Re: [RFC PATCH 16/21] objtool: Add support for CONFIG_CFI_CLANG

Peter Zijlstra
2022-04-29T23:31:51Z

On Fri, Apr 29, 2022 at 01:36:39PM -0700, Sami Tolvanen wrote:

> +static void *kcfi_alloc_hash(unsigned long size)
> +{
> + kcfi_bits = max(10, ilog2(size));
> + kcfi_hash = mmap(NULL, sizeof(struct hlist_head) << kcfi_bits,
> + PROT_READ|PROT_WRITE,
> + MAP_PRIVATE|MAP_ANON, -1, 0);
> + if (kcfi_hash == (void *)-1L) {
> + WARN("mmap fail kcfi_hash");
> + kcfi_hash = NULL;
> + } else if (stats) {
> + printf("kcfi_bits: %d\n", kcfi_bits);
> + }
> +
> + return kcfi_hash;
> +}
> +
> +static void add_kcfi_type(struct kcfi_type *type)
> +{
> + hlist_add_head(&type->hash,
> + &kcfi_hash[hash_min(
> + sec_offset_hash(type->sec, type->offset),
> + kcfi_bits)]);

:se cino=(0:0

Also, I'm thinking you can unwrap some lines at least.

> +}
> +
> +static bool is_kcfi_typeid(struct elf *elf, struct instruction *insn)
> +{
> + struct hlist_head *head;
> + struct kcfi_type *type;
> + struct reloc *reloc;
> +
> + if (!kcfi)
> + return false;
> +
> + /* Compiler-generated annotation in .kcfi_types. */
> + head = &kcfi_hash[hash_min(sec_offset_hash(insn->sec, insn->offset), kcfi_bits)];
> +
> + hlist_for_each_entry(type, head, hash)
> + if (type->sec == insn->sec && type->offset == insn->offset)
> + return true;

missing { }

> +
> + /* Manual annotation (in assembly code). */
> + reloc = find_reloc_by_dest(elf, insn->sec, insn->offset);
> +
> + if (reloc && !strncmp(reloc->sym->name, "__kcfi_typeid_", 14))
> + return true;
> +
> + return false;
> +}
> +
> /*
> * This checks to see if the given function is a "noreturn" function.
> *
> @@ -388,13 +487,18 @@ static int decode_instructions(struct objtool_file *file)
> insn->sec = sec;
> insn->offset = offset;
>
> - ret = arch_decode_instruction(file, sec, offset,
> - sec->sh.sh_size - offset,
> - &insn->len, &insn->type,
> - &insn->immediate,
> - &insn->stack_ops);
> - if (ret)
> - goto err;
> + if (is_kcfi_typeid(file->elf, insn)) {
> + insn->type = INSN_KCFI_TYPEID;
> + insn->len = KCFI_TYPEID_LEN;

Urgh, what does this do for decode speed? This is a hash-lookup for
every single instruction.

Is that kcfi location array sorted by the compiler? Because then you can
keep a running iterator and replace the whole lookup with a simple
equality comparison.

> + } else {
> + ret = arch_decode_instruction(file, sec, offset,
> + sec->sh.sh_size - offset,
> + &insn->len, &insn->type,
> + &insn->immediate,
> + &insn->stack_ops);
> + if (ret)
> + goto err;
> + }
>
> /*
> * By default, "ud2" is a dead end unless otherwise