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

From: David Daney
Date: Wed Mar 01 2017 - 16:28:05 EST


On 03/01/2017 12:02 PM, Jason Baron wrote:


On 03/01/2017 11:40 AM, David Daney wrote:
On 02/28/2017 10:34 PM, Michael Ellerman wrote:
Jason Baron <jbaron@xxxxxxxxxx> 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.


OK, I guess I will send a proper patch e-mail with an updated changelog with the improvements you suggest.

Thanks,
David.