linux-kernel - Re: [PATCH] x86/fpu: Fix invalid FPU ptrace state after execve

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Date:   Thu, 17 Nov 2016 13:31:57 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH] x86/fpu: Fix invalid FPU ptrace state after execve

On 11/16/2016 08:56 AM, Yu-cheng Yu wrote:
> Robert O'Callahan reported that after an execve PTRACE_GETREGSET
> NT_X86_XSTATE continues to return the pre-exec register values
> until the exec'ed task modifies FPU state.  The test code is at
> https://bugzilla.redhat.com/attachment.cgi?id=1164286.
> 
> What is happening is when eagerfpu is enabled, fpu__clear() did
> not properly clear fpstate.  Fix it by doing just that.

Functionally, I think the patch is fine.  just a few
comment/documentation nits.

I think fpu__clear()'s comments are a bit out of date.  Could we make it
clear that it is invalidating both fpregs *and* fpstate?

I also think the

	/* FPU state will be reallocated lazily at the first use. */"

comment was fairly valuable.  Could we find some way to keep it?

The new comment:

> +	/*
> +	 * When eagerfpu is used, make sure fpstate is cleared and initialized.
> +	 */

also kinda implies that the if() block is only messing with fpstate.
Could we make that more clear?  Maybe by commenting the individual lines
inside the if():

> +	if (use_eager_fpu()) {
> +		fpu__activate_curr(fpu);
> +		user_fpu_begin();

instead of having it above?  Maybe something like:

	if (use_eager_fpu()) {
		/* activate and load init fpstate into 'fpu' */
		fpu__activate_curr(fpu);
		/* re-activate fpregs: */
		user_fpu_begin();
		/* take new init fpstate and place in fpregs: */
 		copy_init_fpstate_to_fpregs();
 	}