Re: [PATCH] overflow check calculation in mm/mmap.c is incorrect linux-3.12.38

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Thu, Apr 30 2015, "Reese Faucette" <reesefaucette@gmail.com> wrote:

> When checking for overflow, the code in mm/mmap.c compares the first byte
> *after* the end of mapped region to the start of the region instead of the
> last byte of the mapped region.  This prevents mapping a region which abuts
> the end of physical space, as mmap() incorrectly rejects the region with
> -EOVERFLOW, because pgoff + (len >> PAGE_SHIFT) will be 0, which is <
> pgoff.

Note this comment elsewhere in mmap.c:

 * We don't check here for the merged mmap wrapping around the end of pagecache
 * indices (16TB on ia32) because do_mmap_pgoff() does not permit mmap's which
 * wrap, nor mmaps which cover the final page at index -1UL.

So it seems to be by design.

But I'm also a little confused, since pgoff should be in units of pages (so a
20 bit number on 32bit), and I can't see how adding another 20 bit
number could ever make that overflow. Unless of course some magic power
ensures that pgoffs in the high half get sign-extended.

Rasmus