[PATCH v4 2/2] livepatch: set -f{function,data}-sections compiler option

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

If livepatching support is enabled build the hypervisor with
-f{function,data}-sections compiler options, which is required by the
livepatching tools to detect changes and create livepatches.

This shouldn't result in any functional change on the hypervisor
binary image, but does however require some changes in the linker
script in order to handle that each function and data item will now be
placed into its own section in object files. As a result add catch-all
for .text, .data and .bss in order to merge each individual item
section into the final image.

The main difference will be that .text.startup will end up being part
of .text rather than .init, and thus won't be freed. .text.exit will
also be part of .text rather than dropped. Overall this could make the
image bigger, and package some .text code in a sub-optimal way.

On Arm the .data.read_mostly needs to be moved ahead of the .data
section like it's already done on x86, so the .data.* catch-all
doesn't also include .data.read_mostly. The alignment of
.data.read_mostly also needs to be set to PAGE_SIZE so it doesn't end
up being placed at the tail of a read-only page from the previous
section. While there move the alignment of the .data section ahead of
the section declaration, like it's done for other sections.

The benefit of having CONFIG_LIVEPATCH enable those compiler option
is that the livepatch build tools no longer need to fiddle with the
build system in order to enable them. Note the current livepatch tools
are broken after the recent build changes due to the way they
attempt to set  -f{function,data}-sections.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Acked-by: Julien Grall <jgrall@xxxxxxxxxx> # xen/arm
---
Changes since v3:
 - Place .text.page_aligned ahead of the .text.* catch-all.

Changes since v2:
 - Split the placing of the header code in a separate section to a
   pre-patch.
 - Move Kconfig option to xen/Kconfig.
 - Expand reasoning why .data.read_mostly needs to be moved on Arm.

Changes since v1:
 - Introduce CC_SPLIT_SECTIONS for selecting the compiler options.
 - Drop check for compiler options, all supported versions have them.
 - Re-arrange section placement in .text, to match the default linker
   script.
 - Introduce .text.header to contain the headers bits that must appear
   first in the final binary.
---
It seems on Arm the schedulers and hypfs .data sections should be
moved into read_mostly.
---
Tested by gitlab in order to assert I didn't introduce any regression
on Arm specially.
---
 xen/Kconfig            |  4 ++++
 xen/Makefile           |  2 ++
 xen/arch/arm/xen.lds.S | 41 +++++++++++++++++++++--------------------
 xen/arch/x86/xen.lds.S | 11 ++++++-----
 xen/common/Kconfig     |  1 +
 5 files changed, 34 insertions(+), 25 deletions(-)

diff --git a/xen/Kconfig b/xen/Kconfig
index bcbd2758e5..d134397a0b 100644
--- a/xen/Kconfig
+++ b/xen/Kconfig
@@ -27,6 +27,10 @@ config CLANG_VERSION
 config CC_HAS_VISIBILITY_ATTRIBUTE
        def_bool $(cc-option,-fvisibility=hidden)
 
+# Use -f{function,data}-sections compiler parameters
+config CC_SPLIT_SECTIONS
+       bool
+
 source "arch/$(SRCARCH)/Kconfig"
 
 config DEFCONFIG_LIST
diff --git a/xen/Makefile b/xen/Makefile
index 5c21492d6f..18a4f7e101 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -273,6 +273,8 @@ else
 CFLAGS += -fomit-frame-pointer
 endif
 
+CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections
+
 CFLAGS += -nostdinc -fno-builtin -fno-common
 CFLAGS += -Werror -Wredundant-decls -Wno-pointer-arith
 $(call cc-option-add,CFLAGS,CC,-Wvla)
diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
index 743455a5f9..7921d8fa28 100644
--- a/xen/arch/arm/xen.lds.S
+++ b/xen/arch/arm/xen.lds.S
@@ -36,6 +36,9 @@ SECTIONS
        *(.text.unlikely .text.*_unlikely .text.unlikely.*)
 
        *(.text)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+       *(.text.*)
+#endif
 
        *(.fixup)
        *(.gnu.warning)
@@ -82,10 +85,24 @@ SECTIONS
 #endif
   _erodata = .;                /* End of read-only data */
 
+  . = ALIGN(PAGE_SIZE);
+  .data.read_mostly : {
+       /* Exception table */
+       __start___ex_table = .;
+       *(.ex_table)
+       __stop___ex_table = .;
+
+       /* Pre-exception table */
+       __start___pre_ex_table = .;
+       *(.ex_table.pre)
+       __stop___pre_ex_table = .;
+
+       *(.data.read_mostly)
+  } :text
+
+  . = ALIGN(SMP_CACHE_BYTES);
   .data : {                    /* Data */
-       . = ALIGN(PAGE_SIZE);
        *(.data.page_aligned)
-       *(.data)
        . = ALIGN(8);
        __start_schedulers_array = .;
        *(.data.schedulers)
@@ -98,26 +115,10 @@ SECTIONS
        __paramhypfs_end = .;
 #endif
 
-       *(.data.rel)
-       *(.data.rel.*)
+       *(.data .data.*)
        CONSTRUCTORS
   } :text
 
-  . = ALIGN(SMP_CACHE_BYTES);
-  .data.read_mostly : {
-       /* Exception table */
-       __start___ex_table = .;
-       *(.ex_table)
-       __stop___ex_table = .;
-
-       /* Pre-exception table */
-       __start___pre_ex_table = .;
-       *(.ex_table.pre)
-       __stop___pre_ex_table = .;
-
-       *(.data.read_mostly)
-  } :text
-
   . = ALIGN(8);
   .arch.info : {
       _splatform = .;
@@ -211,7 +212,7 @@ SECTIONS
        *(.bss.percpu.read_mostly)
        . = ALIGN(SMP_CACHE_BYTES);
        __per_cpu_data_end = .;
-       *(.bss)
+       *(.bss .bss.*)
        . = ALIGN(POINTER_ALIGN);
        __bss_end = .;
   } :text
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 960c51eb4c..4103763f63 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -87,9 +87,12 @@ SECTIONS
        *(.text.cold)
        *(.text.unlikely .text.*_unlikely .text.unlikely.*)
 
+       *(.text.page_aligned)
        *(.text)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+       *(.text.*)
+#endif
        *(.text.__x86_indirect_thunk_*)
-       *(.text.page_aligned)
 
        *(.fixup)
        *(.gnu.warning)
@@ -292,9 +295,7 @@ SECTIONS
 
   DECL_SECTION(.data) {
        *(.data.page_aligned)
-       *(.data)
-       *(.data.rel)
-       *(.data.rel.*)
+       *(.data .data.*)
   } PHDR(text)
 
   DECL_SECTION(.bss) {
@@ -309,7 +310,7 @@ SECTIONS
        *(.bss.percpu.read_mostly)
        . = ALIGN(SMP_CACHE_BYTES);
        __per_cpu_data_end = .;
-       *(.bss)
+       *(.bss .bss.*)
        . = ALIGN(POINTER_ALIGN);
        __bss_end = .;
   } PHDR(text)
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 6443943889..d921c74d61 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -354,6 +354,7 @@ config LIVEPATCH
        bool "Live patching support"
        default X86
        depends on "$(XEN_HAS_BUILD_ID)" = "y"
+       select CC_SPLIT_SECTIONS
        ---help---
          Allows a running Xen hypervisor to be dynamically patched using
          binary patches without rebooting. This is primarily used to binarily
-- 
2.34.1