Re: [PATCH] vsprintf: drop comment claiming %n is ignored

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Sep 14, 2013 at 11:49:51AM +0900, Tetsuo Handa wrote:

> Even bad code which has never tested failure case, the authors should already
> know that "seq_printf() returns 0 on success case".

It is designed so that not testing failure case is normal approach for the
majority of users.

>   -	pos += seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
>   +	seq_puts(s, "DMA engine status\n");
>   +	seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
>    
>   - 	return pos;
>   +	return seq_overflow(s) : -1 : 0;
> 
> for keeping the functionality.

ITYM "for keeping the bug".  Read seq_read(), please.  Any negative value
returned by ->show() is a hard error.  It won't be retried with bigger
buffer; read(2) will *fail*.  With -EINVAL, in your case.

We really, really should not return non-zero on overflow.  Moreover, returning
a _positive_ value (SEQ_SKIP, normally, but any positive will do the same thing)
means "silently discard everything ->show() might have produced"

Again, the normal return value of ->show() is 0 and that includes the case of
overflow.  THE ONLY reason to check for overflow early is when subsequent
output of ->show() takes long to generate and we want to skip that and
have seq_read() do realloc-and-call-show-again immediately.  And in that
case the right fix is often to get saner iterator and stop shoving everything
into a single ->show() call...