Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

On Thu, Jul 30, 2020 at 7:24 AM Madhavan T. Venkataraman
[off-list ref] wrote:

Sorry for the delay. I just wanted to think about this a little.
In this email, I will respond to your first suggestion. I will
respond to the rest in separate emails if that is alright with
you.

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" [off-list ref]

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.

Let me state what I have understood about this suggestion. Correct me if
I get anything wrong. If you don't mind, I will also take the liberty
of generalizing and paraphrasing your suggestion.

The goal is to create two page mappings that are adjacent to each other:

- a code page that contains template code for a trampoline. Since the
 template code would tend to be small in size, pack as many of them
 as possible within a page to conserve memory. In other words, create
 an array of the template code fragments. Each element in the array
 would be used for one trampoline instance.

- a data page that contains an array of data elements. Corresponding
 to each code element in the code page, there would be a data element
 in the data page that would contain data that is specific to a
 trampoline instance.

- Code will access data using PC-relative addressing.

The management of the code pages and allocation for each trampoline
instance would all be done in user space.

Is this the general idea?

Yes.

Creating a code page
--------------------

We can do this in one of the following ways:

- Allocate a writable page at run time, write the template code into
  the page and have execute permissions on the page.

- Allocate a writable page at run time, write the template code into
  the page and remap the page with just execute permissions.

- Allocate a writable page at run time, write the template code into
  the page, write the page into a temporary file and map the file with
  execute permissions.

- Include the template code in a code page at build time itself and
  just remap the code page each time you need a code page.

This latter part shouldn't need any special permissions as far as I know.

Pros and Cons
-------------

As long as the OS provides the functionality to do this and the security
subsystem in the OS allows the actions, this is totally feasible. If not,
we need something like trampfd.

As Floren mentioned, libffi does implement something like this for MACH.

In fact, in my libffi changes, I use trampfd only after all the other methods
have failed because of security settings.

But the above approach only solves the problem for this simple type of
trampoline. It does not provide a framework for addressing more complex types
or even other forms of dynamic code.

Also, each application would need to implement this solution for itself
as opposed to relying on one implementation provided by the kernel.

I would argue this is a benefit.  If the whole implementation is in
userspace, there is no ABI compatibility issue.  The user program
contains the trampoline code and the code that uses it.

Trampfd-based solution
----------------------

I outlined an enhancement to trampfd in a response to David Laight. In this
enhancement, the kernel is the one that would set up the code page.

The kernel would call an arch-specific support function to generate the
code required to load registers, push values on the stack and jump to a PC
for a trampoline instance based on its current context. The trampoline
instance data could be baked into the code.

My initial idea was to only have one trampoline instance per page. But I
think I can implement multiple instances per page. I just have to manage
the trampfd file private data and VMA private data accordingly to map an
element in a code page to its trampoline object.

The two approaches are similar except for the detail about who sets up
and manages the trampoline pages. In both approaches, the performance problem
is addressed. But trampfd can be used even when security settings are
restrictive.

Is my solution acceptable?

Perhaps.  In general, before adding a new ABI to the kernel, it's nice
to understand how it's better than doing the same thing in userspace.
Saying that it's easier for user code to work with if it's in the
kernel isn't necessarily an adequate justification.

Why would remapping two pages of actual application text ever fail?