X-Git-Url: http://git.efficios.com/?p=lttng-tools.git;a=blobdiff_plain;f=src%2Fcommon%2Fshm.c;fp=src%2Fcommon%2Fshm.c;h=74b240ccc30898763477ff02b7a09c363bda5262;hp=0000000000000000000000000000000000000000;hb=2463b7879c00298daa79744cdaae82ac061a4ed8;hpb=3a4595c2469472dee1656cde5f8882c2123efd3c diff --git a/src/common/shm.c b/src/common/shm.c new file mode 100644 index 000000000..74b240ccc --- /dev/null +++ b/src/common/shm.c @@ -0,0 +1,233 @@ +/* + * Copyright (C) 2011 David Goulet + * Copyright (C) 2011 Mathieu Desnoyers + * + * SPDX-License-Identifier: GPL-2.0-only + * + */ + +#define _LGPL_SOURCE +#include +#include +#include +#include +#include +#include +#include +#include + +#include + +#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; +}