[PATCH 2/3] UML - Host VDSO fix

[Jeff Dike <jdike@addtoit.com>]

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@addtoit.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	2006-06-17 21:49:35.000000000 -0400
+++ linux-2.6.17/arch/um/os-Linux/elf_aux.c	2007-02-20 10:57:52.000000000 -0500
@@ -40,6 +40,9 @@ __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;

-- 
Work email - jdike at linux dot intel dot com