Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: ftz2/Qu5TR2KAg283CYHg3JsP8d8+CtLBopZZ0iNM9HoLH/3cIYiADcCCSE2tLLCqQb5Y/w3gA
 YE0J2ZZoAfdA==
IronPort-SDR: T7ATyXWAxRPnY4qNGqeC4MFdc2IqrvsjcuyzEPDmiPDg5SVw03FImDPwl1QuEDzbfhFeudpaVJ
 hxDKuzaknNaw==
From:   "Bae, Chang Seok" <chang.seok.bae@intel.com>
To:     Jann Horn <jannh@google.com>
CC:     Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@kernel.org>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "Brown, Len" <len.brown@intel.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the
 first use of dynamic user state
Thread-Topic: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the
 first use of dynamic user state
Thread-Index: AQHXCIQDYHRUMzQ+BU6HvOsZgq/N2KqWqq4AgATTFgA=
Date:   Mon, 29 Mar 2021 18:14:48 +0000
Message-ID: <138A0E3D-4794-43C7-861A-4011534A09A8@intel.com>
References: <20210221185637.19281-1-chang.seok.bae@intel.com>
 <20210221185637.19281-15-chang.seok.bae@intel.com>
 <CAG48ez3hLjwirUoaanDHJ_2h73YruVxfJL88AFVoym7sy02a5w@mail.gmail.com>
In-Reply-To: <CAG48ez3hLjwirUoaanDHJ_2h73YruVxfJL88AFVoym7sy02a5w@mail.gmail.com>
Accept-Language: en-US
Content-Language: en-US
Content-Type: text/plain; charset="us-ascii"
Content-ID: <9534083BFD53A74A96B1FC19D61B170F@namprd11.prod.outlook.com>
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-Network-Message-Id: 2a2c0f68-271b-4c3d-6a88-08d8f2de89ce
X-MS-Exchange-CrossTenant-originalarrivaltime: 29 Mar 2021 18:14:48.0258
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-mailboxtype: HOSTED
X-MS-Exchange-CrossTenant-userprincipalname: tuSLFs/JqIwGDQlMYN+mbTnbhNX0FyxC2mIKP8ta96UpItP+BEe0mDIS+ImiX0KN5FYz/sAVZJnAoa5sb7NG/AJRu1LVOmgVvLp6o4OC6/U=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH0PR11MB5079
X-OriginatorOrg: intel.com
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mar 26, 2021, at 09:34, Jann Horn <jannh@google.com> wrote:
> On Sun, Feb 21, 2021 at 7:56 PM Chang S. Bae <chang.seok.bae@intel.com> w=
rote:
>>=20
>> +       if (handle_xfirstuse_event(&current->thread.fpu))
>> +               return;
>=20
> 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 =3D 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 lim=
ited
  >> 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/