Fix: possible null dereference

[lttng-tools.git] / src / common / fd-tracker / fd-tracker.h

/*
 * Copyright (C) 2018 Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */

#ifndef FD_TRACKER_H
#define FD_TRACKER_H

#include <common/compat/directory-handle.h>
#include <stdint.h>
#include <sys/types.h>

struct fs_handle;
struct fd_tracker;

/*
 * Callback which returns a file descriptor to track through the fd
 * tracker. This callback must not make use of the fd_tracker as a deadlock
 * may occur.
 *
 * The int pointer argument is an output parameter that should be used to return
 * the advertised number of file descriptors.
 *
 * Must return zero on success. Negative values should map to a UNIX error code.
 */
typedef int (*fd_open_cb)(void *, int *out_fds);

/*
 * Callback to allow the user to close a now-untracked file descriptor. This
 * callback must not make use of the fd_tracker as a deadlock may occur.
 *
 * The callback can freely modify the in_fds argument as it is copied by the
 * fd_tracker before being used. The fd tracker assumes in_fds to be closed by
 * the time the callback returns.
 *
 * Must return zero on success. Negative values should map to a UNIX error code.
 */
typedef int (*fd_close_cb)(void *, int *in_fds);

/*
 * Set the maximal number of fds that the process should be allowed to open at
 * any given time. This function must be called before any other of this
 * interface.
 *
 * The unlinked_file_path is an absolute path (which does not need to exist)
 * under which unlinked files will be stored for as long as a reference to them
 * is held.
 */
struct fd_tracker *fd_tracker_create(const char *unlinked_file_path,
		unsigned int capacity);

/* Returns an error if file descriptors are leaked. */
int fd_tracker_destroy(struct fd_tracker *tracker);

/*
 * Open a handle to a suspendable filesystem file descriptor.
 *
 * See OPEN(3) for an explanation of flags and mode. NULL is returned in case of
 * error and errno is left untouched. Note that passing NULL as mode will result
 * in open()'s default behaviour being used (using the process' umask).
 *
 * A fs_handle wraps a file descriptor created by OPEN(3). It is suspendable
 * meaning that the underlying file may be closed at any time unless the
 * handle is marked as being in-use (see fs_handle_get_fd() and
 * fs_handle_put_fd()).
 *
 * If the tracker opted to close the underlying file descriptor, it will
 * be restored to its last known state when it is obtained through
 * the fs_handle's fs_handle_get_fd() method.
 *
 * Note that a suspendable file descriptor can be closed by the fd tracker at
 * anytime when it is not in use. This means that the user should not rely on it
 * being safe to unlink the file. Moreover, concurent modifications to the file
 * (e.g. truncation) may react differently than if the file descriptor was kept
 * open.
 */
struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
		struct lttng_directory_handle *directory,
		const char *path,
		int flags,
		mode_t *mode);

/*
 * Open a tracked unsuspendable file descriptor.
 *
 * This function allows the fd tracker to keep track of unsuspendable
 * file descriptors. A callback, open, is passed to allow the tracker
 * to atomically reserve an entry for a given count of new file descriptors,
 * suspending file descriptors as needed, and invoke the provided callback
 * without ever exceeding the tracker's capacity.
 *
 * fd_count indicates the count of file descriptors that will be opened and
 * returned by the open callback. The storage location at out_fds is assumed
 * to be large enough to hold 'fd_count * sizeof(int)'.
 *
 * Names may be provided to allow easier debugging of file descriptor
 * exhaustions.
 *
 * The callback's return value is returned to the user. Additionally, two
 * negative tracker-specific codes may be returned:
 *   - ENOMEM: allocation of a new entry failed,
 *   - EMFILE: too many unsuspendable fds are opened and the tracker can't
 *             accomodate the request for a new unsuspendable entry.
 */
int fd_tracker_open_unsuspendable_fd(struct fd_tracker *tracker,
		int *out_fds,
		const char **names,
		unsigned int fd_count,
		fd_open_cb open,
		void *data);

/*
 * Close a tracked unsuspendable file descriptor.
 *
 * This function allows the fd tracker to keep track of unsuspendable
 * file descriptors. A callback, close, is passed to allow the tracker
 * to atomically release a file descriptor entry.
 *
 * Returns 0 if the close callback returned success. Returns the value returned
 * by the close callback if it is negative. Additionally, a tracker-specific
 * code may be returned:
 *   - EINVAL: a file descriptor was unknown to the tracker
 *
 * Closed fds are set to -1 in the fds array which, in the event of an error,
 * allows the user to know which file descriptors are no longer being tracked.
 */
int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
		int *fds,
		unsigned int fd_count,
		fd_close_cb close,
		void *data);

/*
 * Log the contents of the fd_tracker.
 */
void fd_tracker_log(struct fd_tracker *tracker);

/*
 * Marks the handle as the most recently used and marks the 'fd' as
 * "in-use". This prevents the tracker from recycling the underlying
 * file descriptor while it is actively being used by a thread.
 *
 * Don't forget that the tracker may be initiating an fd 'suspension'
 * from another thread as the need to free an fd slot may arise from any
 * thread within the daemon.
 *
 * Note that a restorable fd should never be held for longer than
 * strictly necessary (e.g. the duration of a syscall()).
 *
 * Returns the fd on success, otherwise a negative value may be returned
 * if the restoration of the fd failed.
 */
int fs_handle_get_fd(struct fs_handle *handle);

/*
 * Used by the application to signify that it is no longer using the
 * underlying fd and that it may be suspended.
 */
void fs_handle_put_fd(struct fs_handle *handle);

/*
 * Unlink the file associated to an fs_handle. Note that the unlink
 * operation will not be performed immediately. It will only be performed
 * once all references to the underlying file (through other fs_handle objects)
 * have been released.
 *
 * However, note that the file will be renamed so as to provide the observable
 * effect of an unlink(), that is removing a name from the filesystem.
 *
 * Returns 0 on success, otherwise a negative value will be returned
 * if the operation failed.
 */
int fs_handle_unlink(struct fs_handle *handle);

/*
 * Frees the handle and discards the underlying fd.
 */
int fs_handle_close(struct fs_handle *handle);

#endif /* FD_TRACKER_H */