linux-kernel - Re: [PATCH v9 08/26] x86/fpu/xstate: Introduce helpers to manage the XSTATE buffer dynamically

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Message-ID: <YRZDu2Rk+KdRhh1U@zn.tnic>
Date:   Fri, 13 Aug 2021 12:04:43 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Macieira, Thiago" <thiago.macieira@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 08/26] x86/fpu/xstate: Introduce helpers to manage the
 XSTATE buffer dynamically

On Fri, Aug 13, 2021 at 08:04:54AM +0000, Bae, Chang Seok wrote:
> Yes, each state offset in the non-compacted format is fixed in a machine
> regardless of RFBM. So, simply take the size like that.

Comment above it please.

Also, why is this special case needed at all?

> But it might be better to simplify this hunk for readability. I
> suspect its call sites are not that performance-critical.

That's *exactly* what I'm driving at!

> Every task’s state_mask should begin as aligned with the default buffer.
> fpu_clone() sets this for all, except init_task.
> Maybe:
>     “Make sure init_task’s state_mask aligned with its __default_state"

Why "make sure"?

There's nothing to make sure - it is simply so that initially, the FPU
buffer used is the static one, without dynamic states. Just say that
instead.

> How about enlarge_xstate_buffer() or realloc_xstate_buffer()?

realloc is fine along with a proper explanation above it why the realloc
is done/needed.

> The query is intended to check whether the xstate buffer is fully expanded or
> not -- no need to enlarge.
> 
> If the buffer is already the maximum, the code to retrieve XSTATE_BV, this
> call, etc should be skipped there.  
> 
> If the query is moved here, I guess this call site code becomes a bit ugly.

Why does it become ugly?

You simply return early without touching the buffer at all.

> No, it is still pointed by fpu->state and will be freed in the exit path.

Exit path of the task?

All I see is "return -ENOMEM" and no callers of alloc_xstate_buffer()
are calling free_xstate_buffer()...

And looking further into the patchset:

exc_device_not_available does not call free_xstate_buffer() I'm assuming

	force_sig_fault(SIGILL, ILL_ILLOPC,..

later will cause arch_release_task_struct() to happen which will call
free_xstate_buffer(). Yes, no?

I don't see any freeing in xstateregs_set() either, so what's happening
there when it returns -ENOMEM?

I guess there we remain with the old buffer, i.e., the ptrace operation
fails.

Am I close?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette