Re: [RFC PATCH] net: skb: on zero-copy formatted output to skb

[Dmitry Antipov <dmantipov@yandex.ru>]

On Thu, 2026-04-23 at 14:40 +0200, Andrew Lunn wrote:

> 
> A quick look in drivers/bluetooth suggests that none of them care
> about truncation, at least they don't check the return code. Maybe it
> would be better to truncate than return an error?

Hm... I'm not a fan of (silent?) truncation and assume that user who wants to
detect it should be able to do so. If a (bluetooth) driver, or a subsystem,
or whatever else, doesn't care too much, it can wrap it with something like

#define bt_skb_printf(skb, fmt, ...)				\
({								\
	if (unlikely(skb_printf(skb, fmt, __VA_ARGS__) < 0)	\
		bt_warn("output to skb@%p truncated\n", skb));	\
})

> I would also try to make the behaviour consistent with the normal
> sprintf(), or snprintf(). You want to give users some idea what it is
> doing based on its name. It is well known that snprintf() will
> truncate, and does not return an error code. sprintf() will just
> overwrite the end of the buffer and not return an error code etc.
> 
> However calling this skb_snprintf() would be odd, since you don't
> actually pass size. But it hints at what it does.
> 
> So please add some kerneldoc describing what it actually does.

OK, some more neutral name like 'skb_fmt(...)' may be better indeed.

> Please also annotate the const char *fmt, so the compiler can do
> format string checks, parameter counting etc.

Surely '__printf(2, 3)' should be used here.

Dmitry