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

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Message-Id: <20210212044405.4120619-1-drinkcat@chromium.org>
Date:   Fri, 12 Feb 2021 12:43:59 +0800
From:   Nicolas Boichat <drinkcat@...omium.org>
To:     "Darrick J . Wong" <djwong@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Ian Lance Taylor <iant@...gle.com>,
        Luis Lozano <llozano@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Chinner <david@...morbit.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        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: [PATCH 0/6] Add generated flag to filesystem struct to block copy_file_range

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.

[1] http://issuetracker.google.com/issues/178332739
[2] https://lkml.org/lkml/2021/1/25/64
[3] https://lkml.org/lkml/2021/1/26/1736


Nicolas Boichat (6):
  fs: Add flag to file_system_type to indicate content is generated
  proc: Add FS_GENERATED_CONTENT to filesystem flags
  sysfs: Add FS_GENERATED_CONTENT to filesystem flags
  debugfs: Add FS_GENERATED_CONTENT to filesystem flags
  tracefs: Add FS_GENERATED_CONTENT to filesystem flags
  vfs: Disallow copy_file_range on generated file systems

 fs/debugfs/inode.c | 1 +
 fs/proc/root.c     | 2 +-
 fs/read_write.c    | 3 +++
 fs/sysfs/mount.c   | 2 +-
 fs/tracefs/inode.c | 1 +
 include/linux/fs.h | 1 +
 6 files changed, 8 insertions(+), 2 deletions(-)

-- 
2.30.0.478.g8a0d178c01-goog