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Date:   Sun, 30 May 2021 16:18:23 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Justin He <Justin.He@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Luca Coelho <luciano.coelho@intel.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Johannes Berg <johannes.berg@intel.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
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        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>
Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for
 file
Message-ID: <YLOsvz8ZbpjfcuGO@casper.infradead.org>
References: <20210528113951.6225-1-justin.he@arm.com>
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On Fri, May 28, 2021 at 10:06:37PM +0200, Rasmus Villemoes wrote:
> On 28/05/2021 16.22, Justin He wrote:
> > 
> >> From: Matthew Wilcox <willy@infradead.org>
> 
> >> How is it "safer"?  You already have a buffer passed from the caller.
> >> Are you saying that d_path_fast() might overrun a really small buffer
> >> but won't overrun a 256 byte buffer?
> > No, it won't overrun a 256 byte buf. When the full path size is larger than 256, the p->len is < 0 in prepend_name, and this overrun will be
> > dectected in extract_string() with "-ENAMETOOLONG".
> > 
> > Each printk contains 2 vsnprintf. vsnprintf() returns the required size after formatting the string.>
> > 1. vprintk_store() will invoke 1st vsnprintf() will 8 bytes space to get the reserve_size. In this case, the _buf_ could be less than _end_ by design.
> > 2. Then it invokes 2nd printk_sprint()->vscnprintf()->vsnprintf() to really fill the space.
> 
> Please do not assume that printk is the only user of vsnprintf() or the
> only one that would use a given %p<foo> extension.
> 
> Also, is it clear that nothing can change underneath you in between two
> calls to vsnprintf()? IOW, is it certain that the path will fit upon a
> second call using the size returned from the first?

No, but that's also true of %s.  I think vprintk_store() is foolish to
do it this way.