Re: [PATCH v5 1/2] powerpc/32: add stack protector support

Samuel Holland [off-list ref] writes:

Hello all,

On 09/27/18 02:05, Christophe Leroy wrote:
[..snip..]

quoted

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 07d9dce7eda6..45b8eb4d8fe7 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile

@@ -112,6 +112,9 @@ KBUILD_LDFLAGS	+= -m elf$(BITS)$(LDEMULATION)
 KBUILD_ARFLAGS	+= --target=elf$(BITS)-$(GNUTARGET)
 endif
 
+cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
+cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
+
 LDFLAGS_vmlinux-y := -Bstatic
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
 LDFLAGS_vmlinux	:= $(LDFLAGS_vmlinux-y)

@@ -404,6 +407,13 @@ archclean:
 
 archprepare: checkbin
 
+ifdef CONFIG_STACKPROTECTOR
+prepare: stack_protector_prepare
+
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
+endif
+

This breaks when building out-of-tree kernel modules. GCC is not getting passed
the -mstack-protector-guard-offset argument, so the default offset is used. The
kernel then panics the first time a function with stack protector is called.

I'm seeing this on powerpc64. It looks like it was reported for powerpc on
kernel bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201891

Thanks for the bug report.

Alexey also hit this and sent an RFC fix.

  https://patchwork.ozlabs.org/patch/1022751/

But Masahiro thought we could do something less hacky, and hopefully he
can help us come up with something better.

cheers