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

[Matthew Wilcox <willy@infradead.org>]

somehow the linux-fsdevel mailing list got dropped from this revision
of the patch set.  anyone who's following along may wish to refer to
the archives:
https://lore.kernel.org/linux-doc/20210528113951.6225-1-justin.he@arm.com/

On Tue, Jun 01, 2021 at 02:42:15PM +0000, Justin He wrote:
> > 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?
> >
> 
> It almost works except the snprintf case,
> Consider,assuming filp path is 256 bytes, 2 dentries "/root/$long_string":
> snprintf(buffer, 128, "%pD", filp);
> p->len is positive at first, but negative after prepend_name loop.
> So, it will not fill any bytes in _buffer_.
> But in theory, it should fill the beginning 127 bytes and '\0'.

I have a few thoughts ...

1. Do we actually depend on that anywhere?
2. Is that something we should support?
3. We could print the start of the filename, if we do.  So something like
this ...

static void prepend(struct prepend_buffer *p, const char *str, int namelen)
{
	p->len -= namelen;
	if (likely(p->len >= 0)) {
		p->buf -= namelen;
		memcpy(p->buf, str, namelen);
	} else {
		char *s = p->buf;
		int buflen = strlen(p->buf);

		/* The first time we overflow the buffer */
		if (p->len + namelen > 0) {
			p->buf -= p->len + namelen;
			buflen += p->len + namelen;
		}

		if (buflen > namelen) {
			memmove(p->buf + namelen, s, buflen - namelen);
			memcpy(p->buf, str, namelen);
		} else {
			memcpy(p->buf, str, buflen);
		}
	}
}

I haven't tested this; it's probably full of confusion and off-by-one
errors.  But I hope you get the point -- we continue to accumulate
p->len to indicate how many characters we shifted off the right of the
buffer while adding the (start of) the filename on the left.

4. If we want the end of the filename instead, that looks easier:

static void prepend(struct prepend_buffer *p, const char *str, int namelen)
{
	p->len -= namelen;
	if (likely(p->len >= 0)) {
		p->buf -= namelen;
		memcpy(p->buf, str, namelen);
	} else if (p->len + namelen > 0) {
		p->buf -= p->len + namelen;
		memcpy(p->buf, str - p->len, p->len + namelen)
	}
}

But I don't think we want any of this at all.  Just don't put anything
in the buffer if the user didn't supply enough space.  As long as you
get the return value right, they know the string is bad (or they don't
care if the string is bad)