On 22 January 2016 at 04:44, Michael Ellerman [off-list ref] wrote:

On Thu, 2016-01-21 at 14:55 -0800, Kees Cook wrote:

quoted

On Thu, Jan 21, 2016 at 2:50 PM, Andrew Morton
[off-list ref] wrote:

quoted

On Thu, 21 Jan 2016 18:19:43 +0100 Ard Biesheuvel [off-list ref] wrote:

quoted

Similar to how relative extables are implemented, it is possible to emit
the kallsyms table in such a way that it contains offsets relative to some
anchor point in the kernel image rather than absolute addresses. The benefit
is that such table entries are no longer subject to dynamic relocation when
the build time and runtime offsets of the kernel image are different. Also,
on 64-bit architectures, it essentially cuts the size of the address table
in half since offsets can typically be expressed in 32 bits.

Since it is useful for some architectures (like x86) to retain the ability
to emit absolute values as well, this patch adds support for both, by
emitting absolute addresses as positive 32-bit values, and addresses
relative to the lowest encountered relative symbol as negative values, which
are subtracted from the runtime address of this base symbol to produce the
actual address.

Support for the above is enabled by default for all architectures except
IA-64, whose symbols are too far apart to capture in this manner.

I'm not really understanding the benefits of this.  A smaller address
table is nice, but why is it desirable that "such table entries are no
longer subject to dynamic relocation when the build time and runtime
offsets of the kernel image are different"?

IIUC, this means that the relocation work done after decompression now
doesn't have to do relocation updates for all these values, which
means a smaller relocation table as well.

Yep. If I remember the figures rightly it saves ~250K of relocations for the
powerpc build.

For ppc64_defconfig (which has CONFIG_RELOCATABLE=y, i.e., it has a
dynamic relocation section containing a 24-byte RELA entry per
relocated quantity), I got the following numbers

101740 kallsyms entries
397 KB saved in permanent .rodata
2.4 MB saved in __init rela.dyn section
~500 KB saved in compressed image

For arm64, we don't have a compressed image, which is the reason I
need this for my arm64 implementation of CONFIG_RELOCATABLE (for
KASLR), since the RELA overhead goes straight into the distributed
image.

Thanks,
Ard.