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

From: Dave Hansen
Date: Thu Nov 17 2016 - 16:32:06 EST

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
> 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I think fpu__clear()'s comments are a bit out of date.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' */
/* re-activate fpregs: */
/* take new init fpstate and place in fpregs: */