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Subject: Re: [PATCH 3/7] objtool: Add support for intra-function calls
To:     Alexandre Chartre <alexandre.chartre@oracle.com>, x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, jpoimboe@redhat.com,
        peterz@infradead.org, tglx@linutronix.de
References: <20200402082220.808-1-alexandre.chartre@oracle.com>
 <20200402082220.808-4-alexandre.chartre@oracle.com>
From:   Julien Thierry <jthierry@redhat.com>
Message-ID: <db508586-258a-0616-d649-e76e95df9611@redhat.com>
Date:   Thu, 2 Apr 2020 13:53:49 +0100
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Hi Alexandre,

I ran into the limitation of intra-function call for the arm64 support 
but didn't take the time to make a clean patch to support them properly.

Nice to see you've gone through that work :) .

On 4/2/20 9:22 AM, Alexandre Chartre wrote:
> Change objtool to support intra-function calls. An intra-function call
> is represented in objtool as a push onto the stack (of the return

I have to disagree a bit with that. The push onto the stack is true on 
x86, but other architectures might not have that (arm/arm64 have a link 
register that gets set by "bl" instructions and do not modify the stack).

> address), and a jump to the destination address. That way the stack
> information is correctly updated and the call flow is still accurate.
> 
> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> ---
>   tools/objtool/check.c | 73 ++++++++++++++++++++++++++++++++++++++++++-
>   tools/objtool/check.h |  1 +
>   2 files changed, 73 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 214809ac2776..0cec91291d46 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -657,6 +657,18 @@ static int add_call_destinations(struct objtool_file *file)
>   		if (insn->type != INSN_CALL)
>   			continue;
>   
> +		if (insn->intra_function_call) {
> +			dest_off = insn->offset + insn->len + insn->immediate;
> +			insn->jump_dest = find_insn(file, insn->sec, dest_off);
> +			if (insn->jump_dest)
> +				continue;
> +
> +			WARN_FUNC("can't find call dest at %s+0x%lx",
> +				  insn->sec, insn->offset,
> +				  insn->sec->name, dest_off);
> +			return -1;
> +		}
> +
>   		rela = find_rela_by_dest_range(insn->sec, insn->offset,
>   					       insn->len);
>   		if (!rela) {
> @@ -1289,6 +1301,49 @@ static int read_retpoline_hints(struct objtool_file *file)
>   	return 0;
>   }
>   
> +static int read_intra_function_call(struct objtool_file *file)
> +{
> +	struct section *sec;
> +	struct instruction *insn;
> +	struct rela *rela;
> +
> +	sec = find_section_by_name(file->elf,
> +				   ".rela.discard.intra_function_call");

I'm wondering, do we really need to annotate the intra_function_call and 
group the in a section?

Would it be a problem to consider all (static) call instructions with a 
destination that is not the start offset of a symbol to be an 
intra-function call (and set insn->intra_function_call and 
insn->jump_dest accordingly)?

Other than that the logic would stay the same.

> +	if (!sec)
> +		return 0;
> +
> +	list_for_each_entry(rela, &sec->rela_list, list) {
> +		if (rela->sym->type != STT_SECTION) {
> +			WARN("unexpected relocation symbol type in %s",
> +			     sec->name);
> +			return -1;
> +		}
> +
> +		insn = find_insn(file, rela->sym->sec, rela->addend);
> +		if (!insn) {
> +			WARN("bad .discard.intra_function_call entry");
> +			return -1;
> +		}
> +
> +		if (insn->type != INSN_CALL) {
> +			WARN_FUNC("intra_function_call not a call",
> +				  insn->sec, insn->offset);

Nit: This could be slightly confusing with INSN_CALL_DYNAMIC. Maybe just:
	"unsupported instruction for intra-function call " ?

> +			return -1;
> +		}
> +
> +		insn->intra_function_call = true;
> +		/*
> +		 * For the impact on the stack, make an intra-function
> +		 * call behaves like a push of an immediate value (the
> +		 * return address).
> +		 */
> +		insn->stack_op.src.type = OP_SRC_CONST;
> +		insn->stack_op.dest.type = OP_DEST_PUSH;

As commented above, this should be arch dependent.

> +	}
> +
> +	return 0;
> +}
> +
>   static void mark_rodata(struct objtool_file *file)
>   {
>   	struct section *sec;
> @@ -1344,6 +1399,10 @@ static int decode_sections(struct objtool_file *file)
>   	if (ret)
>   		return ret;
>   
> +	ret = read_intra_function_call(file);
> +	if (ret)
> +		return ret;
> +
>   	ret = add_call_destinations(file);
>   	if (ret)
>   		return ret;
> @@ -2092,7 +2151,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>   				return ret;
>   
>   			if (!no_fp && func && !is_fentry_call(insn) &&
> -			    !has_valid_stack_frame(&state)) {
> +			    !has_valid_stack_frame(&state) &&
> +			    !insn->intra_function_call) {
>   				WARN_FUNC("call without frame pointer save/setup",
>   					  sec, insn->offset);
>   				return 1;
> @@ -2101,6 +2161,17 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>   			if (dead_end_function(file, insn->call_dest))
>   				return 0;
>   
> +			if (insn->intra_function_call) {
> +				update_insn_state(insn, &state);
> +				ret = validate_branch(file, func, insn,
> +						      insn->jump_dest, state);
> +				if (ret) {
> +					if (backtrace)
> +						BT_FUNC("(intra-call)", insn);
> +					return ret;
> +				}
> +			}
> +
>   			break;
>   
>   		case INSN_JUMP_CONDITIONAL:
> diff --git a/tools/objtool/check.h b/tools/objtool/check.h
> index cffb23d81782..2bd6d2f46baa 100644
> --- a/tools/objtool/check.h
> +++ b/tools/objtool/check.h
> @@ -35,6 +35,7 @@ struct instruction {
>   	unsigned long immediate;
>   	unsigned int alt_group;
>   	bool dead_end, ignore, hint, save, restore, ignore_alts;
> +	bool intra_function_call;
>   	bool retpoline_safe;
>   	u8 visited;
>   	struct symbol *call_dest;
> 

Thanks,

-- 
Julien Thierry