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

[Rasmus Villemoes]

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?

Rasmus