Re: [RFC PATCH] rseq: x86: implement abort-at-ip extension

----- On Jan 7, 2022, at 8:08 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

----- On Jan 7, 2022, at 5:27 PM, David Laight David.Laight@ACULAB.COM wrote:

quoted

quoted

That being said, there might be an architecture agnostic alternative available.
On abort of a RSEQ_CS_FLAG_ABORT_AT_IP critical section, we could let the kernel
decrement/increment the stack pointer to make room for a pointer (depending if
the
stack grows down or up). It would then store the abort-at-ip value at the top of
stack.

Stack redzone in a leaf function?

Good point!

For x86-64 for instance, which has a redzone of 128 bytes, there are a few
alternatives.
On abort:

Approach A) Move the stack pointer as little as possible

  1. On abort in kernel:
  1.1 Subtract 8 bytes from the stack pointer
  1.2 Store the abort-at-ip value at stack pointer - 128
  2. In abort handler (userspace)
  2.1 user-space knows that it can find the abort-at-ip value at stack pointer -
  128
  2.2 after using that value, the abort handler needs to add 8 bytes to the stack
  pointer

Approach B) Move the stack pointer to skip the entire redzone

  1. On abort in kernel:
  1.1 Subtract 128 bytes from the stack pointer
  1.2 Store the abort-at-ip value at stack pointer - 8 (basically within a new red
  zone)
  2. In abort handler (userspace)
  2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 8
  2.2 after using that value, the abort handler needs to add 128 bytes to the
  stack pointer

Or approach C)

  1. On abort in kernel:
  1.1 Subtract 136 bytes from the stack pointer
  1.2 Store the abort-at-ip value at stack pointer
  2. In abort handler (userspace)
  2.1 user-space knows that it can find the abort-at-ip value at stack pointer, so
      it can pop it from the stack
  2.2 after popping that value, the abort handler needs to add 128 bytes to the stack pointer

So far, approach C seems to be the most elegant IMHO.

Thanks,

Mathieu

I suspect both approaches should work.

Any preference or other ideas ?

Thanks,

Mathieu

quoted

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--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com