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?
