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Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for
 file
To:     Justin He <Justin.He@arm.com>, Matthew Wilcox <willy@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
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        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>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <89fc3919-ca2c-50fd-35e1-33bf3a59b993@rasmusvillemoes.dk>
Date:   Fri, 28 May 2021 22:06:37 +0200
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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?

Rasmus