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SubjectRe: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor
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I am working on a response to this. I will send it soon.

Thanks.

Madhavan

On 7/28/20 12:31 PM, Andy Lutomirski wrote:
>> On Jul 28, 2020, at 6:11 AM, madvenka@linux.microsoft.com wrote:
>>
>> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>>
>> The kernel creates the trampoline mapping without any permissions. When
>> the trampoline is executed by user code, a page fault happens and the
>> kernel gets control. The kernel recognizes that this is a trampoline
>> invocation. It sets up the user registers based on the specified
>> register context, and/or pushes values on the user stack based on the
>> specified stack context, and sets the user PC to the requested target
>> PC. When the kernel returns, execution continues at the target PC.
>> So, the kernel does the work of the trampoline on behalf of the
>> application.
> 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
>
> You would fill a page with a bunch of these, possibly compacted to get
> more per page, and then you would remap as many copies as needed. The
> 'callq trampoline_handler' part would need to be a bit clever to make
> it continue to work despite this remapping. This will be *much*
> faster than trampfd. How much of your use case would it cover? For
> the inverse, it's not too hard to write a bit of asm to set all
> registers and jump somewhere.
>
> 2. Use existing kernel functionality. Raise a signal, modify the
> state, and return from the signal. This is very flexible and may not
> be all that much slower than trampfd.
>
> 3. Use a syscall. Instead of having the kernel handle page faults,
> have the trampoline code push the syscall nr register, load a special
> new syscall nr into the syscall nr register, and do a syscall. On
> x86_64, this would be:
>
> pushq %rax
> movq __NR_magic_trampoline, %rax
> syscall
>
> with some adjustment if the stack slot you're clobbering is important.
>
>
> Also, will using trampfd cause issues with various unwinders? I can
> easily imagine unwinders expecting code to be readable, although this
> is slowly going away for other reasons.
>
> All this being said, I think that the kernel should absolutely add a
> sensible interface for JITs to use to materialize their code. This
> would integrate sanely with LSMs and wouldn't require hacks like using
> files, etc. A cleverly designed JIT interface could function without
> seriailization IPIs, and even lame architectures like x86 could
> potentially avoid shootdown IPIs if the interface copied code instead
> of playing virtual memory games. At its very simplest, this could be:
>
> void *jit_create_code(const void *source, size_t len);
>
> and the result would be a new anonymous mapping that contains exactly
> the code requested. There could also be:
>
> int jittfd_create(...);
>
> that does something similar but creates a memfd. A nicer
> implementation for short JIT sequences would allow appending more code
> to an existing JIT region. On x86, an appendable JIT region would
> start filled with 0xCC, and I bet there's a way to materialize new
> code into a previously 0xcc-filled virtual page wthout any
> synchronization. One approach would be to start with:
>
> <some code>
> 0xcc
> 0xcc
> ...
> 0xcc
>
> and to create a whole new page like:
>
> <some code>
> <some more code>
> 0xcc
> ...
> 0xcc
>
> so that the only difference is that some code changed to some more
> code. Then replace the PTE to swap from the old page to the new page,
> and arrange to avoid freeing the old page until we're sure it's gone
> from all TLBs. This may not work if <some more code> spans a page
> boundary. The #BP fixup would zap the TLB and retry. Even just
> directly copying code over some 0xcc bytes almost works, but there's a
> nasty corner case involving instructions that fetch I$ fetch
> boundaries. I'm not sure to what extent I$ snooping helps.
>
> --Andy

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