Re: [PATCH v3] kallsyms: add support for relative offsets in kallsyms address table

[Andrew Morton]

On Thu, 21 Jan 2016 14:55:00 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> 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.

Makes sense, thanks.  I altered the changelog

: 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 so 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.
: 
: Also, the 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.