Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes

[Jason Baron <jbaron@akamai.com>]

On 03/01/2017 11:40 AM, David Daney wrote:
> On 02/28/2017 10:34 PM, Michael Ellerman wrote:
>> Jason Baron <jbaron@akamai.com> writes:
>> ...
>>> I also checked all the other .ko files and they were properly aligned.
>>> So I think this should hopefully work, and I like that its not a
>>> per-arch fix.
>>>
>>> Sachin, sorry to bother you again, but I'm hoping you can try David's
>>> latest patch to scripts/module-common.lds, just to test in your setup.
>>
>> It does fix the problem.
>>
>> I was reproducing with crc_t10dif:
>>
>> [  695.890552] ------------[ cut here ]------------
>> [  695.890709] WARNING: CPU: 15 PID: 3019 at
>> ../kernel/jump_label.c:287 static_key_set_entries+0x74/0xa0
>> [  695.890710] Modules linked in: crc_t10dif(+) crct10dif_generic
>> crct10dif_common ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat
>> nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 xt_addrtype
>> iptable_filter ip_tables xt_conntrack x_tables nf_nat nf_conntrack
>> bridge stp llc dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio
>> libcrc32c kvm virtio_balloon binfmt_misc autofs4 virtio_net virtio_pci
>> virtio_ring virtio
>>
>> Which had:
>>
>>   [21] __jump_table      PROGBITS        0000000000000000 0004e8
>> 000018 00  WA  0   0  1
>>
>>
>> And now has:
>>
>>   [18] __jump_table      PROGBITS        0000000000000000 0004d0
>> 000018 00  WA  0   0  8
>>
>> And all other modules have an alignment of 8 on __jump_table, as
>> expected.
>>
>> I'm inclined to merge a version of the balign patch for powerpc anyway,
>> just to be on the safe side. I guess the old code was coping fine with
>> the unaligned keys, but it still makes me nervous.
> 
> 
> The original "balign patch" has a couple of problems:
> 
> 1) 4-byte alignment is not sufficient for 64-bit kernels
> 
> 2) It is redundant if the linker script patch is accepted.
> 
> 

The linker script patch seems reasonable to me.

Maybe its worth adding a comment that the alignment is necessary because
the core jump_label makes use of the 2 lsb bits of its __jump_table
pointer due to commit:

3821fd3 jump_label: Reduce the size of struct static_key

Also, in the comment it says that it fixes an oops. We hit a WARN_ON()
not an oops, although bad things are likely to happen when the branch is
updated.

Thanks,

-Jason