[Xen-devel] [PATCH v1 25/27] x86/pie: Add option to build the kernel as PIE

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

Add the CONFIG_X86_PIE option which builds the kernel as a Position
Independent Executable (PIE). The kernel is currently build with the
mcmodel=kernel option which forces it to stay on the top 2G of the
virtual address space. With PIE, the kernel will be able to move below
the current limit.

The --emit-relocs linker option was kept instead of using -pie to limit
the impact on mapped sections. Any incompatible relocation will be
catch by the arch/x86/tools/relocs binary at compile time.

If segment based stack cookies are enabled, try to use the compiler
option to select the segment register. If not available, automatically
enabled global stack cookie in auto mode. Otherwise, recommend
compiler update or global stack cookie option.

Performance/Size impact:
Size of vmlinux (Default configuration):
 File size:
 - PIE disabled: +0.000031%
 - PIE enabled: -3.210% (less relocations)
 .text section:
 - PIE disabled: +0.000644%
 - PIE enabled: +0.837%

Size of vmlinux (Ubuntu configuration):
 File size:
 - PIE disabled: -0.201%
 - PIE enabled: -0.082%
 .text section:
 - PIE disabled: same
 - PIE enabled: +1.319%

Size of vmlinux (Default configuration + ORC):
 File size:
 - PIE enabled: -3.167%
 .text section:
 - PIE enabled: +0.814%

Size of vmlinux (Ubuntu configuration + ORC):
 File size:
 - PIE enabled: -3.167%
 .text section:
 - PIE enabled: +1.26%

The size increase is mainly due to not having access to the 32-bit signed
relocation that can be used with mcmodel=kernel. A small part is due to reduced
optimization for PIE code. This bug [1] was opened with gcc to provide a better
code generation for kernel PIE.

Hackbench (50% and 1600% on thread/process for pipe/sockets):
 - PIE disabled: no significant change (avg +0.1% on latest test).
 - PIE enabled: between -0.50% to +0.86% in average (default and Ubuntu config).

slab_test (average of 10 runs):
 - PIE disabled: no significant change (-2% on latest run, likely noise).
 - PIE enabled: between -1% and +0.8% on latest runs.

Kernbench (average of 10 Half and Optimal runs):
 Elapsed Time:
 - PIE disabled: no significant change (avg -0.239%)
 - PIE enabled: average +0.07%
 System Time:
 - PIE disabled: no significant change (avg -0.277%)
 - PIE enabled: average +0.7%

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82303

Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>

merge PIE
---
 arch/x86/Kconfig  |  7 +++++++
 arch/x86/Makefile | 27 +++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1f2b731c9ec3..bbd28a46ab55 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2148,6 +2148,13 @@ config X86_GLOBAL_STACKPROTECTOR
 
           If unsure, say N
 
+config X86_PIE
+       bool
+       depends on X86_64
+       select DEFAULT_HIDDEN
+       select DYNAMIC_MODULE_BASE
+       select MODULE_REL_CRCS if MODVERSIONS
+
 config HOTPLUG_CPU
        bool "Support for hot-pluggable CPUs"
        depends on SMP
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index b592d57c531b..beae9504c3f4 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -135,7 +135,34 @@ else
 
         KBUILD_CFLAGS += -mno-red-zone
 ifdef CONFIG_X86_PIE
+        KBUILD_CFLAGS += -fPIC
         KBUILD_LDFLAGS_MODULE += -T $(srctree)/arch/x86/kernel/module.lds
+
+ifndef CONFIG_CC_STACKPROTECTOR_NONE
+ifndef CONFIG_X86_GLOBAL_STACKPROTECTOR
+        stackseg-flag := -mstack-protector-guard-reg=%gs
+        ifeq ($(call cc-option-yn,$(stackseg-flag)),n)
+                ifdef CONFIG_CC_STACKPROTECTOR_AUTO
+                        $(warning Cannot use CONFIG_CC_STACKPROTECTOR_* while \
+                                building a position independent kernel. \
+                                Default to global stack protector \
+                                (CONFIG_X86_GLOBAL_STACKPROTECTOR).)
+                        CONFIG_X86_GLOBAL_STACKPROTECTOR := y
+                        KBUILD_CFLAGS += -DCONFIG_X86_GLOBAL_STACKPROTECTOR
+                        KBUILD_AFLAGS += -DCONFIG_X86_GLOBAL_STACKPROTECTOR
+                else
+                        $(error echo Cannot use \
+                                CONFIG_CC_STACKPROTECTOR_(REGULAR|STRONG) \
+                                while building a position independent binary \
+                                Update your compiler or use \
+                                CONFIG_X86_GLOBAL_STACKPROTECTOR)
+                endif
+        else
+                KBUILD_CFLAGS += $(stackseg-flag)
+        endif
+endif
+endif
+
 else
         KBUILD_CFLAGS += -mcmodel=kernel
 endif
-- 
2.15.0.rc0.271.g36b669edcc-goog


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