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Message-ID: <4e779423-395d-5e2e-b641-5604902bf096@oracle.com>
Date: Thu, 2 Apr 2020 15:24:45 +0200
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: Julien Thierry <jthierry@...hat.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
peterz@...radead.org, tglx@...utronix.de
Subject: Re: [PATCH 3/7] objtool: Add support for intra-function calls
On 4/2/20 2:53 PM, Julien Thierry wrote:
> 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).
Correct, this is x86 specific.
>
>> 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@...cle.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)?
Correct, we could automatically detect intra-function calls instead of
having to annotate them. However, I choose to annotate them because I don't
think that's not an expected construct in a "normal" code flow (at least
on x86). So objtool would still issue a warning on intra-function calls
by default, and you can annotate them to indicate if they are expected.
If intra-function calls are frequent on arm then I can add an option to
objtool so it automatically detects them. This way, we won't use the option
on x86 and we have to annotate intra-function call on x86, and you can
use it on arm to automatically detect intra-function calls.
> 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 " ?
Right, I will change that: "intra_function_call not a direct 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.
I will add a arch dependent call. I will also do that for the return
trampoline call case (patch 4).
Thanks,
alex.
>> + }
>> +
>> + 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,
>
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