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Date:   Fri, 28 May 2021 16:22:16 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Justin He <Justin.He@arm.com>
Cc:     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>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        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>,
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Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for
 file
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On Fri, May 28, 2021 at 03:09:28PM +0000, Justin He wrote:
> > I'm not sure why it's so complicated.  p->len records how many bytes
> > are needed for the entire path; can't you just return -p->len ?
> 
> prepend_name() will return at the beginning if p->len is <0 in this case,
> we can't even get the correct full path size if keep __prepend_path unchanged.
> We need another new helper __prepend_path_size() to get the full path size
> regardless of the negative value p->len.

It's a little hard to follow, based on just the patches.  Is there a
git tree somewhere of Al's patches that you're based on?

Seems to me that prepend_name() is just fine because it updates p->len
before returning false:

 static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
 {
 	const char *dname = smp_load_acquire(&name->name); /* ^^^ */
 	u32 dlen = READ_ONCE(name->len);
 	char *s;

 	p->len -= dlen + 1;
 	if (unlikely(p->len < 0))
 		return false;

I think the only change you'd need to make for vsnprintf() is in
prepend_path():

-		if (!prepend_name(&b, &dentry->d_name))
-			break;
+		prepend_name(&b, &dentry->d_name);

Would that hurt anything else?

> More than that, even the 1st vsnprintf could have _end_ > _buf_ in some case:
> What if printk("%pD", filp) ? The 1st vsnprintf has positive (end-buf).

I don't understand the problem ... if p->len is positive, then you
succeeded.  if p->len is negative then -p->len is the expected return
value from vsnprintf().  No?