Re: [PATCH] printk: ringbuffer: Fix truncating buffer size min_t cast

[Petr Mladek]

On Mon 2023-08-14 10:42:26, David Laight wrote:
> From: Kees Cook
> > Sent: 11 August 2023 06:46
> > 
> > If an output buffer size exceeded U16_MAX, the min_t(u16, ...) cast in
> > copy_data() was causing writes to truncate. This manifested as output
> > bytes being skipped, seen as %NUL bytes in pstore dumps when the available
> > record size was larger than 65536. Fix the cast to no longer truncate
> > the calculation.
> > 
> ...
> > diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> > index 2dc4d5a1f1ff..fde338606ce8 100644
> > --- a/kernel/printk/printk_ringbuffer.c
> > +++ b/kernel/printk/printk_ringbuffer.c
> > @@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ring *data_ring,
> >  	if (!buf || !buf_size)
> >  		return true;
> > 
> > -	data_size = min_t(u16, buf_size, len);
> > +	data_size = min_t(unsigned int, buf_size, len);
> 
> I'd noticed that during one of my test compiles while looking
> at making min() less fussy.
> 
> A better fix would be:
> 	data_size = min(buf_size + 0u, len);

This looks like a magic to me. The types are:

	unsigned int data_size;
	unsigned int buf_size;
	u16 len

I would naively expect that

	data_size = min(buf_size, len);

would do the right job and expand @len to "unsigned int".

I do not remember why "min_t" was used. Was it an optimization?
Did we miss the problem with casting "u32" down to "u16"?

I tried to read the discussion at
https://lore.kernel.org/lkml/b6a49ed73aba427ca8bb433763fa94e9@xxxxxxxxxxxxxxxx/
but it is more about "signed" vs. "unsigned" problem. Maybe
it is more complicated that I expected.

> Or put an ack on my patch 3/5 to minmax.h and then min(buf_size, len)
> will be fine (because both arguments are unsigned).

Do you mean
https://lore.kernel.org/lkml/6dc20ac7cb6f4570a0160f076e8362e3@xxxxxxxxxxxxxxxx/ ?
It seems to be just indentation cleanup.

Best Regards,
Petr

PS: I have already pushed the patch because it looked reasonable and
    got testing. I have to admit that I am probably in a pre-vacation
    hurry mode.