re: mm: brk: downgrade mmap_sem to read when shrinking

[Colin Ian King <colin.king@canonical.com>]

Hi,

Static analysis has found a couple of issues as follows:

commit 551f205aff9198e17add1264dd781771d1a2bd9d
Author: Yang Shi <yang.shi@linux.alibaba.com>
Date:   Thu Oct 4 07:43:18 2018 +1000

    mm: brk: downgrade mmap_sem to read when shrinking

Static analysis with CoverityScan has detected an issue in mm/mmap.c,
function do_brk_flags():

                retval = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
                if (retval < 0) {
                        mm->brk = origbrk;
                        goto out;
                } else if (retval == 1)
                        downgraded = true;

retval is unsigned long, so the retval < 0 check is always false, which
looks bogus to me.

Also same kind of issue with:

commit e66477708ec2a764d3add92ca59134e3812da0bb
Author: Yang Shi <yang.shi@linux.alibaba.com>
Date:   Thu Oct 4 07:43:18 2018 +1000

    mm: mremap: downgrade mmap_sem to read when shrinking

                ret = __do_munmap(mm, addr+new_len, old_len - new_len,
                                  &uf_unmap, true);
                if (ret < 0 && old_len != new_len)
                        goto out;
                /* Returning 1 indicates mmap_sem is downgraded to read. */
                else if (ret == 1)
                        downgraded = true;

again, ret is unsigned long, so the comparison with ret < 0 is always false.

Detected by CoverityScan, CID#1473794, CID#1473791 "Unsigned compared
against 0".

Colin