Date: Sun, 2 Aug 2020 13:56:01 +0200 From: Pavel Machek <pavel@....cz> To: David Laight <David.Laight@...LAB.COM> Cc: 'Andy Lutomirski' <luto@...nel.org>, "madvenka@...ux.microsoft.com" <madvenka@...ux.microsoft.com>, 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 Hi! > > This is quite clever, but now I???m wondering just how much kernel help > > is really needed. In your series, the trampoline is an non-executable > > page. I can think of at least two alternative approaches, and I'd > > like to know the pros and cons. > > > > 1. Entirely userspace: a return trampoline would be something like: > > > > 1: > > pushq %rax > > pushq %rbc > > pushq %rcx > > ... > > pushq %r15 > > movq %rsp, %rdi # pointer to saved regs > > leaq 1b(%rip), %rsi # pointer to the trampoline itself > > callq trampoline_handler # see below > > For nested calls (where the trampoline needs to pass the > original stack frame to the nested function) I think you > just need a page full of: > mov $0, scratch_reg; jmp trampoline_handler I believe you could do with mov %pc, scratch_reg; jmp ... That has advantage of being able to share single physical page across multiple virtual pages... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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