linux-kernel - Re: [PATCH v3 06/21] x86/fpu/xstate: Calculate and remember dynamic xstate buffer sizes

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Date:   Wed, 27 Jan 2021 01:23:35 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...e.de>
CC:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <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>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 06/21] x86/fpu/xstate: Calculate and remember dynamic
 xstate buffer sizes

On Jan 22, 2021, at 03:44, Borislav Petkov <bp@...e.de> wrote:
> On Wed, Dec 23, 2020 at 07:57:02AM -0800, Chang S. Bae wrote:
>> The xstate buffer is currently in-line with static size. To accommodatea
> 
> "in-line" doesn't fit in this context, especially since "inline"
> is a keyword with another meaning. Please replace it with a better
> formulation in this patch.

How about ‘embedded’?,
    “The xstate buffer is currently embedded into struct fpu with static size."

>> -extern unsigned int fpu_kernel_xstate_size;
>> +extern unsigned int fpu_kernel_xstate_min_size;
>> +extern unsigned int fpu_kernel_xstate_max_size;
> 
> Is it time to group this into a struct so that all those settings go
> together instead in single variables?
> 
> struct fpu_xstate {
> 	unsigned int min_size, max_size;
> 	unsigned int user_size;
> 	...
> };
> 
> etc.

<snip>

> And since we're probably going to start querying different aspects about
> the buffer, instead of exporting all kinds of variables in the future,
> maybe this should be a single exported function called
> 
> get_xstate_buffer_attr(typedef buffer_attr)
> 
> which gives you what you wanna know about it... For example:
> 
> get_xstate_buffer_attr(MIN_SIZE);
> get_xstate_buffer_attr(MAX_SIZE);
> ...

Okay. I will prepare a separate cleanup patch that can be applied at the end
of the series. Will post the change in this thread at first.

>> /* Whitelist the FPU state from the task_struct for hardened usercopy. */
>> -static inline void arch_thread_struct_whitelist(unsigned long *offset,
>> -						unsigned long *size)
>> -{
>> -	*offset = offsetof(struct thread_struct, fpu.state);
>> -	*size = fpu_kernel_xstate_size;
>> -}
>> +extern void arch_thread_struct_whitelist(unsigned long *offset, unsigned long *size);
> 
> What's that move for?

One of my drafts had some internal helper to be called in there. No reason
prior to applying the get_xstate_buffer_attr() helper. But with it, better to
move this out of this header file I think.

>> @@ -627,13 +627,18 @@ static void check_xstate_against_struct(int nr)
>>  */
> 
> <-- There's a comment over this function that might need adjustment.

Do you mean an empty line? (Just want to clarify.)

>> static void do_extra_xstate_size_checks(void)
>> {

<snip>

>> 	if (boot_cpu_has(X86_FEATURE_XSAVES))
> 
> using_compacted_format()
> 
> FPU code needs to agree on one helper and not use both. :-\

Agreed. I will prepare a patch. At least will post the diff here.

<snip>

>> +	/* Ensure we have the supported in-line space: */
> 
> Who's "we"?

How about:
    “Ensure the size fits in the statically-allocated buffer:"

>> +	if (!is_supported_xstate_size(fpu_kernel_xstate_min_size))
>> +		return -EINVAL;

No excuse, just pointing out the upstream code has “we” there [1].

Thanks,
Chang

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/xstate.c#n752