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Date:   Fri, 26 Jun 2020 15:37:14 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 8/9] fs: don't allow kernel reads and writes without
 iter ops
Message-ID: <20200626133714.GB12646@lst.de>
References: <20200626075836.1998185-1-hch@lst.de> <20200626075836.1998185-9-hch@lst.de> <20200626122752.GN4332@42.do-not-panic.com>
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On Fri, Jun 26, 2020 at 12:27:52PM +0000, Luis Chamberlain wrote:
> On Fri, Jun 26, 2020 at 09:58:35AM +0200, Christoph Hellwig wrote:
> > diff --git a/fs/read_write.c b/fs/read_write.c
> > index e765c95ff3440d..ae463bcadb6906 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -420,6 +420,18 @@ ssize_t iter_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos,
> >  	return ret;
> >  }
> >  
> > +static void warn_unsupported(struct file *file, const char *op)
> > +{
> > +	char pathname[128], *path;
> 
> Why 128? How about kstrdup_quotable_file()?

This is in the read/write path for the case where we did not end up
calling in the driver.  This is far less stack usage than any read/write
method would have used eventually.