Re: [PATCH 3/7] objtool: Add support for intra-function calls

[Alexandre Chartre <alexandre.chartre@oracle.com>]

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@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)?

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,
>