lkml.org 
[lkml]   [2022]   [Jan]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH] rseq: x86: implement abort-at-ip extension
----- 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

>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT,
> UK
> Registration No: 1397386 (Wales)

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

\
 
 \ /
  Last update: 2022-01-08 02:09    [W:0.057 / U:0.568 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site