[PATCH] Allow i386 crash kernels to handle x86_64 dumps.

[Ian Campbell <ian.campbell@xensource.com>]

From: Ian Campbell <ian.campbell@xensource.com>

The specific case I am encountering is kdump under Xen with a 64 bit
hypervisor and 32 bit kernel/userspace. The dump created is 64 bit due to
the hypervisor but the dump kernel is 32 bit for maximum compatibility.

It's possibly less likely to be useful in a purely native scenario but
I see no reason to disallow it.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>
Acked-by: Vivek Goyal <vgoyal@in.ibm.com>
---

 fs/proc/vmcore.c           |    2 +-
 include/asm-i386/kexec.h   |    3 +++
 include/linux/crash_dump.h |    8 ++++++++
 3 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index d960507..523e109 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -514,7 +514,7 @@ static int __init parse_crash_elf64_headers(void)
 	/* Do some basic Verification. */
 	if (memcmp(ehdr.e_ident, ELFMAG, SELFMAG) != 0 ||
 		(ehdr.e_type != ET_CORE) ||
-		!elf_check_arch(&ehdr) ||
+		!vmcore_elf_check_arch(&ehdr) ||
 		ehdr.e_ident[EI_CLASS] != ELFCLASS64 ||
 		ehdr.e_ident[EI_VERSION] != EV_CURRENT ||
 		ehdr.e_version != EV_CURRENT ||
diff --git a/include/asm-i386/kexec.h b/include/asm-i386/kexec.h
index 4dfc9f5..c76737e 100644
--- a/include/asm-i386/kexec.h
+++ b/include/asm-i386/kexec.h
@@ -47,6 +47,9 @@
 /* The native architecture */
 #define KEXEC_ARCH KEXEC_ARCH_386
 
+/* We can also handle crash dumps from 64 bit kernel. */
+#define vmcore_elf_check_arch_cross(x) ((x)->e_machine == EM_X86_64)
+
 #define MAX_NOTE_BYTES 1024
 
 /* CPU does not save ss and esp on stack if execution is already
diff --git a/include/linux/crash_dump.h b/include/linux/crash_dump.h
index 3250365..db60dac 100644
--- a/include/linux/crash_dump.h
+++ b/include/linux/crash_dump.h
@@ -14,5 +14,13 @@ extern ssize_t copy_oldmem_page(unsigned long, char *, size_t,
 extern const struct file_operations proc_vmcore_operations;
 extern struct proc_dir_entry *proc_vmcore;
 
+/* Architecture code defines this if there are other possible ELF
+ * machine types, e.g. on bi-arch capable hardware. */
+#ifndef vmcore_elf_check_arch_cross(x)
+#define vmcore_elf_check_arch_cross(x) 0
+#endif
+
+#define vmcore_elf_check_arch(x) (elf_check_arch(x) || vmcore_elf_check_arch_cross(x))
+
 #endif /* CONFIG_CRASH_DUMP */
 #endif /* LINUX_CRASHDUMP_H */