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

[Reese Faucette]

Rasmus-
I think you're right - I was not paying close enough attention.  In this
case, I think the real culprit appears to be something like libc incorrectly
sign-extending the mmap() offset argument.  Calling mmap() with an offset of
0xf80000000 is resulting in a pg_off of 0xffff8000 when the syscall wrappers
convert the mmap() call to mmap_pgoff().  This is on MIPS linux - I'll dig
further on the user side to see why the offset is being sign extended.  The
proper casts *seem* to be in place in libc, but its plainly not doing what
it's supposed to.
-reese

> -----Original Message-----
> From: Rasmus Villemoes [mailto:linux@xxxxxxxxxxxxxxxxxx]
> Sent: Friday, May 08, 2015 2:46 AM
> To: Reese Faucette
> Cc: linux-kernel@xxxxxxxxxxxxxxx; alan@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH] overflow check calculation in mm/mmap.c is incorrect
> linux-3.12.38
> 
> On Thu, Apr 30 2015, "Reese Faucette" <reesefaucette@xxxxxxxxx> 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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