src/common/fd-tracker/fd-tracker.h

   1 /*
   2  * Copyright (C) 2018 Jérémie Galarneau <jeremie.galarneau@efficios.com>
   3  *
   4  * SPDX-License-Identifier: GPL-2.0-only
   5  *
   6  */
   7
   8 #ifndef FD_TRACKER_H
   9 #define FD_TRACKER_H
  10
  11 #include <common/compat/directory-handle.h>
  12 #include <common/macros.h>
  13 #include <stdint.h>
  14 #include <sys/types.h>
  15
  16 struct fs_handle;
  17 struct fd_tracker;
  18
  19 /*
  20  * Callback which returns a file descriptor to track through the fd
  21  * tracker. This callback must not make use of the fd_tracker as a deadlock
  22  * may occur.
  23  *
  24  * The int pointer argument is an output parameter that should be used to return
  25  * the advertised number of file descriptors.
  26  *
  27  * Must return zero on success. Negative values should map to a UNIX error code.
  28  */
  29 typedef int (*fd_open_cb)(void *, int *out_fds);
  30
  31 /*
  32  * Callback to allow the user to close a now-untracked file descriptor. This
  33  * callback must not make use of the fd_tracker as a deadlock may occur.
  34  *
  35  * The callback can freely modify the in_fds argument as it is copied by the
  36  * fd_tracker before being used. The fd tracker assumes in_fds to be closed by
  37  * the time the callback returns.
  38  *
  39  * Must return zero on success. Negative values should map to a UNIX error code.
  40  */
  41 typedef int (*fd_close_cb)(void *, int *in_fds);
  42
  43 /*
  44  * Set the maximal number of fds that the process should be allowed to open at
  45  * any given time. This function must be called before any other of this
  46  * interface.
  47  *
  48  * The unlinked_file_path is an absolute path (which does not need to exist)
  49  * under which unlinked files will be stored for as long as a reference to them
  50  * is held.
  51  */
  52 LTTNG_HIDDEN
  53 struct fd_tracker *fd_tracker_create(const char *unlinked_file_path,
  54                 unsigned int capacity);
  55
  56 /* Returns an error if file descriptors are leaked. */
  57 LTTNG_HIDDEN
  58 int fd_tracker_destroy(struct fd_tracker *tracker);
  59
  60 /*
  61  * Open a handle to a suspendable filesystem file descriptor.
  62  *
  63  * See OPEN(3) for an explanation of flags and mode. NULL is returned in case of
  64  * error and errno is left untouched. Note that passing NULL as mode will result
  65  * in open()'s default behaviour being used (using the process' umask).
  66  *
  67  * A fs_handle wraps a file descriptor created by OPEN(3). It is suspendable
  68  * meaning that the underlying file may be closed at any time unless the
  69  * handle is marked as being in-use (see fs_handle_get_fd() and
  70  * fs_handle_put_fd()).
  71  *
  72  * If the tracker opted to close the underlying file descriptor, it will
  73  * be restored to its last known state when it is obtained through
  74  * the fs_handle's fs_handle_get_fd() method.
  75  *
  76  * Note that a suspendable file descriptor can be closed by the fd tracker at
  77  * anytime when it is not in use. This means that the user should not rely on it
  78  * being safe to unlink the file. Moreover, concurent modifications to the file
  79  * (e.g. truncation) may react differently than if the file descriptor was kept
  80  * open.
  81  */
  82 LTTNG_HIDDEN
  83 struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
  84                 struct lttng_directory_handle *directory,
  85                 const char *path,
  86                 int flags,
  87                 mode_t *mode);
  88
  89 /*
  90  * Open a tracked unsuspendable file descriptor.
  91  *
  92  * This function allows the fd tracker to keep track of unsuspendable
  93  * file descriptors. A callback, open, is passed to allow the tracker
  94  * to atomically reserve an entry for a given count of new file descriptors,
  95  * suspending file descriptors as needed, and invoke the provided callback
  96  * without ever exceeding the tracker's capacity.
  97  *
  98  * fd_count indicates the count of file descriptors that will be opened and
  99  * returned by the open callback. The storage location at out_fds is assumed
 100  * to be large enough to hold 'fd_count * sizeof(int)'.
 101  *
 102  * Names may be provided to allow easier debugging of file descriptor
 103  * exhaustions.
 104  *
 105  * The callback's return value is returned to the user. Additionally, two
 106  * negative tracker-specific codes may be returned:
 107  *   - ENOMEM: allocation of a new entry failed,
 108  *   - EMFILE: too many unsuspendable fds are opened and the tracker can't
 109  *             accomodate the request for a new unsuspendable entry.
 110  */
 111 LTTNG_HIDDEN
 112 int fd_tracker_open_unsuspendable_fd(struct fd_tracker *tracker,
 113                 int *out_fds,
 114                 const char **names,
 115                 unsigned int fd_count,
 116                 fd_open_cb open,
 117                 void *data);
 118
 119 /*
 120  * Close a tracked unsuspendable file descriptor.
 121  *
 122  * This function allows the fd tracker to keep track of unsuspendable
 123  * file descriptors. A callback, close, is passed to allow the tracker
 124  * to atomically release a file descriptor entry.
 125  *
 126  * Returns 0 if the close callback returned success. Returns the value returned
 127  * by the close callback if it is negative. Additionally, a tracker-specific
 128  * code may be returned:
 129  *   - EINVAL: a file descriptor was unknown to the tracker
 130  *
 131  * Closed fds are set to -1 in the fds array which, in the event of an error,
 132  * allows the user to know which file descriptors are no longer being tracked.
 133  */
 134 LTTNG_HIDDEN
 135 int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
 136                 int *fds,
 137                 unsigned int fd_count,
 138                 fd_close_cb close,
 139                 void *data);
 140
 141 /*
 142  * Log the contents of the fd_tracker.
 143  */
 144 LTTNG_HIDDEN
 145 void fd_tracker_log(struct fd_tracker *tracker);
 146
 147 /*
 148  * Marks the handle as the most recently used and marks the 'fd' as
 149  * "in-use". This prevents the tracker from recycling the underlying
 150  * file descriptor while it is actively being used by a thread.
 151  *
 152  * Don't forget that the tracker may be initiating an fd 'suspension'
 153  * from another thread as the need to free an fd slot may arise from any
 154  * thread within the daemon.
 155  *
 156  * Note that a restorable fd should never be held for longer than
 157  * strictly necessary (e.g. the duration of a syscall()).
 158  *
 159  * Returns the fd on success, otherwise a negative value may be returned
 160  * if the restoration of the fd failed.
 161  */
 162 LTTNG_HIDDEN
 163 int fs_handle_get_fd(struct fs_handle *handle);
 164
 165 /*
 166  * Used by the application to signify that it is no longer using the
 167  * underlying fd and that it may be suspended.
 168  */
 169 LTTNG_HIDDEN
 170 void fs_handle_put_fd(struct fs_handle *handle);
 171
 172 /*
 173  * Unlink the file associated to an fs_handle. Note that the unlink
 174  * operation will not be performed immediately. It will only be performed
 175  * once all references to the underlying file (through other fs_handle objects)
 176  * have been released.
 177  *
 178  * However, note that the file will be renamed so as to provide the observable
 179  * effect of an unlink(), that is removing a name from the filesystem.
 180  *
 181  * Returns 0 on success, otherwise a negative value will be returned
 182  * if the operation failed.
 183  */
 184 LTTNG_HIDDEN
 185 int fs_handle_unlink(struct fs_handle *handle);
 186
 187 /*
 188  * Frees the handle and discards the underlying fd.
 189  */
 190 LTTNG_HIDDEN
 191 int fs_handle_close(struct fs_handle *handle);
 192
 193 #endif /* FD_TRACKER_H */