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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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

>> 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

I suspect both approaches should work.

Any preference or other ideas ?

Thanks,

Mathieu

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