linux-kernel - Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

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Date:   Wed, 23 Sep 2020 14:49:30 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Solar Designer <solar@...nwall.com>
Cc:     Florian Weimer <fweimer@...hat.com>, Pavel Machek <pavel@....cz>,
        madvenka@...ux.microsoft.com, kernel-hardening@...ts.openwall.com,
        linux-api@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, oleg@...hat.com,
        x86@...nel.org, luto@...nel.org, David.Laight@...LAB.COM,
        mark.rutland@....com, mic@...ikod.net,
        Rich Felker <dalias@...c.org>
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

On Wed, Sep 23, 2020 at 08:11:36PM +0200, Solar Designer wrote:
> On Wed, Sep 23, 2020 at 04:39:31PM +0200, Florian Weimer wrote:
> > * Solar Designer:
> > 
> > > While I share my opinion here, I don't mean that to block Madhavan's
> > > work.  I'd rather defer to people more knowledgeable in current userland
> > > and ABI issues/limitations and plans on dealing with those, especially
> > > to Florian Weimer.  I haven't seen Florian say anything specific for or
> > > against Madhavan's proposal, and I'd like to.  (Have I missed that?)
> 
> [...]
> > I think it's unnecessary for the libffi use case.
> [...]
> 
> > I don't know if kernel support could
> > make sense in this context, but it would be a completely different
> > patch.
> 
> Thanks.  Are there currently relevant use cases where the proposed
> trampfd would be useful and likely actually made use of by userland -
> e.g., specific userland project developers saying they'd use it, or
> Madhavan intending to develop and contribute userland patches?
> 
> Alexander

The trampoline it provides in this version can be implemented completely
in userspace. The kernel part of it is essentially just providing a way
to do text relocations without needing a WX mapping, but the text
relocations would be unnecessary in the first place if the trampoline
was position-independent code.