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

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Date:   Mon, 31 May 2021 00:39:50 +0000
From:   Justin He <Justin.He@....com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Johannes Berg <johannes.berg@...el.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>
Subject: RE: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for
 file



> -----Original Message-----
> From: Matthew Wilcox <willy@...radead.org>
> Sent: Friday, May 28, 2021 11:22 PM
> To: Justin He <Justin.He@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>; Petr Mladek
> <pmladek@...e.com>; Steven Rostedt <rostedt@...dmis.org>; Sergey
> Senozhatsky <senozhatsky@...omium.org>; Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com>; Rasmus Villemoes
> <linux@...musvillemoes.dk>; Jonathan Corbet <corbet@....net>; Alexander
> Viro <viro@...iv.linux.org.uk>; Luca Coelho <luciano.coelho@...el.com>;
> Kalle Valo <kvalo@...eaurora.org>; David S. Miller <davem@...emloft.net>;
> Jakub Kicinski <kuba@...nel.org>; Heiko Carstens <hca@...ux.ibm.com>;
> Vasily Gorbik <gor@...ux.ibm.com>; Christian Borntraeger
> <borntraeger@...ibm.com>; Johannes Berg <johannes.berg@...el.com>; linux-
> doc@...r.kernel.org; linux-kernel@...r.kernel.org; linux-
> wireless@...r.kernel.org; netdev@...r.kernel.org; linux-
> s390@...r.kernel.org
> 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?

The git tree of Al's patches is at:
https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/log/?h=work.d_path

>
> 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?
I will try your suggestion soon.

>
> > 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?

There are 3 cases I once met in my debugging:
1. p->len is positive but too small (e.g. end-buf is 6). In first prepend_name
loop p-len-=dlen, then p->len is negative

2. p->len is negative at the very beginning (i.e. end-buf is negative)

3. p->len positive and large enough. Typically the 2nd vsnprintf of printk


--
Cheers,
Justin (Jia He)


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