linux-kernel - Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

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Message-ID: <138A0E3D-4794-43C7-861A-4011534A09A8@intel.com>
Date:   Mon, 29 Mar 2021 18:14:48 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Jann Horn <jannh@...gle.com>
CC:     Borislav Petkov <bp@...e.de>, Andy Lutomirski <luto@...nel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "the arch/x86 maintainers" <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>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the
 first use of dynamic user state

On Mar 26, 2021, at 09:34, Jann Horn <jannh@...gle.com> wrote:
> On Sun, Feb 21, 2021 at 7:56 PM Chang S. Bae <chang.seok.bae@...el.com> wrote:
>> 
>> +       if (handle_xfirstuse_event(&current->thread.fpu))
>> +               return;
> 
> What happens if handle_xfirstuse_event() fails because vmalloc()
> failed in alloc_xstate_buffer()? I think that should probably kill the
> task with something like force_sig() - but as far as I can tell, at
> the moment, it will instead end up at die(), which should only be used
> for kernel bugs.

This question was raised on v1 before [1].

In the end, people suggested to handle the failure, e.g., with tracepoints or
stats. So, proposed this on the allocation site:

+	state_ptr = vmalloc(newsz);
+	if (!state_ptr) {
+		trace_x86_fpu_xstate_alloc_failed(fpu);
+		return -ENOMEM;
+	}

Also, I tried to justify this to Boris [2]:

  >> Maybe it is possible to backtrack this allocation failure out of #NM
  >> handling. But the tracepoint can provide a clear context, although limited
  >> to those using it.

  > Yes, add it when it is really needed. Not slapping it proactively and
  > hoping for any potential usage.

Let me know if you have a better way.

Thanks,
Chang

[1] https://lore.kernel.org/lkml/c4669d5f-11b8-3879-562c-78a791b86229@intel.com/
[2] https://lore.kernel.org/lkml/20210204131002.GA17068@zn.tnic/