-----Original Message-----
From: Matthew Wilcox <willy@infradead.org>
Sent: Friday, May 28, 2021 11:22 PM
To: Justin He <redacted>
Cc: Linus Torvalds <torvalds@linux-foundation.org>; Petr Mladek
[off-list ref]; Steven Rostedt [off-list ref]; Sergey
Senozhatsky [off-list ref]; Andy Shevchenko
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Kalle Valo [off-list ref]; David S. Miller [off-list ref];
Jakub Kicinski [off-list ref]; Heiko Carstens [off-list ref];
Vasily Gorbik [off-list ref]; Christian Borntraeger
[off-list ref]; Johannes Berg [off-list ref]; linux-
doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
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s390@vger.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:

quoted

quoted

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.

quoted

We need another new helper __prepend_path_size() to get the full path

size

quoted

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.

quoted

More than that, even the 1st vsnprintf could have _end_ > _buf_ in some

case:

quoted

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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