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

[Matthew Wilcox]

On Fri, May 28, 2021 at 10:06:37PM +0200, Rasmus Villemoes wrote:
> On 28/05/2021 16.22, Justin He wrote:
> > 
> >> From: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> 
> >> How is it "safer"?  You already have a buffer passed from the caller.
> >> Are you saying that d_path_fast() might overrun a really small buffer
> >> but won't overrun a 256 byte buffer?
> > No, it won't overrun a 256 byte buf. When the full path size is larger than 256, the p->len is < 0 in prepend_name, and this overrun will be
> > dectected in extract_string() with "-ENAMETOOLONG".
> > 
> > Each printk contains 2 vsnprintf. vsnprintf() returns the required size after formatting the string.>
> > 1. vprintk_store() will invoke 1st vsnprintf() will 8 bytes space to get the reserve_size. In this case, the _buf_ could be less than _end_ by design.
> > 2. Then it invokes 2nd printk_sprint()->vscnprintf()->vsnprintf() to really fill the space.
> 
> Please do not assume that printk is the only user of vsnprintf() or the
> only one that would use a given %p<foo> extension.
> 
> Also, is it clear that nothing can change underneath you in between two
> calls to vsnprintf()? IOW, is it certain that the path will fit upon a
> second call using the size returned from the first?

No, but that's also true of %s.  I think vprintk_store() is foolish to
do it this way.