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Subject: Re: [PATCH 2/7] objtool: Allow branches within the same alternative.
To:     Julien Thierry <jthierry@redhat.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>
 <20200402082220.808-3-alexandre.chartre@oracle.com>
 <50e8a5d8-7cb4-f25c-9657-eb11038bd0b5@redhat.com>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <0824494b-15a9-f810-e81e-003d3d3b85cd@oracle.com>
Date:   Thu, 2 Apr 2020 14:38:16 +0200
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On 4/2/20 2:03 PM, Julien Thierry wrote:
> Hi Alexandre,
> 
> I ran into the same issue for the arm64 work:
> https://lkml.org/lkml/2020/1/9/656

Thanks for the reference, I didn't notice that change, but I saw a more
recent one where you were just removing the branch to alternative check
(https://lkml.org/lkml/2020/3/25/151).

> Your solution seems nicer however.
> 
> On 4/2/20 9:22 AM, Alexandre Chartre wrote:
>> Currently objtool prevents any branch to an alternative. While preventing
>> branching from the outside to the middle of an alternative makes perfect
>> sense, branching within the same alternative should be allowed. To do so,
>> identify each alternative and check that a branch to an alternative comes
>> from the same alternative.
>>
>> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
>> ---
>> ? tools/objtool/check.c | 19 +++++++++++++------
>> ? tools/objtool/check.h |? 3 ++-
>> ? 2 files changed, 15 insertions(+), 7 deletions(-)
>>
>> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
>> index 708562fb89e1..214809ac2776 100644
>> --- a/tools/objtool/check.c
>> +++ b/tools/objtool/check.c
>> @@ -712,7 +712,9 @@ static int handle_group_alt(struct objtool_file *file,
>> ????????????????? struct instruction *orig_insn,
>> ????????????????? struct instruction **new_insn)
>> ? {
>> +??? static unsigned int alt_group_next_index = 1;
>> ????? struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = NULL;
>> +??? unsigned int alt_group = alt_group_next_index++;
>> ????? unsigned long dest_off;
>> ????? last_orig_insn = NULL;
>> @@ -721,7 +723,7 @@ static int handle_group_alt(struct objtool_file *file,
>> ????????? if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
>> ????????????? break;
>> -??????? insn->alt_group = true;
>> +??????? insn->alt_group = alt_group;
>> ????????? last_orig_insn = insn;
>> ????? }
>> @@ -1942,6 +1944,7 @@ static int validate_sibling_call(struct instruction *insn, struct insn_state *st
>> ?? * tools/objtool/Documentation/stack-validation.txt.
>> ?? */
>> ? static int validate_branch(struct objtool_file *file, struct symbol *func,
>> +?????????????? struct instruction *from,
> 
> Maybe instead of passing a new instruction pointer, just the
> alt_group could be passed? 0 Meaning it was not in an alt group> 
>> ???????????????? struct instruction *first, struct insn_state state)
>> ? {
>> ????? struct alternative *alt;
>> @@ -1953,7 +1956,9 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>> ????? insn = first;
>> ????? sec = insn->sec;
>> -??? if (insn->alt_group && list_empty(&insn->alts)) {
>> +??? if (insn->alt_group &&
>> +??????? (!from || from->alt_group != insn->alt_group) &&
>> +??????? list_empty(&insn->alts)) {
> 
> This would become
> 
>  ????if (insn->alt_group != alt_group && list_empty(&insn->alts))
> 
> And the recursive validate_branch() calls would just take
> insn->alt_group as parameter (and the calls in validate_functions()
> and validate_unwind_hints() would take 0).
> 
> Any opinions on that?

Yes, that would work too. I choose to pass the instruction pointer because
I was thinking that having the "from" instruction might be useful in the
future if there's a need to do additional check about the origin of the
branch.

alex.


>> ????????? WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
>> ??????????????? sec, insn->offset);
>> ????????? return 1;
>> @@ -2035,7 +2040,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>> ????????????????? if (alt->skip_orig)
>> ????????????????????? skip_orig = true;
>> -??????????????? ret = validate_branch(file, func, alt->insn, state);
>> +??????????????? ret = validate_branch(file, func,
>> +????????????????????????????? NULL, alt->insn, state);
>> ????????????????? if (ret) {
>> ????????????????????? if (backtrace)
>> ????????????????????????? BT_FUNC("(alt)", insn);
>> @@ -2105,7 +2111,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>> ????????????????????? return ret;
>> ????????????? } else if (insn->jump_dest) {
>> -??????????????? ret = validate_branch(file, func,
>> +??????????????? ret = validate_branch(file, func, insn,
>> ??????????????????????????????? insn->jump_dest, state);
>> ????????????????? if (ret) {
>> ????????????????????? if (backtrace)
>> @@ -2236,7 +2242,8 @@ static int validate_unwind_hints(struct objtool_file *file)
>> ????? for_each_insn(file, insn) {
>> ????????? if (insn->hint && !insn->visited) {
>> -??????????? ret = validate_branch(file, insn->func, insn, state);
>> +??????????? ret = validate_branch(file, insn->func,
>> +????????????????????????? NULL, insn, state);
>> ????????????? if (ret && backtrace)
>> ????????????????? BT_FUNC("<=== (hint)", insn);
>> ????????????? warnings += ret;
>> @@ -2377,7 +2384,7 @@ static int validate_functions(struct objtool_file *file)
>> ????????????? state.uaccess = func->uaccess_safe;
>> -??????????? ret = validate_branch(file, func, insn, state);
>> +??????????? ret = validate_branch(file, func, NULL, insn, state);
>> ????????????? if (ret && backtrace)
>> ????????????????? BT_FUNC("<=== (func)", insn);
>> ????????????? warnings += ret;
>> diff --git a/tools/objtool/check.h b/tools/objtool/check.h
>> index 6d875ca6fce0..cffb23d81782 100644
>> --- a/tools/objtool/check.h
>> +++ b/tools/objtool/check.h
>> @@ -33,7 +33,8 @@ struct instruction {
>> ????? unsigned int len;
>> ????? enum insn_type type;
>> ????? unsigned long immediate;
>> -??? bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
>> +??? unsigned int alt_group;
>> +??? bool dead_end, ignore, hint, save, restore, ignore_alts;
>> ????? bool retpoline_safe;
>> ????? u8 visited;
>> ????? struct symbol *call_dest;
>>
> 
> Cheers,
>