From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ffbox0-bg.mplayerhq.hu (ffbox0-bg.ffmpeg.org [79.124.17.100]) by master.gitmailbox.com (Postfix) with ESMTP id E4D774B416 for ; Wed, 5 Jun 2024 12:24:03 +0000 (UTC) Received: from [127.0.1.1] (localhost [127.0.0.1]) by ffbox0-bg.mplayerhq.hu (Postfix) with ESMTP id A61F668D68F; Wed, 5 Jun 2024 15:24:01 +0300 (EEST) Received: from haasn.dev (haasn.dev [78.46.187.166]) by ffbox0-bg.mplayerhq.hu (Postfix) with ESMTP id F185968D231 for ; Wed, 5 Jun 2024 15:23:54 +0300 (EEST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=haasn.xyz; s=mail; t=1717590234; bh=RQjaP+WX1zKGP5PxE2A63JIcTTDRW8RBDrnXf0lUfIs=; h=Date:From:To:Subject:In-Reply-To:References:From; b=CeMAkyXtmY0ehZV6yKOh/Hn/ry4Zd/rJtyAdNC1t+zOH0kjSOo1+ukPvHtpw6JulA li6EouR6U+JOCzrXE/Rg3ey57jGq9iIwrOp749DuMwHipUsVCmgc11a4VBX5eLQ2iv SGv+Rf0IM6lvXn9cnXWAIUe10tJohEw9pA3Bchlk= Received: from haasn.dev (unknown [10.30.0.2]) by haasn.dev (Postfix) with ESMTP id A1CF54052B for ; Wed, 5 Jun 2024 14:23:54 +0200 (CEST) Date: Wed, 5 Jun 2024 14:23:54 +0200 Message-ID: <20240605142354.GB72613@haasn.xyz> From: Niklas Haas To: ffmpeg-devel@ffmpeg.org In-Reply-To: References: <20240605095959.17135-1-ffmpeg@haasn.xyz> MIME-Version: 1.0 Content-Disposition: inline Subject: Re: [FFmpeg-devel] [PATCH] avcodec/dovi_rpudec: replace brittle struct copying code X-BeenThere: ffmpeg-devel@ffmpeg.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: FFmpeg development discussions and patches List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: FFmpeg development discussions and patches Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: ffmpeg-devel-bounces@ffmpeg.org Sender: "ffmpeg-devel" Archived-At: List-Archive: List-Post: On Wed, 05 Jun 2024 12:07:08 +0200 Andreas Rheinhardt wrote: > Niklas Haas: > > From: Niklas Haas > > > > This code was unnecessarily trying to be robust against downgrades of > > libavutil (relative to the version libavcodec was compiled against), but > > in the process, ended up with very brittle code that is easy to > > accidentally forget to update when adding new fields. > > > > Instead, do the obvious thing and just directly copy the parts of the > > struct known at compile time. Since it is not generally supported to > > link against a version of libavutil older than the version libavcodec > > was compiled against, the struct shrinking externally is not a case we > > need to be worrying about. > > The exact opposite is true: The code is trying to be robust against > upgrades of libavutil. The reason for this is potential trailing padding > in the structures that are copied here. It may be used for actual stuff > in a future libavutil and the approach you use here allows the compiler > to clobber it. > > (How would this code be robust against downgrades of libavutil at all? > There is no check here that sizeof of the side data is big enough to > contain everything we expect it to contain.) I should clearly not write code immediately after waking up. Yes, true, the only thing this logic is trying to accomplish is being robust against the struct gaining extra padding in the future. That said, I still think the code as written is brittle and I'm not sure it's providing anything useful. What is the likelihood of this struct being extended in a way that does not affect the encoder, vs. the likelihood of this struct being extended but somebody forgetting to bump the equivalent "last field" entry in this file? Anecdotally, the latter has already happened once. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".