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 struct fd_tracker *fd_tracker_create(const char *unlinked_file_path,
  53                 unsigned int capacity);
  54
  55 /* Returns an error if file descriptors are leaked. */
  56 int fd_tracker_destroy(struct fd_tracker *tracker);
  57
  58 /*
  59  * Open a handle to a suspendable filesystem file descriptor.
  60  *
  61  * See OPEN(3) for an explanation of flags and mode. NULL is returned in case of
  62  * error and errno is left untouched. Note that passing NULL as mode will result
  63  * in open()'s default behaviour being used (using the process' umask).
  64  *
  65  * A fs_handle wraps a file descriptor created by OPEN(3). It is suspendable
  66  * meaning that the underlying file may be closed at any time unless the
  67  * handle is marked as being in-use (see fs_handle_get_fd() and
  68  * fs_handle_put_fd()).
  69  *
  70  * If the tracker opted to close the underlying file descriptor, it will
  71  * be restored to its last known state when it is obtained through
  72  * the fs_handle's fs_handle_get_fd() method.
  73  *
  74  * Note that a suspendable file descriptor can be closed by the fd tracker at
  75  * anytime when it is not in use. This means that the user should not rely on it
  76  * being safe to unlink the file. Moreover, concurent modifications to the file
  77  * (e.g. truncation) may react differently than if the file descriptor was kept
  78  * open.
  79  */
  80 struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
  81                 struct lttng_directory_handle *directory,
  82                 const char *path,
  83                 int flags,
  84                 mode_t *mode);
  85
  86 /*
  87  * Open a tracked unsuspendable file descriptor.
  88  *
  89  * This function allows the fd tracker to keep track of unsuspendable
  90  * file descriptors. A callback, open, is passed to allow the tracker
  91  * to atomically reserve an entry for a given count of new file descriptors,
  92  * suspending file descriptors as needed, and invoke the provided callback
  93  * without ever exceeding the tracker's capacity.
  94  *
  95  * fd_count indicates the count of file descriptors that will be opened and
  96  * returned by the open callback. The storage location at out_fds is assumed
  97  * to be large enough to hold 'fd_count * sizeof(int)'.
  98  *
  99  * Names may be provided to allow easier debugging of file descriptor
 100  * exhaustions.
 101  *
 102  * The callback's return value is returned to the user. Additionally, two
 103  * negative tracker-specific codes may be returned:
 104  *   - ENOMEM: allocation of a new entry failed,
 105  *   - EMFILE: too many unsuspendable fds are opened and the tracker can't
 106  *             accommodates the request for a new unsuspendable entry.
 107  */
 108 int fd_tracker_open_unsuspendable_fd(struct fd_tracker *tracker,
 109                 int *out_fds,
 110                 const char **names,
 111                 unsigned int fd_count,
 112                 fd_open_cb open,
 113                 void *data);
 114
 115 /*
 116  * Close a tracked unsuspendable file descriptor.
 117  *
 118  * This function allows the fd tracker to keep track of unsuspendable
 119  * file descriptors. A callback, close, is passed to allow the tracker
 120  * to atomically release a file descriptor entry.
 121  *
 122  * Returns 0 if the close callback returned success. Returns the value returned
 123  * by the close callback if it is negative. Additionally, a tracker-specific
 124  * code may be returned:
 125  *   - EINVAL: a file descriptor was unknown to the tracker
 126  *
 127  * Closed fds are set to -1 in the fds array which, in the event of an error,
 128  * allows the user to know which file descriptors are no longer being tracked.
 129  */
 130 int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
 131                 int *fds,
 132                 unsigned int fd_count,
 133                 fd_close_cb close,
 134                 void *data);
 135
 136 /*
 137  * Log the contents of the fd_tracker.
 138  */
 139 void fd_tracker_log(struct fd_tracker *tracker);
 140
 141 #endif /* FD_TRACKER_H */