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

[Justin He]

Hi Matthew

> -----Original Message-----
> From: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Sent: Friday, May 28, 2021 11:22 PM
> To: Justin He <Justin.He@xxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>; Petr Mladek
> <pmladek@xxxxxxxx>; Steven Rostedt <rostedt@xxxxxxxxxxx>; Sergey
> Senozhatsky <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx>; Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; Alexander
> Viro <viro@xxxxxxxxxxxxxxxxxx>; Luca Coelho <luciano.coelho@xxxxxxxxx>;
> Kalle Valo <kvalo@xxxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>;
> Jakub Kicinski <kuba@xxxxxxxxxx>; Heiko Carstens <hca@xxxxxxxxxxxxx>;
> Vasily Gorbik <gor@xxxxxxxxxxxxx>; Christian Borntraeger
> <borntraeger@xxxxxxxxxx>; Johannes Berg <johannes.berg@xxxxxxxxx>; linux-
> doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> wireless@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-
> s390@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path
> for file
>
> 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'.

What do you think of it?

--
Cheers,
Justin (Jia He)


> > 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?

IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.