linux-kernel - Re: FatELF patches...

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Date:	Mon, 2 Nov 2009 10:14:15 -0500 (EST)
From:	"Ryan C. Gordon" <icculus@...ulus.org>
To:	Valdis.Kletnieks@...edu
cc:	Måns Rullgård <mans@...sr.com>,
	linux-kernel@...r.kernel.org
Subject: Re: FatELF patches...

> > think if Ubuntu did this as a distribution-wide policy, then people would 
> > probably choose a different distribution.
> 
> Hmm.. so let's see - people compiling stuff for themselves won't use this
> feature.  And if a distro uses it, users would probably go to a different
> distro.

I probably wasn't clear when I said "distribution-wide policy" followed by 
a "then again." I meant there would be backlash if the distribution glued 
the whole system together, instead of just binaries that made sense to do 
it to.

And, again, there's a third use-case besides compiling your programs and 
getting them from the package manager, and FatELF is meant to address 
that.

> Actually, they can't nuke the /lib{32,64} directories unless *all* binaries
> are using FatELF - as long as there's any binaries doing things The Old Way,
> you need to keep the supporting binaries around.

Binaries don't refer directly to /libXX, they count on ld.so to tapdance 
on their behalf. My virtual machine example left the dirs there as 
symlinks to /lib, but they could probably just go away directly.

> Don't forget you take that hit once for each shared library involved.  Plus

That happens in user space in ld.so, so it's not a kernel problem in any 
case, but still...we're talking about, what? Twenty more branch 
instructions per-process?

> I'm not sure if there's hidden gotchas lurking in there (is there code that
> assumes that if executable code is mmap'ed, it's only done so in one arch?

The current code sets up file mappings based on the offset of the desired 
ELF binary.

> Or will a FatELF glibc.so screw up somebody's refcounts if it's mapped
> in both 32 and 64 bit modes?

Whose refcounts would this screw up? If there's a possible bug, I'd like 
to make sure it gets resolved, of course.

--ryan.

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