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SubjectRe: [RFC v5 12/57] objtool: check: Allow jumps from an alternative group to itself
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On 1/21/20 5:33 PM, Josh Poimboeuf wrote:
> On Thu, Jan 09, 2020 at 04:02:15PM +0000, Julien Thierry wrote:
>> Alternatives can contain instructions that jump to another instruction
>> in the same alternative group. This is actually a common pattern on
>> arm64.
>>
>> Keep track of instructions jumping within their own alternative group
>> and carry on validating such branches.
>>
>> Signed-off-by: Julien Thierry <jthierry@redhat.com>
>> ---
>> tools/objtool/check.c | 48 ++++++++++++++++++++++++++++++++++---------
>> tools/objtool/check.h | 1 +
>> 2 files changed, 39 insertions(+), 10 deletions(-)
>>
>> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
>> index 8f2ff030936d..c7b3f1e2a628 100644
>> --- a/tools/objtool/check.c
>> +++ b/tools/objtool/check.c
>> @@ -722,6 +722,14 @@ static int handle_group_alt(struct objtool_file *file,
>> sec_for_each_insn_from(file, insn) {
>> if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
>> break;
>> + /* Is insn a jump to an instruction within the alt_group */
>> + if (insn->jump_dest && insn->sec == insn->jump_dest->sec &&
>> + (insn->type == INSN_JUMP_CONDITIONAL ||
>> + insn->type == INSN_JUMP_UNCONDITIONAL)) {
>> + dest_off = insn->jump_dest->offset;
>> + insn->intra_group_jump = special_alt->orig_off <= dest_off &&
>> + dest_off < special_alt->orig_off + special_alt->orig_len;
>> + }
>
> This patch adds some complexity, just so we can keep the
>
> "don't know how to handle branch to middle of alternative instruction group"
>
> warning for the case where code from outside an alternative insn group
> is branching to inside the group. But I've never actually seen that
> case in practice, and I get the feeling that warning isn't very useful
> or realistic.
>
> How about we just remove the warning?
>

I'm happy to remove it.

I was trying to look for a less intrusive place to only check for
instructions jumping into alternatives. add_jump_destinations() could be
an option, but it would require to duplicate some stuff done in
add_special_section_alts(). So maybe just ignoring for now can be fine.

Thanks,

--
Julien Thierry

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