uprobes: Introduce arch_uprobe->ixol

Currently xol_get_insn_slot() assumes that we should simply copy
arch_uprobe->insn[] which is (ignoring arch_uprobe_analyze_insn)
just the copy of the original insn.

This is not true for arm which needs to create another insn to
execute it out-of-line.

So this patch simply adds the new member, ->ixol into the union.
This doesn't make any difference for x86 and powerpc, but arm
can divorce insn/ixol and initialize the correct xol insn in
arch_uprobe_analyze_insn().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

arch/powerpc/include/asm/uprobes.h

@@ -37,6 +37,7 @@ typedef ppc_opcode_t uprobe_opcode_t;
 struct arch_uprobe {
 	union {
 		u8	insn[MAX_UINSN_BYTES];
+		u8	ixol[MAX_UINSN_BYTES];
 		u32	ainsn;
 	};
 };

arch/x86/include/asm/uprobes.h

@@ -35,7 +35,10 @@ typedef u8 uprobe_opcode_t;
 
 struct arch_uprobe {
 	u16				fixups;
+	union {
 		u8			insn[MAX_UINSN_BYTES];
+		u8			ixol[MAX_UINSN_BYTES];
+	};
 #ifdef CONFIG_X86_64
 	unsigned long			rip_rela_target_address;
 #endif

kernel/events/uprobes.c

@@ -1264,7 +1264,8 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
 		return 0;
 
 	/* Initialize the slot */
-	copy_to_page(area->page, xol_vaddr, uprobe->arch.insn, MAX_UINSN_BYTES);
+	copy_to_page(area->page, xol_vaddr,
+			uprobe->arch.ixol, sizeof(uprobe->arch.ixol));
 	/*
 	 * We probably need flush_icache_user_range() but it needs vma.
 	 * This should work on supported architectures too.