SHMLBA and compat tasks

[Arun Sharma <arun.sharma@intel.com>]

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The current Linux implementation of shmat() insists on SHMLBA alignment
even when shmflg & SHM_RND == 0. This is not consistent with the man pages
and the single UNIX spec, which require only a page-aligned address.

man page:

> If shmaddr isn’t NULL and SHM_RND is asserted  in  shmflg,  the  attach
> occurs at the address equal to shmaddr rounded down to the nearest mul-
> tiple of SHMLBA.  Otherwise shmaddr must be a page-aligned  address  at
> which the attach occurs.

This is not a problem for many platforms because SHMLBA == PAGE_SIZE. It's
also not a problem for 64 bit programs running on 64 bit platforms,
because arch_get_unmapped_area() typically guarantees SHMLBA alignment,
even when SHM_RND is not asserted.

However, for 32 bit compatiblity programs, an innocuous sequence such as:

p = shmat(id, 0, shmflg)  /* shmflag & SHM_RND == 0 */
/* p is page-aligned, but may not be SHMLBA aligned */
shmdt(p)
shmat(id, p, shmflg) == -EINVAL ??

can fail. The attached patch fixes the problem for ia64. Can other arch
maintainers confirm that it doesn't break anything for you ?

	-Arun

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--- linux/ipc/shm.c.orig	2003-04-28 11:19:24.000000000 -0700
+++ linux/ipc/shm.c	2003-05-09 13:26:13.000000000 -0700
@@ -600,7 +600,7 @@
 		if (addr & (SHMLBA-1)) {
 			if (shmflg & SHM_RND)
 				addr &= ~(SHMLBA-1);	   /* round down */
-			else
+			else if (addr & ~PAGE_MASK)
 				return -EINVAL;
 		}
 		flags = MAP_SHARED | MAP_FIXED;