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Subject: Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative groups
To:     Miroslav Benes <mbenes@suse.cz>
Cc:     linux-kernel@vger.kernel.org, jpoimboe@redhat.com,
        peterz@infradead.org, raphael.gault@arm.com
References: <20200327152847.15294-1-jthierry@redhat.com>
 <20200327152847.15294-5-jthierry@redhat.com>
 <alpine.LSU.2.21.2004011450100.15809@pobox.suse.cz>
From:   Julien Thierry <jthierry@redhat.com>
Message-ID: <1f53ee68-3bcc-7de9-beb9-df812b2e3613@redhat.com>
Date:   Wed, 1 Apr 2020 14:43:22 +0100
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On 4/1/20 1:53 PM, Miroslav Benes wrote:
> On Fri, 27 Mar 2020, Julien Thierry wrote:
> 
>> Atlernative section can contain entries for alternatives with no
>> instructions. Objtool will currently crash when handling such an entry.
>>
>> Just skip that entry, but still give a warning to discourage useless
>> entries.
>>
>> Signed-off-by: Julien Thierry <jthierry@redhat.com>
>> ---
>>   tools/objtool/check.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
>> index 74353b2c39ce..5c03460f1f07 100644
>> --- a/tools/objtool/check.c
>> +++ b/tools/objtool/check.c
>> @@ -904,6 +904,12 @@ static int add_special_section_alts(struct objtool_file *file)
>>   		}
>>   
>>   		if (special_alt->group) {
>> +			if (!special_alt->orig_len) {
>> +				WARN_FUNC("empty alternative entry",
>> +					  orig_insn->sec, orig_insn->offset);
>> +				continue;
>> +			}
>> +
>>   			ret = handle_group_alt(file, special_alt, orig_insn,
>>   					       &new_insn);
>>   			if (ret)
> 
> Probably the first time I am looking at alternatives handling in objtool,
> so I must be missing something, but is this even possible now? I mean
> get_alt_entry() in special.c sets alt->orig_len when alt->group is true
> (which means .alternatives section) to something which cannot be zero.
> 

What I see is:

	if (alt->group) {
		alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
						   entry->orig_len);
		alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
                                                   entry->new_len);
	}


And as far as I can tell, "alt->orig_len" can be 0 if the entry in the 
.altinstructions section of the .o file has the length set to 0.

I don't know how the alternative section generation works on x86, but on 
arm64 it's just a computed assembly offset which can be 0.

> Is this a preparatory patch for arm64, where this could happen? If yes, it
> would be better to mention it in the changelog.
> 

It used to happen on arm64, but the fix [1] was picked.

I can add that link to the commit if necessary.

[1] https://lkml.org/lkml/2020/1/9/708

Cheers,

-- 
Julien Thierry