[PATCH v2] modversions: treat symbol CRCs as 32 bit quantities on 64 bit archs
Michael Ellerman
mpe at ellerman.id.au
Wed Oct 26 03:07:59 PDT 2016
Hi Ard,
I like the concept, but ...
Ard Biesheuvel <ard.biesheuvel at linaro.org> writes:
> The symbol CRCs are emitted as ELF symbols, which allows us to easily
> populate the kcrctab sections by relying on the linker to associate
> each kcrctab slot with the correct value.
>
> This has two downsides:
> - given that the CRCs are treated as pointers, we waste 4 bytes for
> each CRC on 64 bit architectures,
> - on architectures that support runtime relocation, a relocation entry is
> emitted for each CRC value, which may take up 24 bytes of __init space
> (on ELF64 systems)
>
> This comes down to a x8 overhead in [uncompressed] kernel size. In addition,
> each relocation has to be reverted before the CRC value can be used.
>
> Switching to explicit 32 bit values on 64 bit architectures fixes both
> issues, since 32 bit values are not treated as relocatable quantities on
> ELF64 systems, even if the value ultimately resolves to a linker supplied
> value.
Are we sure that part is true? ("not treated as relocatable")
A quick test build on powerpc gives me:
WARNING: 6829 bad relocations
c000000000ca3748 R_PPC64_ADDR16 *ABS*+0x0000000013f53da6
c000000000ca374a R_PPC64_ADDR16 *ABS*+0x00000000f7272059
c000000000ca374c R_PPC64_ADDR16 *ABS*+0x0000000002013d36
c000000000ca374e R_PPC64_ADDR16 *ABS*+0x00000000a59dffc8
...
Which is coming from our relocs_check.sh script, which checks that the
generated relocations are ones we know how to handle.
And when I try to boot it I get:
virtio: disagrees about version of symbol module_layout
virtio: disagrees about version of symbol module_layout
scsi_mod: disagrees about version of symbol module_layout
And it can't find my root file system (unsurprisingly as it's on scsi).
Will try and investigate more tomorrow.
cheers
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