Re: [PATCH v3 0/3] Add stack protector

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/12/2024 12:13 am, Volodymyr Babchuk wrote:
> Hello Jan,
>
> Jan Beulich <jbeulich@xxxxxxxx> writes:
>
>> On 11.12.2024 03:04, Volodymyr Babchuk wrote:
>>> Both GCC and Clang support -fstack-protector feature, which add stack
>>> canaries to functions where stack corruption is possible. This series
>>> makes possible to use this feature in Xen. I tested this on ARM64 and
>>> it is working as intended. Tested both with GCC and Clang.
>>>
>>> It is hard to enable this feature on x86, as GCC stores stack canary
>>> in %fs:40 by default, but Xen can't use %fs for various reasons. It is
>>> possibly to change stack canary location new newer GCC versions, but
>>> this will change minimal GCC requirement, which is also hard due to
>>> various reasons. So, this series focus mostly on ARM and RISCV.
>> Why exactly would it not be possible to offer the feature when new enough
>> gcc is in use?
> It is possible to use this feature with a modern enough GCC, yes. Are
> you suggesting to make HAS_STACK_PROTECTOR dependent on GCC_VERSION for
> x86 platform?

(With the knowledge that this is a disputed Kconfig pattern, and will
need rebasing), the way I want this to work is simply:

diff --git a/xen/Makefile b/xen/Makefile
index 0de0101fd0bf..5d0a88fb3c3f 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -434,6 +434,9 @@ endif
 
 ifeq ($(CONFIG_STACK_PROTECTOR),y)
 CFLAGS += -fstack-protector
+ifeq ($(CONFIG_X86),y)
+CFLAGS += -mstack-protector-guard=global
+endif
 else
 CFLAGS += -fno-stack-protector
 endif
diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 9cdd04721afa..7951ca908b36 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -28,6 +28,7 @@ config X86
        select HAS_PCI_MSI
        select HAS_PIRQ
        select HAS_SCHED_GRANULARITY
+       select HAS_STACK_PROTECTOR if
$(cc-option,-mstack-protector-guard=global)
        select HAS_UBSAN
        select HAS_VMAP
        select HAS_VPCI if HVM



Sadly, it doesn't build.  I get a handful of:

prelink.o: in function `cmdline_parse':
/home/andrew/xen.git/xen/common/kernel.c:216:(.init.text+0x20f2): failed
to convert GOTPCREL relocation against '__stack_chk_guard'; relink with
--no-relax
/home/andrew/xen.git/xen/common/kernel.c:230:(.init.text+0x246f): failed
to convert GOTPCREL relocation against '__stack_chk_guard'; relink with
--no-relax

which is more toolchain-whispering than I feel like doing tonight.

~Andrew