linux-kernel - Re: [PATCH 17/23] initramfs: switch initramfs unpacking to struct file based APIs

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Message-ID: <20200727025507.GC795125@ZenIV.linux.org.uk>
Date:   Mon, 27 Jul 2020 03:55:07 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Song Liu <song@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-raid@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 17/23] initramfs: switch initramfs unpacking to struct
 file based APIs

On Tue, Jul 14, 2020 at 09:04:21PM +0200, Christoph Hellwig wrote:

> -		ssize_t rv = ksys_write(fd, p, count);
> +		ssize_t rv = kernel_write(file, p, count, &file->f_pos);

No.  Sure, that'll work for ramfs with nobody else playing with those.
However, this is the wrong way to do such things; do *NOT* pass the
address of file->f_pos to anything.  The few places that still do that
are wrong.

As a general rule, ->read() and ->write() instances should never be
given &file->f_pos.  Address of a local variable - sure, no problem.
Copy it back into ->f_pos when they are done?  Also fine.  But not
this,

Keep that offset in a variable (static in file, argument of xwrite(),
whatever).