[PATCH 10/35] x86/jump_label: Implement arch_static_assert()

[Peter Zijlstra]

Implement the static (branch) assertion. It simply emits the address
of the next instruction into a special section which objtool will read
and validate against either __jump_table or .altinstructions.

Use like:

        if (static_branch_likely(_key)) {
                arch_static_assert();
                /* do stuff */
        }

Or

        if (static_cpu_has(_feat)) {
                arch_static_assert();
                /* do stuff */
        }

Cc: Borislav Petkov <b...@alien8.de>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Josh Poimboeuf <jpoim...@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 arch/x86/include/asm/jump_label.h |   23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -62,6 +62,29 @@ static __always_inline bool arch_static_
        return true;
 }
 
+/*
+ * Annotation for objtool; asserts that the previous instruction is the
+ * jump_label patch site. Or rather, that the next instruction is a static
+ * branch target.
+ *
+ * Use like:
+ *
+ *     if (static_branch_likely(key)) {
+ *             arch_static_assert();
+ *             do_code();
+ *     }
+ *
+ * Also works with static_cpu_has().
+ */
+static __always_inline void arch_static_assert(void)
+{
+       asm volatile ("1:\n\t"
+                     ".pushsection .discard.jump_assert \n\t"
+                     _ASM_ALIGN  "\n\t"
+                     _ASM_PTR "1b \n\t"
+                     ".popsection \n\t");
+}
+
 #ifdef CONFIG_X86_64
 typedef u64 jump_label_t;
 #else