[PATCH] uml: fix compilation and execution with hardened GCC

To make some half-assembly stubs compile, disable various "hardened" GCC
features:

*) we can't make it build PIC code as we need %ebx to do syscalls and GCC
   wants it free for PIC

*) we can't leave stack protection as the stub is moved (not relocated!) in
   memory so the RIP-relative access to the canary tries reading from an
   unmapped address and causes a segfault, since we move the stub of various
   megabytes (the exact amount will be decided at runtime) away from the
   link-time address.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/um/kernel/skas/Makefile b/arch/um/kernel/skas/Makefile
index 57181a920d48..ad842964707a 100644
--- a/arch/um/kernel/skas/Makefile
+++ b/arch/um/kernel/skas/Makefile
@@ -11,4 +11,11 @@ USER_OBJS := clone.o
 include arch/um/scripts/Makefile.rules
 
 # clone.o is in the stub, so it can't be built with profiling
-$(obj)/clone.o : c_flags = -Wp,-MD,$(depfile) $(call unprofile,$(USER_CFLAGS))
+# GCC hardened also auto-enables -fpic, but we need %ebx so it can't work ->
+# disable it
+
+CFLAGS_clone.o := $(CFLAGS_NO_HARDENING)
+
+# since we're setting c_flags we _must_ add $(CFLAGS_$(*F).o).
+
+$(obj)/clone.o : c_flags = -Wp,-MD,$(depfile) $(call unprofile,$(USER_CFLAGS)) $(CFLAGS_$(*F).o)