linux-kernel - RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

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Date:   Tue, 4 Aug 2020 14:44:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     David Laight <David.Laight@...LAB.COM>,
        'Mark Rutland' <mark.rutland@....com>,
        "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
CC:     Andy Lutomirski <luto@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "LSM List" <linux-security-module@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>
Subject: RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

> > > If you look at the libffi reference patch I have included, the architecture
> > > specific changes to use trampfd just involve a single C function call to
> > > a common code function.
> 
> No idea what libffi is, but it must surely be simpler to
> rewrite it to avoid nested function definitions.
> 
> Or find a book from the 1960s on how to do recursive
> calls and nested functions in FORTRAN-IV.

FWIW it is probably as simple as:
1) Put all the 'variables' the nested function accesses into a struct.
2) Add a field for the address of the 'nested' function.
3) Pass the address of the structure down instead of the
   address of the function.

If you aren't in control of the call sites then add the
structure to a linked list on a thread-local variable.

	David

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