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98b6cf102b
lookup_fast_for_open() only hands an O_CREAT open a cached dentry if
it's positive. Otherwise the open goes on to lookup_open(), which takes
the directory lock and creates the file. It tests for that with
!dentry->d_inode, but traverse_mounts() later decides with the type in
d_flags. And when a dentry is instantiated __d_set_inode_and_type()
stores ->d_inode first and only then releases the type into ->d_flags.
So a refwalk that took its reference to a negative dentry right before
another task instantiates it can find ->d_inode already set in
lookup_fast_for_open(), go on to step_into(), and still find the
negative type in traverse_mounts(). The open then fails with -ENOENT,
which an O_CREAT open must never do. rcuwalk doesn't get that far, the
d_seq check in step_into() catches the instantiation.
Use d_is_negative() so both look at the type. While the type is still
negative the open takes lookup_open() and waits for the directory lock
like any other O_CREAT open of a negative dentry.
On its own the window is a couple of instructions in the creating task,
so it takes an interrupt or a preempted vcpu in exactly the right spot.
I haven't caught it without help in about 600 million racing O_CREAT
opens.
The debug patch below stalls the creating task between the two stores
for names starting with "ocrace", the way an interrupt landing there
would. The reproducer below opens "ocrace" with O_CREAT through
symlinks on a strictatime tmpfs: following them updates their atime,
which drops the walk out of rcuwalk, so the name is always looked up in
refwalk. With
echo 20 > /proc/sys/fs/ocrace_instantiate_us
./ocreat_enoent -t 60
on a KASAN kernel open() failed with -ENOENT 43 times in 4.9 million
calls. With this patch it never failed in 4.8 million calls. Tracing
step_into_slowpath() during such a run, each of the 98 -ENOENTs it
returned came right after the same task had entered it with a dentry
that was still typed negative.
// SPDX-License-Identifier: GPL-2.0
/*
* ocreat_enoent: open(O_CREAT) racing the creation of the same name.
*
* Writers unlink "ocrace" and create it again. Readers open it with
* O_CREAT through a symlink of their own on a strictatime tmpfs.
* Following the symlink updates its atime, which drops the walk out of
* rcu mode, so the name is always looked up in refwalk. An O_CREAT open
* must never fail with ENOENT.
*
* -p arms a cpu-clock perf timer with that period in ns on every writer,
* so that interrupts land at random points in it.
*
* usage: ocreat_enoent [-t secs] [-w writers] [-r readers] [-p ns]
*/
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <stdatomic.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <linux/perf_event.h>
#define DIR "/tmp/oc"
static atomic_int stop;
static atomic_long opens, creates, enoent, other;
static long period;
static void die(const char *what)
{
fprintf(stderr, "FATAL %s: %s\n", what, strerror(errno));
exit(2);
}
static void *writer(void *arg)
{
(void)arg;
if (period) {
struct perf_event_attr attr = {
.type = PERF_TYPE_SOFTWARE,
.size = sizeof(attr),
.config = PERF_COUNT_SW_CPU_CLOCK,
.sample_period = period,
};
if (syscall(SYS_perf_event_open, &attr, 0, -1, -1, 0) < 0)
die("perf_event_open");
}
while (!atomic_load(&stop)) {
int fd;
unlink(DIR "/ocrace");
fd = open(DIR "/ocrace", O_CREAT | O_RDWR | O_CLOEXEC, 0644);
if (fd >= 0)
close(fd);
atomic_fetch_add(&creates, 1);
}
return NULL;
}
static void *reader(void *arg)
{
char lnk[64];
snprintf(lnk, sizeof(lnk), DIR "/lnk.%ld", (long)arg);
while (!atomic_load(&stop)) {
int fd = open(lnk, O_CREAT | O_RDWR | O_CLOEXEC, 0644);
if (fd >= 0)
close(fd);
else if (errno == ENOENT)
atomic_fetch_add(&enoent, 1);
else
atomic_fetch_add(&other, 1);
atomic_fetch_add(&opens, 1);
}
return NULL;
}
int main(int argc, char **argv)
{
int secs = 60, nw = 4, nr = 12, c, n = 0;
pthread_t t[256];
while ((c = getopt(argc, argv, "t:w:r:p:")) != -1) {
switch (c) {
case 't': secs = atoi(optarg); break;
case 'w': nw = atoi(optarg); break;
case 'r': nr = atoi(optarg); break;
case 'p': period = atol(optarg); break;
default: return 2;
}
}
if (nw + nr > 256)
return 2;
mkdir(DIR, 0755);
if (mount("oc", DIR, "tmpfs", MS_STRICTATIME, "mode=0755"))
die("tmpfs");
for (long i = 0; i < nr; i++) {
char lnk[64];
snprintf(lnk, sizeof(lnk), DIR "/lnk.%ld", i);
if (symlink("ocrace", lnk))
die("symlink");
}
for (long i = 0; i < nw; i++)
if (pthread_create(&t[n++], NULL, writer, NULL))
die("writer");
for (long i = 0; i < nr; i++)
if (pthread_create(&t[n++], NULL, reader, (void *)i))
die("reader");
sleep(secs);
atomic_store(&stop, 1);
for (int i = 0; i < n; i++)
pthread_join(t[i], NULL);
printf("%ds: %ld opens, %ld creates, ENOENT %ld, other errors %ld\n",
secs, atomic_load(&opens), atomic_load(&creates),
atomic_load(&enoent), atomic_load(&other));
umount2(DIR, MNT_DETACH);
return atomic_load(&enoent) ? 1 : 0;
}
diff --git a/fs/dcache.c b/fs/dcache.c
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -32,9 +32,13 @@
#include <linux/bit_spinlock.h>
#include <linux/rculist_bl.h>
#include <linux/list_lru.h>
+#include <linux/delay.h>
#include "internal.h"
#include "mount.h"
+/* debug: stall "ocrace*" names between ->d_inode and the type */
+static int sysctl_ocrace_instantiate_us;
+
#include <asm/runtime-const.h>
/*
@@ -200,6 +204,13 @@
}
static const struct ctl_table fs_dcache_sysctls[] = {
+ {
+ .procname = "ocrace_instantiate_us",
+ .data = &sysctl_ocrace_instantiate_us,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec,
+ },
{
.procname = "dentry-state",
.data = &dentry_stat,
@@ -405,6 +416,10 @@
unsigned flags;
dentry->d_inode = inode;
+ /* debug: an interrupt landing right here */
+ if (unlikely(READ_ONCE(sysctl_ocrace_instantiate_us)) &&
+ !strncmp(dentry->d_name.name, "ocrace", 6))
+ udelay(READ_ONCE(sysctl_ocrace_instantiate_us));
flags = READ_ONCE(dentry->d_flags);
flags &= ~DCACHE_ENTRY_TYPE;
flags |= type_flags;
Fixes:
|
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer: Getting started with kernel development * Academic Researcher: Studying kernel internals and architecture * Security Expert: Hardening and vulnerability analysis * Backport/Maintenance Engineer: Maintaining stable kernels * System Administrator: Configuring and troubleshooting * Maintainer: Leading subsystems and reviewing patches * Hardware Vendor: Writing drivers for new hardware * Distribution Maintainer: Packaging kernels for distros * AI Coding Assistant: LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst