Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

[Mathieu Desnoyers]

On 5/30/23 10:25, Mathieu Desnoyers wrote:
> On 5/30/23 04:20, Florian Weimer wrote:
[...]
> > The challenge is that once we put stuff at fixed offsets, we can't
> > transparently fix it later.  It would need more auxv entries with
> > further offsets, or accessing this data through some indirection,
> > perhaps via vDSO helpers.
>
> Perhaps this is more flexibility/complexity than we really need. One 
> possible approach would be to split struct rseq into sub-structures, e.g.:
>
> rseq_len = overall size of all sub-structures.
> auxv AT_RSEQ_ALIGN = 256
>
> auxv AT_RSEQ_FEATURE_SIZE = size of first portion of struct rseq,
>                              at most 256 bytes, meant to contain fields
>                              stored/loaded from the thread doing the
>                              registration.
> auxv AT_RSEQ_SHARED_FEATURE_SIZE =
>                              size of 2nd portion of struct rseq,
>                              starts at offset 256, at most 256 bytes,
>                              meant to contain fields stored/loaded by
>                              any thread.
>
> Then we have this layout:
>
> struct rseq {
>    struct rseq_local {
>      /* Fields accessed from local thread. */
>
>    } __attribute__((aligned((256));
>    struct rseq_shared {
>      /* Shared fields. */
>
>    } __attribute__((aligned(256));
> } __attribute__((aligned(256));
>
> And if someday AT_RSEQ_FEATURE_SIZE needs to grow over 256 bytes
> (32 * u64), we can still extend with a new auxv entry after the "shared"
> features.

Actually, after giving it some more thoughts, I think we can do better:

- Add a sys_rseq() rseq_flag RSEQ_FLAG_SHARED, which changes the 
behavior of sys_rseq() to expect an additional "struct rseq_shared *" 
argument.

- Introduce auxv AT_RSEQ_SHARED_FEATURE_SIZE.

This way, it's up to the libc to decide how to allocate its private vs 
shared rseq structures.

The auxv "AT_RSEQ_ALIGN" would dictate the minimal alignment required 
for both private and shared rseq structures.

I don't think we need to express the size of the rseq_shared memory area 
allocated by libc because we know that it needs to be large enough to 
handle the shared feature size.

Thoughts ?

Thanks,

Mathieu

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