Date:   Thu, 2 Apr 2020 14:38:41 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.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 2:24 PM, Alexandre Chartre wrote:
> 
> 
> 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.
> 

That makes sense. Maybe we can just allow them in !file->c_file, I don't 
think gcc generates such call on arm64, so I think we'd only have that 
in assembly.

If people prefer to keep the annotation, would you mind having a 
"ANNOTATE_INTRA_FUNCTION_CALL" macro in include/linux/frame.h to add the 
label and the reference to the right section?
This way it could be reused for other archs.

> 
>> 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).
> 

Thank you!

-- 
Julien Thierry

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