[PATCH v2 1/5] x86/livepatch: set function alignment to ensure minimal function size

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

The minimal function size requirements for livepatch are either 5 bytes (for
jmp) or 9 bytes (for endbr + jmp).  Ensure that functions are always at least
that size by requesting the compiled to align the functions to 8 or 16 bytes,
depending on whether Xen is build with IBT support.

Note that it's possible for the compiler to end up using a higher function
alignment regardless of the passed value, so this change just make sure that
the minimum required for livepatch to work is present.

Since the option (-falign-functions) is supported by both minimal required
compiler versions of clang and gcc there's no need to add a test to check for
its presence.

The alignment is currently only implemented for livepatch on x86, I'm unsure
whether ARM has a mandatory function alignment high enough to cover for the
space required by the replacement instruction(s).

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - New in this version.
---
 xen/arch/x86/Kconfig     | 6 ++++++
 xen/arch/x86/Makefile    | 2 ++
 xen/arch/x86/livepatch.c | 4 ++++
 3 files changed, 12 insertions(+)

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 1acdffc51c22..612a4acf079b 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -47,6 +47,12 @@ config HAS_CC_CET_IBT
        # Retpoline check to work around 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93654
        def_bool $(cc-option,-fcf-protection=branch -mmanual-endbr 
-mindirect-branch=thunk-extern) && $(as-instr,endbr64)
 
+# Set function alignment to ensure enough padding available
+config CC_FUNCTION_ALIGNMENT
+       int
+       default 16 if LIVEPATCH && XEN_IBT
+       default 8 if LIVEPATCH
+
 menu "Architecture Features"
 
 source "arch/Kconfig"
diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index f3abdf9cd111..f629157086d0 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -82,6 +82,8 @@ obj-$(CONFIG_COMPAT) += x86_64/platform_hypercall.o
 obj-y += sysctl.o
 endif
 
+CFLAGS-$(CONFIG_LIVEPATCH) += -falign-functions=$(CONFIG_CC_FUNCTION_ALIGNMENT)
+
 extra-y += asm-macros.i
 extra-y += xen.lds
 
diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
index ee539f001b73..4a6ba09e0ec5 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -109,6 +109,10 @@ int arch_livepatch_verify_func(const struct livepatch_func 
*func)
          */
         uint8_t needed = ARCH_PATCH_INSN_SIZE;
 
+        BUILD_BUG_ON(ARCH_PATCH_INSN_SIZE +
+                     (IS_ENABLED(CONIFG_XEN_IBT) ? ENDBR64_LEN : 0) >
+                     CONFIG_CC_FUNCTION_ALIGNMENT);
+
         if ( is_endbr64(func->old_addr) || is_endbr64_poison(func->old_addr) )
             needed += ENDBR64_LEN;
 
-- 
2.43.0