Re: [LTP] mmapstress03 weirdness? (fwd)

[Geert Uytterhoeven]

On Tue, 22 Sep 2009, Benjamin Herrenschmidt wrote:
> On Mon, 2009-09-21 at 15:40 +0200, Geert Uytterhoeven wrote:
> > With 32-bit userland, this boils down to:
> > | mmap addr 0x7fff0000 size 0x7fff0000
> > | mmap returned 0x7fff0000
> > 
> > i.e. mmap() succeeds, but (1) the test expects it to fail, so the test returns
> > TFAIL, but (2) ltp-pan still reports that the tests passed?
> 
> What is the output of /proc/<pid>/maps after that mmap ?

| 00100000-00120000 r-xp 00100000 00:00 0                                  [vdso]
| 0f470000-0f5d0000 r-xp 00000000 03:03 56852565                           /lib/libc-2.5.so
| 0f5d0000-0f5e0000 r--p 00160000 03:03 56852565                           /lib/libc-2.5.so
| 0f5e0000-0f5f0000 rw-p 00170000 03:03 56852565                           /lib/libc-2.5.so
| 0ffc0000-0ffe0000 r-xp 00000000 03:03 56852482                           /lib/ld-2.5.so
| 0ffe0000-0fff0000 r--p 00010000 03:03 56852482                           /lib/ld-2.5.so
| 0fff0000-10000000 rw-p 00020000 03:03 56852482                           /lib/ld-2.5.so
| 10000000-10010000 r-xp 00000000 03:03 65571126                           /tmp/a.out
| 10010000-10020000 rw-p 00000000 03:03 65571126                           /tmp/a.out
| 7fff0000-fffe0000 rw-s 00000000 00:09 5580806                            /dev/zero (deleted)

I.e. the big mmap() took out the stack mapping, which was previously at:

| ffa00000-ffb50000 rw-p ffa00000 00:00 0                                  [stack]

> With a 64-bit kernel, 32-bit userspace has access to the entire 4G
> address space, so mapping 2G-64k at the 2G-64k point can work, provided
> you aren't overlapping an existing mapping such as the stack.
> 
> > In addition, sometimes mmapstress03 fails due to SEGV. I created a small test
> > program that just does the above mmap(), and depending on the distro and what
> > else I print later it crashes with a SEGV, too. Probably this happens because
> > the mmap() did succeed, and corrupted some existing mappings, cfr. the notes
> > for MAP_FIXED:
> 
> That's possible.
> 
> >        MAP_FIXED
> >               Donât  interpret  addr  as  a hint: place the mapping at exactly
> >               that address.  addr must be a multiple of the page size.  If the
> >               memory  region  specified  by addr and len overlaps pages of any
> >               existing mapping(s), then the overlapped part  of  the  existing
> >               mapping(s)  will  be discarded.  If the specified address cannot
> >               be used, mmap() will fail.  Because requiring  a  fixed  address
> >               for  a  mapping is less portable, the use of this option is disâ
> >               couraged.
> 
> Yeah, I suppose the test might be wiping out its own stack for example

Indeed.

> IE. I think that test is just bogus :-)
> 
> > JFYI, with 64-bit userland, this boils down to:
> > 
> > | mmap addr 0x7fffffffffff0000 size 0x7fffffffffff0000
> > | mmap returned 0xffffffffffffffff
> > 
> > i.e. mmap() fails as expected, and the test succeeds.
> 
> Right because on 64-bit userspace, you only are allowed something like
> 16T of address space.
> 
> > Does all of this sound OK?
> > Thanks for your comments!
> 
> Yes, I think so far, it's just bogus tests :-)

Thanks for the confirmation, Segher and Ben!

With kind regards,

Geert Uytterhoeven
Software Architect
Techsoft Centre

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