Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for file

[Matthew Wilcox <willy@infradead.org>]

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?