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Date:   Mon, 16 Nov 2020 12:35:25 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexei Starovoitov <ast@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: Garbage data while reading via usermode driver?
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On Mon, Nov 16, 2020 at 08:11:04PM +0900, Tetsuo Handa wrote:
> Hello.
> 
> Below is a loadable kernel module which attempts to read (for example) /proc/interrupts from
> kernel using usermode driver interface. What is strange is that the total bytes obtained by
> doing "wc -c /proc/interrupts" from userspace's shell and trying to insmod this kernel module
> differs; for unknown reason, kernel_read() returns "#!/bin/cat /proc/interrupts\n" (28 bytes)
> at the end of input.

Because /bin/cat writes it out ;-)

$ echo "#!/bin/echo foo" >/tmp/a
$ chmod +x /tmp/a
$ /tmp/a
foo /tmp/a
$

IOW, same way #!/bin/sh -e in the beginning of /tmp/foo.sh results in exec
of /bin/sh with -e and /tmp/foo.sh in the arguments, #!/bin/cat /proc/interrupts
in /tmp/bar.sh will result in exec of /bin/cat with /proc/interrupts and
/tmp/bar.sh in parameters.  With cat(1) doing what it's supposed to do.