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 #ifdef __cplusplus
  17 extern "C" {
  18 #endif
  19
  20 struct fs_handle;
  21 struct fd_tracker;
  22
  23 /*
  24  * Callback which returns a file descriptor to track through the fd
  25  * tracker. This callback must not make use of the fd_tracker as a deadlock
  26  * may occur.
  27  *
  28  * The int pointer argument is an output parameter that should be used to return
  29  * the advertised number of file descriptors.
  30  *
  31  * Must return zero on success. Negative values should map to a UNIX error code.
  32  */
  33 typedef int (*fd_open_cb)(void *, int *out_fds);
  34
  35 /*
  36  * Callback to allow the user to close a now-untracked file descriptor. This
  37  * callback must not make use of the fd_tracker as a deadlock may occur.
  38  *
  39  * The callback can freely modify the in_fds argument as it is copied by the
  40  * fd_tracker before being used. The fd tracker assumes in_fds to be closed by
  41  * the time the callback returns.
  42  *
  43  * Must return zero on success. Negative values should map to a UNIX error code.
  44  */
  45 typedef int (*fd_close_cb)(void *, int *in_fds);
  46
  47 /*
  48  * Set the maximal number of fds that the process should be allowed to open at
  49  * any given time. This function must be called before any other of this
  50  * interface.
  51  *
  52  * The unlinked_file_path is an absolute path (which does not need to exist)
  53  * under which unlinked files will be stored for as long as a reference to them
  54  * is held.
  55  */
  56 struct fd_tracker *fd_tracker_create(const char *unlinked_file_path,
  57                 unsigned int capacity);
  58
  59 /* Returns an error if file descriptors are leaked. */
  60 int fd_tracker_destroy(struct fd_tracker *tracker);
  61
  62 /*
  63  * Open a handle to a suspendable filesystem file descriptor.
  64  *
  65  * See OPEN(3) for an explanation of flags and mode. NULL is returned in case of
  66  * error and errno is left untouched. Note that passing NULL as mode will result
  67  * in open()'s default behaviour being used (using the process' umask).
  68  *
  69  * A fs_handle wraps a file descriptor created by OPEN(3). It is suspendable
  70  * meaning that the underlying file may be closed at any time unless the
  71  * handle is marked as being in-use (see fs_handle_get_fd() and
  72  * fs_handle_put_fd()).
  73  *
  74  * If the tracker opted to close the underlying file descriptor, it will
  75  * be restored to its last known state when it is obtained through
  76  * the fs_handle's fs_handle_get_fd() method.
  77  *
  78  * Note that a suspendable file descriptor can be closed by the fd tracker at
  79  * anytime when it is not in use. This means that the user should not rely on it
  80  * being safe to unlink the file. Moreover, concurent modifications to the file
  81  * (e.g. truncation) may react differently than if the file descriptor was kept
  82  * open.
  83  */
  84 struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
  85                 struct lttng_directory_handle *directory,
  86                 const char *path,
  87                 int flags,
  88                 mode_t *mode);
  89
  90 /*
  91  * Open a tracked unsuspendable file descriptor.
  92  *
  93  * This function allows the fd tracker to keep track of unsuspendable
  94  * file descriptors. A callback, open, is passed to allow the tracker
  95  * to atomically reserve an entry for a given count of new file descriptors,
  96  * suspending file descriptors as needed, and invoke the provided callback
  97  * without ever exceeding the tracker's capacity.
  98  *
  99  * fd_count indicates the count of file descriptors that will be opened and
 100  * returned by the open callback. The storage location at out_fds is assumed
 101  * to be large enough to hold 'fd_count * sizeof(int)'.
 102  *
 103  * Names may be provided to allow easier debugging of file descriptor
 104  * exhaustions.
 105  *
 106  * The callback's return value is returned to the user. Additionally, two
 107  * negative tracker-specific codes may be returned:
 108  *   - ENOMEM: allocation of a new entry failed,
 109  *   - EMFILE: too many unsuspendable fds are opened and the tracker can't
 110  *             accommodates the request for a new unsuspendable entry.
 111  */
 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 int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
 135                 int *fds,
 136                 unsigned int fd_count,
 137                 fd_close_cb close,
 138                 void *data);
 139
 140 /*
 141  * Log the contents of the fd_tracker.
 142  */
 143 void fd_tracker_log(struct fd_tracker *tracker);
 144
 145 #ifdef __cplusplus
 146 }
 147 #endif
 148
 149 #endif /* FD_TRACKER_H */