linux-kernel - Re: [PATCH 0/6] Add generated flag to filesystem struct to block copy_file

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Date:   Sun, 14 Feb 2021 23:09:02 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     "Darrick J . Wong" <djwong@...nel.org>,
        Ian Lance Taylor <iant@...gle.com>,
        Luis Lozano <llozano@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Chinner <david@...morbit.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Add generated flag to filesystem struct to block
 copy_file_range

On Fri, Feb 12, 2021 at 12:43:59PM +0800, Nicolas Boichat wrote:
> We hit an issue when upgrading Go compiler from 1.13 to 1.15 [1],
> as we use Go's `io.Copy` to copy the content of
> `/sys/kernel/debug/tracing/trace` to a temporary file.
> 
> Under the hood, Go 1.15 uses `copy_file_range` syscall to
> optimize the copy operation. However, that fails to copy any
> content when the input file is from tracefs, with an apparent
> size of 0 (but there is still content when you `cat` it, of
> course).
> 
> >From discussions in [2][3], it is clear that copy_file_range
> cannot be properly implemented on filesystems where the content
> is generated at runtime: the file size is incorrect (because it
> is unknown before the content is generated), and seeking in such
> files (as required by partial writes) is unlikely to work
> correctly.
> 
> With this patch, Go's `io.Copy` gracefully falls back to a normal
> read/write file copy.
> 
> I'm not 100% sure which stable tree this should go in, I'd say
> at least >=5.3 since this is what introduced support for
> cross-filesystem copy_file_range (and where most users are
> somewhat likely to hit this issue). But let's discuss the patch
> series first.

No.  This is *NOT* an fs-wide flag.  Decision regarding the
usability of copy_file_range() is on per-file basis.

The real constraint is "can freely seek back and expect to
find consistent data".  That is what's violated for seq_file.
And frankly, I would rather add a flag and have seq_open()
(and other suckers, if any) clear it.  With check being
"has both FMODE_PREAD and this new flag".