linux-kernel - Re: [PATCH] Add non-zero module sections to sysfs

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Message-ID: <1365245524.4298.14.camel@dabdike.spectrum.wifi>
Date:	Sat, 06 Apr 2013 11:52:04 +0100
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Sebastian Wankerl <sisewank@....cs.fau.de>,
	linux-kernel@...r.kernel.org,
	Philip Kranz <philip.kranz@...glemail.com>,
	i4passt@...ts.informatik.uni-erlangen.de,
	linux-parisc@...r.kernel.org, corbet@....net
Subject: Re: [PATCH] Add non-zero module sections to sysfs

On Sat, 2013-04-06 at 15:22 +1030, Rusty Russell wrote:
> > The problem is our assumption that section names be unique.  This
> > assumption is wrong.  The ELF spec says (version 1.1 page 1-15): "An
> > object file may have more than one section with the same name."  We need
> > to fix the kernel not to rely on a bogus assumption ... but we had no
> > idea how to do that in a way that preserved the backwards compatibility
> > of sections subdirectory.
> >
> > I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, but now
> > the problem has got attention, can we fix it properly?
> 
> Yep.  The original patch didn't go through me, or we would have had this
> discussion back then...
>
> The use of section names in sysfs goes back to one Mr. Corbet.  Why did
> he do it that way?  Because gdb's add-symbol-file makes the same
> assumption.  So if we fixed the sysfs somehow, it still wouldn't be
> useful, since there's no way to tell gdb :(
> 
> The real answer don't use -ffunction-sections on modules: probably not
> as important as the rest of the kernel.  And the new shiny is
> -flto anyway.
> 
> And that leaves us with a PA-RISC specific issue, for which we should
> move the fix to PA-RISC.
> 
> Thoughts?

Well, we don't have much of a choice.  Our ELF stub jump on 32 bits is a
PCREL17.  That means once a module size is over 128k there's a chance we
might not be able to link it because the jump is too big for the
instruction.  IPV6 is one such big module today, but I'm sure there are
others.  The only way I know to fix this is to allow the linker to
insert stubs between functions, so we only fail at linking if a single
function is >128k big.  The way to do this is -ffunction-sections,
unless there's something else we could do (all we really need is a way
to ensure we can insert ELF stubs every 128k).

We're not the only architecture that has these problems: frv, metag and
score seem to as well.

James


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