp b/src/libcamera/dma_buf_allocator.cpp\n> index d8c62dd67..c673d23ec 100644\n> --- a/src/libcamera/dma_buf_allocator.cpp\n> +++ b/src/libcamera/dma_buf_allocator.cpp\n> @@ -100,7 +100,7 @@ DmaBufAllocator::DmaBufAllocator(DmaBufAllocatorFlags type)\n> \t\tif (!(type & info.type))\n> \t\t\tcontinue;\n> \n> -\t\tint ret = ::open(info.deviceNodeName, O_RDWR | O_CLOEXEC, 0);\n> +\t\tint ret = ::open(info.deviceNodeName, O_RDONLY | O_CLOEXEC, 0);\n> \t\tif (ret < 0) {\n> \t\t\tret = errno;\n> \t\t\tLOG(DmaBufAllocator, Debug)","headers":{"Return-Path":"","X-Original-To":"parsemail@patchwork.libcamera.org","Delivered-To":"parsemail@patchwork.libcamera.org","Received":["from lancelot.ideasonboard.com (lancelot.ideasonboard.com\n\t[92.243.16.209])\n\tby patchwork.libcamera.org (Postfix) with ESMTPS id 1EE3DC3318\n\tfor ;\n\tThu, 9 Jul 2026 19:47:21 +0000 (UTC)","from lancelot.ideasonboard.com (localhost [IPv6:::1])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTP id 2B4C266104;\n\tThu, 9 Jul 2026 21:47:20 +0200 (CEST)","from perceval.ideasonboard.com (perceval.ideasonboard.com\n\t[IPv6:2001:4b98:dc2:55:216:3eff:fef7:d647])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTPS id 762BC660E3\n\tfor ;\n\tThu, 9 Jul 2026 21:47:18 +0200 (CEST)","from killaraus.ideasonboard.com\n\t(2001-14ba-70f3-e800--a06.rev.dnainternet.fi\n\t[IPv6:2001:14ba:70f3:e800::a06])\n\tby perceval.ideasonboard.com (Postfix) with ESMTPSA id 9100F289;\n\tThu, 9 Jul 2026 21:46:27 +0200 (CEST)"],"Authentication-Results":"lancelot.ideasonboard.com; dkim=pass (1024-bit key;\n\tunprotected) header.d=ideasonboard.com header.i=@ideasonboard.com\n\theader.b=\"khR/B31k\"; dkim-atps=neutral","DKIM-Signature":"v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com;\n\ts=mail; t=1783626387;\n\tbh=KNZDjunpP3TEjlaO01OqFNUxV+FD80mKQ/+ek+QsNBI=;\n\th=Date:From:To:Cc:Subject:References:In-Reply-To:From;\n\tb=khR/B31k/do7gc4iNIaU+h6u4BjihB8Ow47rdavKe9sMFlJki/t6zd2z1ogwgq+ws\n\tfIbpQvo4iLIzaubavoeMiAtaQ6FgU9TGRXeU9m8xkaCMJBykwUXkH7iC+z5TrO0JIt\n\tRaC5loTNlBdlKYigO+N+0m7bcDvGkGvQ4pcJ372s=","Date":"Thu, 9 Jul 2026 22:47:16 +0300","From":"Laurent Pinchart ","To":"\"T.J. Mercier\" ","Cc":"libcamera-devel@lists.libcamera.org","Subject":"Re: [PATCH] DmaBufAllocator: Open heap device files O_RDONLY","Message-ID":"<20260709194716.GA1052680@killaraus.ideasonboard.com>","References":"<20260709183049.2282221-1-tjmercier@google.com>","MIME-Version":"1.0","Content-Type":"text/plain; charset=utf-8","Content-Disposition":"inline","In-Reply-To":"<20260709183049.2282221-1-tjmercier@google.com>","X-BeenThere":"libcamera-devel@lists.libcamera.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"","List-Unsubscribe":",\n\t","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n\t","Errors-To":"libcamera-devel-bounces@lists.libcamera.org","Sender":"\"libcamera-devel\" "}},{"id":39661,"web_url":"https://patchwork.libcamera.org/comment/39661/","msgid":"","date":"2026-07-09T20:04:42","subject":"Re: [PATCH] DmaBufAllocator: Open heap device files O_RDONLY","submitter":{"id":385,"url":"https://patchwork.libcamera.org/api/people/385/","name":"T.J. Mercier","email":"tjmercier@google.com"},"content":"On Thu, Jul 9, 2026 at 12:47 PM Laurent Pinchart\n wrote:\n>\n> On Thu, Jul 09, 2026 at 11:30:45AM -0700, T.J. Mercier wrote:\n> > The write access mode is not required to issue ioctls and allocate\n> > dma-bufs from heaps. Applications should be opening them as O_RDONLY.\n>\n> Is that explictly documented somewhere, or just the current behaviour ?\n> I suppose it won't change as that would introduce regressions.\n\nHi, Unfortunately I don't think it's documented anywhere. Even the\nkernel selftests are mixed (which some users were pointing at), but\nI've addressed those here:\nhttps://lore.kernel.org/all/20260701192210.2997769-1-tjmercier@google.com/\n\nThe file descriptor is really all that's needed, however it is\nobtained. The ioctl handler in the dma-heap driver intentionally\nperforms no permission checks.\n\n> > Permission errors can be encountered while attempting to open() files in\n> > /dev/dma_heap/* on systems where write permissions are not available. So\n> > apply the principle of least privilege and remove the write access mode.\n>\n> I'm curious, what systems have you encountered read-only dma-buf heap\n> device nodes on ?\n\nAndroid: https://cs.android.com/android/platform/superproject/+/android-latest-release:system/core/rootdir/ueventd.rc;l=55-57\n\n> > Signed-off-by: T.J. Mercier \n> > ---\n> > src/libcamera/dma_buf_allocator.cpp | 2 +-\n> > 1 file changed, 1 insertion(+), 1 deletion(-)\n> >\n> > diff --git a/src/libcamera/dma_buf_allocator.cpp b/src/libcamera/dma_buf_allocator.cpp\n> > index d8c62dd67..c673d23ec 100644\n> > --- a/src/libcamera/dma_buf_allocator.cpp\n> > +++ b/src/libcamera/dma_buf_allocator.cpp\n> > @@ -100,7 +100,7 @@ DmaBufAllocator::DmaBufAllocator(DmaBufAllocatorFlags type)\n> > if (!(type & info.type))\n> > continue;\n> >\n> > - int ret = ::open(info.deviceNodeName, O_RDWR | O_CLOEXEC, 0);\n> > + int ret = ::open(info.deviceNodeName, O_RDONLY | O_CLOEXEC, 0);\n> > if (ret < 0) {\n> > ret = errno;\n> > LOG(DmaBufAllocator, Debug)\n>\n> --\n> Regards,\n>\n> Laurent Pinchart","headers":{"Return-Path":"","X-Original-To":"parsemail@patchwork.libcamera.org","Delivered-To":"parsemail@patchwork.libcamera.org","Received":["from lancelot.ideasonboard.com (lancelot.ideasonboard.com\n\t[92.243.16.209])\n\tby patchwork.libcamera.org (Postfix) with ESMTPS id C665DC3318\n\tfor ;\n\tThu, 9 Jul 2026 20:04:56 +0000 (UTC)","from lancelot.ideasonboard.com (localhost [IPv6:::1])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTP id 196DA660F9;\n\tThu, 9 Jul 2026 22:04:56 +0200 (CEST)","from mail-wm1-x336.google.com (mail-wm1-x336.google.com\n\t[IPv6:2a00:1450:4864:20::336])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTPS id 88C6E660E3\n\tfor ;\n\tThu, 9 Jul 2026 22:04:55 +0200 (CEST)","by mail-wm1-x336.google.com with SMTP id\n\t5b1f17b1804b1-493c556ada3so13955e9.0\n\tfor ;\n\tThu, 09 Jul 2026 13:04:55 -0700 (PDT)"],"Authentication-Results":"lancelot.ideasonboard.com; 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Thu, 09 Jul 2026\n\t13:04:54 -0700 (PDT)","MIME-Version":"1.0","References":"<20260709183049.2282221-1-tjmercier@google.com>\n\t<20260709194716.GA1052680@killaraus.ideasonboard.com>","In-Reply-To":"<20260709194716.GA1052680@killaraus.ideasonboard.com>","From":"\"T.J. Mercier\" ","Date":"Thu, 9 Jul 2026 13:04:42 -0700","X-Gm-Features":"AUfX_myn7nahk1xySqKoG6dSJMGSS0u5IbRiN7BHQCDxh6a2hL8vIhOYQ2YW5wI","Message-ID":"","Subject":"Re: [PATCH] DmaBufAllocator: Open heap device files O_RDONLY","To":"Laurent Pinchart ","Cc":"libcamera-devel@lists.libcamera.org","Content-Type":"text/plain; charset=\"UTF-8\"","Content-Transfer-Encoding":"quoted-printable","X-BeenThere":"libcamera-devel@lists.libcamera.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"","List-Unsubscribe":",\n\t","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n\t","Errors-To":"libcamera-devel-bounces@lists.libcamera.org","Sender":"\"libcamera-devel\" "}}]