On Fri, May 28, 2021 at 10:06:37PM +0200, Rasmus Villemoes wrote:

On 28/05/2021 16.22, Justin He wrote:

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From: Matthew Wilcox <willy@infradead.org>

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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.