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

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Date:   Mon, 21 Nov 2016 10:40:51 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     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>,
        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


* Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> 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 I think fpu__clear()'s comments are a bit out of date.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();
>  	}

I agree with these suggestions - but I'll apply the simple patch to x86/urgent - 
which can then be backported as far as necessary, and then resolve the conflict 
with the v4.10 tip:x86/fpu branch, and on top of that we can fix these details, 
ok?

In particular I don't like it how non-obvious the semantics are from the function 
names. I think we should try to improve the nomenclature instead of adding 
comments to every line.

Thanks,

	Ingo