From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 40621 invoked by alias); 11 Oct 2019 03:48:34 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Received: (qmail 39472 invoked by uid 89); 11 Oct 2019 03:48:33 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-25.3 required=5.0 tests=AWL,BAYES_00,GIT_PATCH_0,GIT_PATCH_1,GIT_PATCH_2,GIT_PATCH_3,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.1 spammy=HX-Languages-Length:2851 X-HELO: mx2a.mailbox.org Date: Fri, 11 Oct 2019 03:48:00 -0000 From: Aleksa Sarai To: Michael Ellerman Cc: mingo@redhat.com, peterz@infradead.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, christian@brauner.io, keescook@chromium.org, linux@rasmusvillemoes.dk, viro@zeniv.linux.org.uk, torvalds@linux-foundation.org, libc-alpha@sourceware.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] usercopy: Avoid soft lockups in test_check_nonzero_user() Message-ID: <20191011034810.xkmz3e4l5ezxvq57@yavin.dot.cyphar.com> References: <20191010114007.o3bygjf4jlfk242e@yavin.dot.cyphar.com> <20191011022447.24249-1-mpe@ellerman.id.au> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="xu7xtsni3al6cjf6" Content-Disposition: inline In-Reply-To: <20191011022447.24249-1-mpe@ellerman.id.au> X-SW-Source: 2019-10/txt/msg00329.txt.bz2 --xu7xtsni3al6cjf6 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Content-length: 2912 On 2019-10-11, Michael Ellerman wrote: > On a machine with a 64K PAGE_SIZE, the nested for loops in > test_check_nonzero_user() can lead to soft lockups, eg: >=20 > watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611] > Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum = virtio_balloon ip_tables x_tables autofs4 > CPU: 4 PID: 611 Comm: modprobe Tainted: G L 5.4.0-rc1-gc= c-8.2.0-00001-gf5a1a536fa14-dirty #1151 > ... > NIP __might_sleep+0x20/0xc0 > LR __might_fault+0x40/0x60 > Call Trace: > check_zeroed_user+0x12c/0x200 > test_user_copy_init+0x67c/0x1210 [test_user_copy] > do_one_initcall+0x60/0x340 > do_init_module+0x7c/0x2f0 > load_module+0x2d94/0x30e0 > __do_sys_finit_module+0xc8/0x150 > system_call+0x5c/0x68 >=20 > Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead > tweak it to only scan a 1024 byte region, but make it cross the > page boundary. >=20 > Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper") > Suggested-by: Aleksa Sarai > Signed-off-by: Michael Ellerman > --- > lib/test_user_copy.c | 23 ++++++++++++++++++++--- > 1 file changed, 20 insertions(+), 3 deletions(-) >=20 > How does this look? It runs in < 1s on my machine here. >=20 > cheers >=20 > diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c > index 950ee88cd6ac..9fb6bc609d4c 100644 > --- a/lib/test_user_copy.c > +++ b/lib/test_user_copy.c > @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size) > static int test_check_nonzero_user(char *kmem, char __user *umem, size_t= size) > { > int ret =3D 0; > - size_t start, end, i; > - size_t zero_start =3D size / 4; > - size_t zero_end =3D size - zero_start; > + size_t start, end, i, zero_start, zero_end; > + > + if (test(size < 1024, "buffer too small")) > + return -EINVAL; > + > + /* > + * We want to cross a page boundary to exercise the code more > + * effectively. We assume the buffer we're passed has a page boundary at > + * size / 2. We also don't want to make the size we scan too large, > + * otherwise the test can take a long time and cause soft lockups. So > + * scan a 1024 byte region across the page boundary. > + */ > + start =3D size / 2 - 512; > + size =3D 1024; I don't think it's necessary to do "size / 2" here -- you can just use PAGE_SIZE directly and check above that "size =3D=3D 2*PAGE_SIZE" (not that this check is exceptionally necessary -- since there's only one caller of this function and it's in the same file). > + > + kmem +=3D start; > + umem +=3D start; > + > + zero_start =3D size / 4; > + zero_end =3D size - zero_start; >=20=20 > /* > * We conduct a series of check_nonzero_user() tests on a block of memo= ry --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --xu7xtsni3al6cjf6 Content-Type: application/pgp-signature; name="signature.asc" Content-length: 228 -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXZ/7dgAKCRCdlLljIbnQ EgboAQDJ8CE0F2TLUINi1TVmrSnff05vx9rNqKgWqX6FRhmwZQD/fTHxCU6/MBBj t/OvAUCvDgAeGUi++GX8ZCl5uljK2AE= =DY8J -----END PGP SIGNATURE----- --xu7xtsni3al6cjf6--