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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PR4eStJu; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 36A71606D830; Mon, 2 Nov 2026 14:52:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PR4eStJu Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9FECC1F00A1B; Mon, 2 Nov 2026 14:51:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793631126; bh=iJ84nEP4YY4K28kLfEZAA/6j0sLXVTf/aEe1LCZ8e14=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=PR4eStJuCoR+mECGMaIVXtQASozW+a7AOmyTO4iG4xy4nIzb4PWW2BKxvYmoNRuHs x2Lm2Rfp+v90UcW719VC0BYV4O8DTyukQ9z+uecUIg7pY28G7drnWgQgVWiNUS+/ck xxhSirH9ZyCaYNU7/1qRcYlti7eFvPDGGsReRJYcEPKXsJ5iSGH7SZJURK2R0hC4aT hXX/7O0Tf8cPki+pg2H6SZ7pXnNxkptKTlJYs8cIZrx/PHwXHyzeBPMp/y/AzKyvOE N8mqzLmWmEBFNIMmXqg5H4OevB0bDwM2UM7fMf6GDHFGBq7FkN5sBI9EBYQrhUrLbY N6tJxPjUYmLBA== From: Christian Brauner Date: Mon, 02 Nov 2026 15:48:48 +0100 Subject: [PATCH RFC 08/16] selftests/proc: check that a faulting reader doesn't hold up removal X-Mailing-List: linux-fbdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261102-work-procfs-core-v1-8-75df35e4f8b8@kernel.org> References: <20261102-work-procfs-core-v1-0-75df35e4f8b8@kernel.org> In-Reply-To: <20261102-work-procfs-core-v1-0-75df35e4f8b8@kernel.org> To: linux-fsdevel@vger.kernel.org Cc: Linus Torvalds , Jann Horn , Josef Bacik , Chris Mason , Jeff Layton , NeilBrown , Eric Dumazet , Thomas Gleixner , Al Viro , Jonathan Corbet , Christoph Hellwig , Greg Kroah-Hartman , Joel Granados , Kees Cook , Jan Kara , Jan Kara , linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-acpi@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, linux-um@lists.infradead.org, linux-wireless@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, ntfs3@lists.linux.dev, platform-driver-x86@vger.kernel.org, coreteam@netfilter.org, kexec@lists.infradead.org, linux-pci@vger.kernel.org, linux-sound@vger.kernel.org, netfilter-devel@vger.kernel.org, dri-devel@lists.freedesktop.org, ibm-acpi-devel@lists.sourceforge.net, jfs-discussion@lists.sourceforge.net, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-cifs@vger.kernel.org, linux-fbdev@vger.kernel.org, linux-hwmon@vger.kernel.org, linux-mm@kvack.org, linux-sh@vger.kernel.org, linux-usb@vger.kernel.org, samba-technical@lists.samba.org, sparclinux@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-doc@vger.kernel.org, linux-serial@vger.kernel.org, linux-atm-general@lists.sourceforge.net, Takashi Iwai , linux-input@vger.kernel.org, linux-raid@vger.kernel.org, linux-ext4@vger.kernel.org, linux-modules@vger.kernel.org, linux-debuggers@vger.kernel.org, "Christian Brauner (Amutable)" , Chuck Lever X-Mailer: b4 0.17-dev Read /proc/net/dev_snmp6/ of a tun device into a buffer whose first page is backed by userfaultfd and remove the device while the reader sleeps in its copy to userspace: - removing the entry doesn't wait for the reader - the reader still gets what was printed before the removal - and the next read fails with -EIO The entry is removed under RTNL, so a regression stalls the test for five seconds before it fails. Signed-off-by: Christian Brauner (Amutable) --- .../selftests/proc/proc-remove-slow-reader.c | 133 +++++++++++++++++++++ 1 file changed, 133 insertions(+) diff --git a/tools/testing/selftests/proc/proc-remove-slow-reader.c b/tools/testing/selftests/proc/proc-remove-slow-reader.c new file mode 100644 index 000000000000..a201645beca9 --- /dev/null +++ b/tools/testing/selftests/proc/proc-remove-slow-reader.c @@ -0,0 +1,133 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * A read that faults while copying to userspace must not hold up removal of + * the /proc entry. /proc/net/dev_snmp6/ of a tun device is removed under + * RTNL when the device's file is closed. + */ +#define _GNU_SOURCE +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#include "../kselftest_harness.h" + +#define TUN_NAME "procslow0" +#define SNMP6_PATH "/proc/net/dev_snmp6/" TUN_NAME +#define READ_PAGES 16 +#define TIMEOUT_MS 5000 + +struct reader { + int fd; + char *buf; + size_t len; + ssize_t ret; +}; + +struct closer { + int tun; + int done; +}; + +static void *read_thread(void *arg) +{ + struct reader *r = arg; + + r->ret = read(r->fd, r->buf, r->len); + return NULL; +} + +static void *close_thread(void *arg) +{ + struct closer *c = arg; + ssize_t ret; + + close(c->tun); + ret = write(c->done, "x", 1); + return (void *)ret; +} + +TEST(remove_while_reader_faults) +{ + struct uffdio_api api = { .api = UFFD_API }; + struct uffdio_register reg = {}; + struct uffdio_zeropage zero = {}; + long psz = sysconf(_SC_PAGESIZE); + struct ifreq ifr = {}; + struct reader r = {}; + struct closer c = {}; + struct uffd_msg msg; + struct pollfd pfd; + pthread_t rt, ct; + int uffd, done[2]; + char *buf; + + if (unshare(CLONE_NEWNET)) + SKIP(return, "unshare(CLONE_NEWNET): %s", strerror(errno)); + + c.tun = open("/dev/net/tun", O_RDWR | O_CLOEXEC); + if (c.tun < 0) + SKIP(return, "/dev/net/tun: %s", strerror(errno)); + strncpy(ifr.ifr_name, TUN_NAME, IFNAMSIZ - 1); + ifr.ifr_flags = IFF_TUN | IFF_NO_PI; + ASSERT_EQ(ioctl(c.tun, TUNSETIFF, &ifr), 0); + + r.fd = open(SNMP6_PATH, O_RDONLY | O_CLOEXEC); + if (r.fd < 0) + SKIP(return, SNMP6_PATH ": %s", strerror(errno)); + + uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK); + if (uffd < 0) + SKIP(return, "userfaultfd: %s", strerror(errno)); + ASSERT_EQ(ioctl(uffd, UFFDIO_API, &api), 0); + + /* Only the first page of the buffer goes through userfaultfd. */ + buf = mmap(NULL, READ_PAGES * psz, PROT_READ | PROT_WRITE, + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); + ASSERT_NE(buf, MAP_FAILED); + reg.range.start = (unsigned long)buf; + reg.range.len = psz; + reg.mode = UFFDIO_REGISTER_MODE_MISSING; + ASSERT_EQ(ioctl(uffd, UFFDIO_REGISTER, ®), 0); + + r.buf = buf; + r.len = READ_PAGES * psz; + ASSERT_EQ(pthread_create(&rt, NULL, read_thread, &r), 0); + + /* The reader now sleeps in its copy to userspace. */ + pfd = (struct pollfd){ .fd = uffd, .events = POLLIN }; + ASSERT_EQ(poll(&pfd, 1, TIMEOUT_MS), 1); + ASSERT_EQ(read(uffd, &msg, sizeof(msg)), sizeof(msg)); + ASSERT_EQ(msg.event, UFFD_EVENT_PAGEFAULT); + + /* Closing the device removes the file that is being read. */ + ASSERT_EQ(pipe2(done, O_CLOEXEC), 0); + c.done = done[1]; + ASSERT_EQ(pthread_create(&ct, NULL, close_thread, &c), 0); + pfd = (struct pollfd){ .fd = done[0], .events = POLLIN }; + EXPECT_EQ(poll(&pfd, 1, TIMEOUT_MS), 1) { + TH_LOG("removing " SNMP6_PATH " waited for the reader"); + } + + zero.range.start = (unsigned long)buf; + zero.range.len = psz; + ASSERT_EQ(ioctl(uffd, UFFDIO_ZEROPAGE, &zero), 0); + ASSERT_EQ(pthread_join(rt, NULL), 0); + ASSERT_EQ(pthread_join(ct, NULL), 0); + + /* What was printed before the removal still arrives, then nothing. */ + EXPECT_GT(r.ret, 0); + EXPECT_EQ(read(r.fd, buf, psz), -1); + EXPECT_EQ(errno, EIO); +} + +TEST_HARNESS_MAIN -- 2.53.0