Messages in this thread | | | From | Rasmus Villemoes <> | Subject | Re: [PATCH] overflow check calculation in mm/mmap.c is incorrect linux-3.12.38 | Date | Fri, 08 May 2015 11:46:21 +0200 |
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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
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