Re: don't let mmap allocate down to zero

[Mikael Pettersson]

William Lee Irwin III writes:
 > On Wed, Jan 26, 2005 at 11:18:08AM -0500, Rik van Riel wrote:
 > >> With some programs the 2.6 kernel can end up allocating memory
 > >> at address zero, for a non-MAP_FIXED mmap call!  This causes
 > >> problems with some programs and is generally rude to do. This
 > >> simple patch fixes the problem in my tests.
 > >> Make sure that we don't allocate memory all the way down to zero,
 > >> so the NULL pointer never gets covered up with anonymous memory
 > >> and we don't end up violating the C standard.
 > >> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
 > 
 > On Wed, Jan 26, 2005 at 09:25:38AM -0800, William Lee Irwin III wrote:
 > > SHLIB_BASE does not appear to be present in 2.6.9; perhaps something
 > > else is going on.
 > > I think we are better off:
 > > 	(a) checking for hitting zero explicitly as opposed to
 > > 		enforcing a randomly-chosen lower limit for addresses
 > > 	(b) enforcing vma allocation above FIRST_USER_PGD_NR*PGDIR_SIZE,
 > > 		to which SHLIB_BASE bears no relation.
 > 
 > There's a long discussion here, in which no one appears to have noticed
 > that SHLIB_BASE does not exist in mainline. Is anyone else awake here?

About the only kernel-level enforcement I would feel comfortable with is
to have non-fixed mmap()s refuse to grab the _page_ at address 0. Any range
larger than this is policy, and hence needs a user-space override mechanism.

Also, if user-space wants to catch accesses in a larger region above 0 then
it can do that itself, by checking the result of mmap(), or by doing a fixed
mmap() at address 0 with suitable size and rwx protection disabled.

/Mikael
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