[PATCH] UML - host VDSO fix

[PATCH] UML - host VDSO fix

[Jeff Dike]

This fixes a problem seen by a number of people running UML on newer host
kernels.  init would hang with an infinite segfault loop.

It turns out that the host kernel was providing a AT_SYSINFO_EHDR of
0xffffe000, which faked UML into believing that the host VDSO page could be
reused.  However, AT_SYSINFO pointed into the middle of the address space, and
was unmapped as a result.  Because UML was providing AT_SYSINFO_EHDR and
AT_SYSINFO to its own processes, these would branch to nowhere when trying to
use the VDSO.

The fix is to also check the location of AT_SYSINFO when deciding whether to
use the host's VDSO.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
--
 arch/um/os-Linux/elf_aux.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-2.6.17/arch/um/os-Linux/elf_aux.c
===================================================================
--- linux-2.6.17.orig/arch/um/os-Linux/elf_aux.c	2007-02-23 15:00:51.000000000 -0500
+++ linux-2.6.17/arch/um/os-Linux/elf_aux.c	2007-02-23 15:09:58.000000000 -0500
@@ -39,6 +39,9 @@ __init void scan_elf_aux( char **envp)
 		switch ( auxv->a_type ) {
 			case AT_SYSINFO:
 				__kernel_vsyscall = auxv->a_un.a_val;
+				/* See if the page is under TASK_SIZE */
+				if (__kernel_vsyscall < (unsigned long) envp)
+					__kernel_vsyscall = 0;
 				break;
 			case AT_SYSINFO_EHDR:
 				vsyscall_ehdr = auxv->a_un.a_val;