objtool: Constify arch_decode_instruction()

Mostly straightforward constification, except that WARN_FUNC()
needs a writable pointer while we have read-only pointers,
so deflect this to WARN().
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200422103205.61900-4-mingo@kernel.org

tools/objtool/arch.h

@@ -72,7 +72,7 @@ struct instruction;
 
 void arch_initial_func_cfi_state(struct cfi_init_state *state);
 
-int arch_decode_instruction(struct elf *elf, struct section *sec,
+int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 			    unsigned long offset, unsigned int maxlen,
 			    unsigned int *len, enum insn_type *type,
 			    unsigned long *immediate,

tools/objtool/arch/x86/decode.c

@@ -27,7 +27,7 @@ static unsigned char op_to_cfi_reg[][2] = {
 	{CFI_DI, CFI_R15},
 };
 
-static int is_x86_64(struct elf *elf)
+static int is_x86_64(const struct elf *elf)
 {
 	switch (elf->ehdr.e_machine) {
 	case EM_X86_64:
@@ -77,7 +77,7 @@ unsigned long arch_jump_destination(struct instruction *insn)
 	return insn->offset + insn->len + insn->immediate;
 }
 
-int arch_decode_instruction(struct elf *elf, struct section *sec,
+int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 			    unsigned long offset, unsigned int maxlen,
 			    unsigned int *len, enum insn_type *type,
 			    unsigned long *immediate,
@@ -98,7 +98,7 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 	insn_get_length(&insn);
 
 	if (!insn_complete(&insn)) {
-		WARN_FUNC("can't decode instruction", sec, offset);
+		WARN("can't decode instruction at %s:0x%lx", sec->name, offset);
 		return -1;
 	}