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

[David Daney]

On 02/28/2017 11:05 AM, David Daney wrote:
> On 02/28/2017 10:39 AM, Jason Baron wrote:
> >
[...]
> > > I suspect that the alignment of the __jump_table section in the .ko
> > > files is not correct, and you are seeing some sort of problem due to
> > > that.
> >
> > Hi,
> >
> > Yes, if you look at the trace that Sachin sent the module being loaded
> > that does the WARN_ON() is nfsd.ko.
> >
> > That module from Sachin's trace has:
> >
> >   [31] __jump_table      PROGBITS        0000000000000000 03fd77 0000c0
> > 18 WAM  0   0  1
>
> The problem is then the section alignment (last column) for power.
>
> On mips with no patches applied, we get:
>
>   [17] __jump_table      PROGBITS        0000000000000000 00d2c0 000048
> 00  WA  0   0  8
>
> Look, proper alignment!
>
> The question I have is why do the power ".llong" and ".long" assembler
> directives not force section alignment?  Is there an alternative that
> could be used that would result in the proper alignment?  Would ".word"
> work?
>
> If not, then I would say patch only power with your balign thing. 8-byte
> alignment for 64-bit kernel, 4-byte alignment for 32-bit kernel

I think the proper fix is either:

A) Modify scripts/module-common.lds to force __jump_table alignment for 
all architectures.

B) Add arch/powerpc/kernel/module.lds to force __jump_table alignment 
for powerpc only.

David.



> > So its not the size but rather the start offset '03fd77', that is the
> > problem here. That is what the WARN_ON triggers on, that the start of
> > the table is not 4-byte aligned.
> >
> > Using a ppc cross-compiler and the ENTSIZE patch that line does not
> > change, however if I use the initial patch posted in this thread, the
> > start does align to 4-bytes and thus the warning goes away, as Sachin
> > verified. In fact, without the patch I found several modules that don't
> > start at the proper alignment, however with the patch that started this
> > thread they were all properly aligned.
> >
> > In terms of the '.balign' causing holes, we originally added the
> > '_ASM_ALIGN' to x86 for precisely this reason. See commit:
> > ef64789 jump label: Add _ASM_ALIGN for x86 and x86_64 and discussion.
> >
> > In addition, we have a lot of runtime with the .balign in the tree and
> > I'm not aware of any holes in the table. I think the code would blow up
> > pretty badly if there were.
> >
> > A number of arches were already using the '.balign', and the patch I
> > proposed simply added it to remaining ones, now that we added a
> > WARN_ON() to catch this condition.
> >
> > Thanks,
> >
> > -Jason
>
>
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