Re: [PATCH] fs/read_write.c: Fix a broken signed integer overflow check.

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

On Mon, Feb 07, 2022 at 02:58:59PM +0000, Al Viro wrote:
> On Mon, Feb 07, 2022 at 02:07:11PM +0200, Ari Sundholm wrote:
> > The function generic_copy_file_checks() checks that the ends of the
> > input and output file ranges do not overflow. Unfortunately, there is
> > an issue with the check itself.
> > 
> > Due to the integer promotion rules in C, the expressions
> > (pos_in + count) and (pos_out + count) have an unsigned type because
> > the count variable has the type uint64_t. Thus, in many cases where we
> > should detect signed integer overflow to have occurred (and thus one or
> > more of the ranges being invalid), the expressions will instead be
> > interpreted as large unsigned integers. This means the check is broken.
> 
> I must be slow this morning, but... which values of pos_in and count are
> caught by your check, but not by the original?
> 
> > -	if (pos_in + count < pos_in || pos_out + count < pos_out)
> > +	if ((loff_t)(pos_in + count) < pos_in ||
> > +			(loff_t)(pos_out + count) < pos_out)
> 
> Example, please.  Why do you need that comparison to be signed?

Note that we explicitly truncate count so we won't get past the EOF of
file_in right below that check and the check in generic_write_check_limits()
truncates count so we won't get past ->s_maxbytes on the filesystem we
are writing to.

If both source and destination allow arbitrary offsets, we should not
fail on copy that crosses from 2^63-1 to 2^63.  Your variant will do
just that.

It's multiples of 2^64 that we should never attempt to cross, no matter
what.

IOW, what values of pos_in, pos_out, count, input file size and output
filesystem file size limit do you think should be rejected with
-EOVERFLOW here?