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Cc:     "psodagud@codeaurora.org" <psodagud@codeaurora.org>
From:   Siddharth Gupta <sidgup@codeaurora.org>
Subject: PROBLEM: Firmware loader fallback mechanism no longer works with
 sendfile
Message-ID: <7e6f44b1-a0d2-d1d1-9c11-dcea163f8f03@codeaurora.org>
Date:   Mon, 4 Jan 2021 14:43:45 -0800
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Hi all,

With the introduction of the filesystem change "fs: don't allow splice 
read/write without explicit ops"[1] the fallback mechanism of the 
firmware loader[2] no longer works when using sendfile[3] from the 
userspace.

Since the binary attributes don't support splice_{read,write} functions 
the calls to splice_{read,write} used the default kernel_{read,write} 
functions. With the above change this results in an -EINVAL return from 
do_splice_from[4].

This essentially means that sendfile will not work for any binary 
attribute in the sysfs.

Thanks,
Siddharth

[1]: 
https://github.com/torvalds/linux/commit/36e2c7421f02a22f71c9283e55fdb672a9eb58e7#diff-70c49af2ed5805fc1406ed6e6532d6a029ada1abd90cca6442711b9cecd4d523
[2]: 
https://github.com/torvalds/linux/blob/master/drivers/base/firmware_loader/main.c#L831
[3]: https://github.com/torvalds/linux/blob/master/fs/read_write.c#L1257
[4]: https://github.com/torvalds/linux/blob/master/fs/splice.c#L753