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

[Justin He <Justin.He@arm.com>]

Hi Matthew

> -----Original Message-----
> From: Matthew Wilcox <willy@infradead.org>
> Sent: Friday, May 28, 2021 11:22 PM
> To: Justin He <Justin.He@arm.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>; Petr Mladek
> <pmladek@suse.com>; Steven Rostedt <rostedt@goodmis.org>; Sergey
> Senozhatsky <senozhatsky@chromium.org>; Andy Shevchenko
> <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
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> Kalle Valo <kvalo@codeaurora.org>; David S. Miller <davem@davemloft.net>;
> Jakub Kicinski <kuba@kernel.org>; Heiko Carstens <hca@linux.ibm.com>;
> Vasily Gorbik <gor@linux.ibm.com>; Christian Borntraeger
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> 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?

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