linux-kernel - Re: [RFC PATCH 1/4] splice: Fix corruption of spliced data after splice() returns

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Date:   Wed, 19 Jul 2023 19:41:07 -0400
From:   Matt Whitlock <kernel@...twhitlock.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>, <netdev@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        Jens Axboe <axboe@...nel.dk>, <linux-fsdevel@...ck.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/4] splice: Fix corruption of spliced data after splice() returns

On Wednesday, 19 July 2023 19:20:32 EDT, Linus Torvalds wrote:
>> On Wednesday, 19 July 2023 16:16:07 EDT, Linus Torvalds wrote:
>>> The *ONLY* reason for splice() existing is for zero-copy.
>> 
>> The very first sentence of splice(2) reads: "splice() moves data between
>> two file descriptors without copying between kernel address space and user
>> address space." Thus, it is not unreasonable to believe that the point of
>> splice is to avoid copying between user-space and kernel-space.
>
> I'm not at all opposed to clarifying the documentation.

Then that is my request. This entire complaint/discussion/argument would 
have been avoided if splice(2) had contained a sentence like this one from 
sendfile(2):

"If out_fd refers to a socket or pipe with zero-copy support, callers must 
ensure the transferred portions of the file referred to by in_fd remain 
unmodified until the reader on the other end of out_fd has consumed the 
transferred data."

That is a clear warning of the perils of the implementation under the hood, 
and it could/should be copied, more or less verbatim, to splice(2).