--- /dev/null
+/*
+ * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
+ * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ *
+ */
+
+#define _LGPL_SOURCE
+#include <fcntl.h>
+#include <limits.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <urcu.h>
+
+#include <common/error.h>
+
+#include "shm.h"
+
+/*
+ * Using fork to set umask in the child process (not multi-thread safe). We
+ * deal with the shm_open vs ftruncate race (happening when the sessiond owns
+ * the shm and does not let everybody modify it, to ensure safety against
+ * shm_unlink) by simply letting the mmap fail and retrying after a few
+ * seconds. For global shm, everybody has rw access to it until the sessiond
+ * starts.
+ */
+static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
+{
+ int wait_shm_fd, ret;
+ mode_t mode;
+
+ assert(shm_path);
+
+ /* Default permissions */
+ mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
+
+ /*
+ * Change owner of the shm path.
+ */
+ if (global) {
+ /*
+ * If global session daemon, any application can
+ * register. Make it initially writeable so applications
+ * registering concurrently can do ftruncate() by
+ * themselves.
+ */
+ mode |= S_IROTH | S_IWOTH;
+ }
+
+ /*
+ * We're alone in a child process, so we can modify the process-wide
+ * umask.
+ */
+ umask(~mode);
+
+ /*
+ * Try creating shm (or get rw access). We don't do an exclusive open,
+ * because we allow other processes to create+ftruncate it concurrently.
+ *
+ * A sysctl, fs.protected_regular may prevent the session daemon from
+ * opening a previously created shm when the O_CREAT flag is provided.
+ * Systemd enables this ABI-breaking change by default since v241.
+ *
+ * First, attempt to use the create-or-open semantic that is
+ * desired here. If this fails with EACCES, work around this broken
+ * behaviour and attempt to open the shm without the O_CREAT flag.
+ *
+ * The two attempts are made in this order since applications are
+ * expected to race with the session daemon to create this shm.
+ * Attempting an shm_open() without the O_CREAT flag first could fail
+ * because the file doesn't exist. It could then be created by an
+ * application, which would cause a second try with the O_CREAT flag to
+ * fail with EACCES.
+ *
+ * Note that this introduces a new failure mode where a user could
+ * launch an application (creating the shm) and unlink the shm while
+ * the session daemon is launching, causing the second attempt
+ * to fail. This is not recovered-from as unlinking the shm will
+ * prevent userspace tracing from succeeding anyhow: the sessiond would
+ * use a now-unlinked shm, while the next application would create
+ * a new named shm.
+ */
+ wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
+ if (wait_shm_fd < 0) {
+ if (errno == EACCES) {
+ /* Work around sysctl fs.protected_regular. */
+ DBG("shm_open of %s returned EACCES, this may be caused "
+ "by the fs.protected_regular sysctl. "
+ "Attempting to open the shm without "
+ "creating it.", shm_path);
+ wait_shm_fd = shm_open(shm_path, O_RDWR, mode);
+ }
+ if (wait_shm_fd < 0) {
+ PERROR("Failed to open wait shm at %s", shm_path);
+ goto error;
+ }
+ }
+
+ ret = ftruncate(wait_shm_fd, mmap_size);
+ if (ret < 0) {
+ PERROR("ftruncate wait shm");
+ exit(EXIT_FAILURE);
+ }
+
+#ifndef __FreeBSD__
+ if (global) {
+ ret = fchown(wait_shm_fd, 0, 0);
+ if (ret < 0) {
+ PERROR("fchown");
+ exit(EXIT_FAILURE);
+ }
+ /*
+ * If global session daemon, any application can
+ * register so the shm needs to be set in read-only mode
+ * for others.
+ */
+ mode &= ~S_IWOTH;
+ ret = fchmod(wait_shm_fd, mode);
+ if (ret < 0) {
+ PERROR("fchmod");
+ exit(EXIT_FAILURE);
+ }
+ } else {
+ ret = fchown(wait_shm_fd, getuid(), getgid());
+ if (ret < 0) {
+ PERROR("fchown");
+ exit(EXIT_FAILURE);
+ }
+ }
+#else
+#warning "FreeBSD does not support setting file mode on shm FD."
+#endif
+
+ DBG("Got the wait shm fd %d", wait_shm_fd);
+
+ return wait_shm_fd;
+
+error:
+ DBG("Failing to get the wait shm fd");
+
+ return -1;
+}
+
+/*
+ * Return the wait shm mmap for UST application notification. The global
+ * variable is used to indicate if the the session daemon is global
+ * (root:tracing) or running with an unprivileged user.
+ *
+ * This returned value is used by futex_wait_update() in futex.c to WAKE all
+ * waiters which are UST application waiting for a session daemon.
+ */
+char *shm_ust_get_mmap(char *shm_path, int global)
+{
+ size_t mmap_size;
+ int wait_shm_fd, ret;
+ char *wait_shm_mmap;
+ long sys_page_size;
+
+ assert(shm_path);
+
+ sys_page_size = sysconf(_SC_PAGE_SIZE);
+ if (sys_page_size < 0) {
+ PERROR("sysconf PAGE_SIZE");
+ goto error;
+ }
+ mmap_size = sys_page_size;
+
+ wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
+ if (wait_shm_fd < 0) {
+ goto error;
+ }
+
+ wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
+ MAP_SHARED, wait_shm_fd, 0);
+
+ /* close shm fd immediately after taking the mmap reference */
+ ret = close(wait_shm_fd);
+ if (ret) {
+ PERROR("Error closing fd");
+ }
+
+ if (wait_shm_mmap == MAP_FAILED) {
+ DBG("mmap error (can be caused by race with ust).");
+ goto error;
+ }
+
+ return wait_shm_mmap;
+
+error:
+ return NULL;
+}
+
+/*
+ * shm_create_anonymous is never called concurrently within a process.
+ */
+int shm_create_anonymous(const char *owner_name)
+{
+ char tmp_name[NAME_MAX];
+ int shmfd, ret;
+
+ ret = snprintf(tmp_name, NAME_MAX, "/shm-%s-%d", owner_name, getpid());
+ if (ret < 0) {
+ PERROR("snprintf");
+ return -1;
+ }
+ /*
+ * Allocate shm, and immediately unlink its shm oject, keeping only the
+ * file descriptor as a reference to the object.
+ */
+ shmfd = shm_open(tmp_name, O_CREAT | O_EXCL | O_RDWR, 0700);
+ if (shmfd < 0) {
+ PERROR("shm_open");
+ goto error_shm_open;
+ }
+ ret = shm_unlink(tmp_name);
+ if (ret < 0 && errno != ENOENT) {
+ PERROR("shm_unlink");
+ goto error_shm_release;
+ }
+ return shmfd;
+
+error_shm_release:
+ ret = close(shmfd);
+ if (ret) {
+ PERROR("close");
+ }
+error_shm_open:
+ return -1;
+}
projects / lttng-tools.git / blobdiff