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

From: Andrew Morton
Date: Fri Jan 22 2016 - 18:34:35 EST

On Thu, 21 Jan 2016 18:19:43 +0100 Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:

> 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.

scripts/kallsyms.c: In function 'record_relative_base':
scripts/kallsyms.c:744: error: 'ULLONG_MAX' undeclared (first use in this function)
scripts/kallsyms.c:744: error: (Each undeclared identifier is reported only once
scripts/kallsyms.c:744: error: for each function it appears in.)

That's with (ancient) glibc-headers-2.5-3. It appears that limits.h's
ULLONG_MAX requires "#ifdef __USE_ISOC99". I'm not sure what's the
correct way of turning this on.