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+/*++
+/* NAME
+/* edit_file 3
+/* SUMMARY
+/* simple cooperative file updating protocol
+/* SYNOPSIS
+/* #include <edit_file.h>
+/*
+/* typedef struct {
+/* .in +4
+/* char *tmp_path; /* temp. pathname */
+/* VSTREAM *tmp_fp; /* temp. stream */
+/* /* private members... */
+/* .in -4
+/* } EDIT_FILE;
+/*
+/* EDIT_FILE *edit_file_open(original_path, output_flags, output_mode)
+/* const char *original_path;
+/* int output_flags;
+/* mode_t output_mode;
+/*
+/* int edit_file_close(edit_file)
+/* EDIT_FILE *edit_file;
+/*
+/* void edit_file_cleanup(edit_file)
+/* EDIT_FILE *edit_file;
+/* DESCRIPTION
+/* This module implements a simple protocol for cooperative
+/* processes to update one file. The idea is to 1) create a
+/* new file under a deterministic temporary pathname, 2)
+/* populate the new file with updated information, and 3)
+/* rename the new file into the place of the original file.
+/* This module provides 1) and 3), and leaves 2) to the
+/* application. The temporary pathname is deterministic to
+/* avoid accumulation of thrash after program crashes.
+/*
+/* edit_file_open() implements the first phase of the protocol.
+/* It creates or opens an output file with a deterministic
+/* temporary pathname, obtained by appending the suffix defined
+/* with EDIT_FILE_SUFFIX to the specified original file pathname.
+/* The original file itself is not opened. edit_file_open()
+/* then locks the output file for exclusive access, and verifies
+/* that the file still exists under the temporary pathname.
+/* At this point in the protocol, the current process controls
+/* both the output file content and its temporary pathname.
+/*
+/* In the second phase, the application opens the original
+/* file if needed, and updates the output file via the
+/* \fBtmp_fp\fR member of the EDIT_FILE data structure. This
+/* phase is not implemented by the edit_file() module.
+/*
+/* edit_file_close() implements the third and final phase of
+/* the protocol. It flushes the output file to persistent
+/* storage, and renames the output file from its temporary
+/* pathname into the place of the original file. When any of
+/* these operations fails, edit_file_close() behaves as if
+/* edit_file_cleanup() was called. Regardless of whether these
+/* operations succeed, edit_file_close() releases the exclusive
+/* lock, closes the output file, and frees up memory that was
+/* allocated by edit_file_open().
+/*
+/* edit_file_cleanup() aborts the protocol. It discards the
+/* output file, releases the exclusive lock, closes the output
+/* file, and frees up memory that was allocated by edit_file_open().
+/*
+/* Arguments:
+/* .IP original_path
+/* The pathname of the original file that will be replaced by
+/* the output file. The temporary pathname for the output file
+/* is obtained by appending the suffix defined with EDIT_FILE_SUFFIX
+/* to a copy of the specified original file pathname, and is
+/* made available via the \fBtmp_path\fR member of the EDIT_FILE
+/* data structure.
+/* .IP output_flags
+/* Flags for opening the output file. These are as with open(2),
+/* except that the O_TRUNC flag is ignored. edit_file_open()
+/* always truncates the output file after it has obtained
+/* exclusive control over the output file content and temporary
+/* pathname.
+/* .IP output_mode
+/* Permissions for the output file. These are as with open(2),
+/* except that the output file is initially created with no
+/* group or other access permissions. The specified output
+/* file permissions are applied by edit_file_close().
+/* .IP edit_file
+/* Pointer to data structure that is returned upon successful
+/* completion by edit_file_open(), and that must be passed to
+/* edit_file_close() or edit_file_cleanup().
+/* DIAGNOSTICS
+/* Fatal errors: memory allocation failure, fstat() failure,
+/* unlink() failure, lock failure, ftruncate() failure.
+/*
+/* edit_file_open() immediately returns a null pointer when
+/* it cannot open the output file.
+/*
+/* edit_file_close() returns zero on success, VSTREAM_EOF on
+/* failure.
+/*
+/* With both functions, the global errno variable indicates
+/* the nature of the problem. All errors are relative to the
+/* temporary output's pathname. With both functions, this
+/* pathname is not available via the EDIT_FILE data structure,
+/* because that structure was already destroyed, or not created.
+/* BUGS
+/* In the non-error case, edit_file_open() will not return
+/* until it obtains exclusive control over the output file
+/* content and temporary pathname. Applications that are
+/* concerned about deadlock should protect the edit_file_open()
+/* call with a watchdog timer.
+/*
+/* When interrupted, edit_file_close() may leave behind a
+/* world-readable output file under the temporary pathname.
+/* On some systems this can be used to inflict a shared-lock
+/* DOS on the protocol. Applications that are concerned about
+/* maximal safety should protect the edit_file_close() call
+/* with sigdelay() and sigresume() calls, but this introduces
+/* the risk that the program will get stuck forever.
+/* LICENSE
+/* .ad
+/* .fi
+/* The Secure Mailer license must be distributed with this software.
+/* AUTHOR(S)
+/* Based on code originally by:
+/* Victor Duchovni
+/* Morgan Stanley
+/*
+/* Packaged into one module with minor improvements by:
+/* Wietse Venema
+/* IBM T.J. Watson Research
+/* P.O. Box 704
+/* Yorktown Heights, NY 10598, USA
+/*--*/
+
+/* System library. */
+
+#include <sys_defs.h>
+#include <sys/stat.h>
+#include <stdio.h> /* rename(2) */
+#include <errno.h>
+
+ /*
+ * This mask selects all permission bits in the st_mode stat data. There is
+ * no portable definition (unlike S_IFMT, which is defined for the file type
+ * bits). For example, BSD / Linux have ALLPERMS, while Solaris has S_IAMB.
+ */
+#define FILE_PERM_MASK \
+ (S_ISUID | S_ISGID | S_ISVTX | S_IRWXU | S_IRWXG | S_IRWXO)
+
+/* Utility Library. */
+
+#include <msg.h>
+#include <vstream.h>
+#include <mymalloc.h>
+#include <stringops.h>
+#include <myflock.h>
+#include <edit_file.h>
+#include <warn_stat.h>
+
+ /*
+ * Do we reuse and truncate an output file that persists after a crash, or
+ * do we unlink it and create a new file?
+ */
+#define EDIT_FILE_REUSE_AFTER_CRASH
+
+ /*
+ * Protocol internals: the temporary file permissions.
+ */
+#define EDIT_FILE_MODE (S_IRUSR | S_IWUSR) /* temp file mode */
+
+ /*
+ * Make complex operations more readable. We could use functions, instead.
+ * The main thing is that we keep the _alloc and _free code together.
+ */
+#define EDIT_FILE_ALLOC(ep, path, mode) do { \
+ (ep) = (EDIT_FILE *) mymalloc(sizeof(EDIT_FILE)); \
+ (ep)->final_path = mystrdup(path); \
+ (ep)->final_mode = (mode); \
+ (ep)->tmp_path = concatenate((path), EDIT_FILE_SUFFIX, (char *) 0); \
+ (ep)->tmp_fp = 0; \
+ } while (0)
+
+#define EDIT_FILE_FREE(ep) do { \
+ myfree((ep)->final_path); \
+ myfree((ep)->tmp_path); \
+ myfree((void *) (ep)); \
+ } while (0)
+
+/* edit_file_open - open and lock file with deterministic temporary pathname */
+
+EDIT_FILE *edit_file_open(const char *path, int flags, mode_t mode)
+{
+ struct stat before_lock;
+ struct stat after_lock;
+ int saved_errno;
+ EDIT_FILE *ep;
+
+ /*
+ * Initialize. Do not bother to optimize for the error case.
+ */
+ EDIT_FILE_ALLOC(ep, path, mode);
+
+ /*
+ * As long as the output file can be opened under the temporary pathname,
+ * this code can loop or block forever.
+ *
+ * Applications that are concerned about deadlock should protect the
+ * edit_file_open() call with a watchdog timer.
+ */
+ for ( /* void */ ; /* void */ ; (void) vstream_fclose(ep->tmp_fp)) {
+
+ /*
+ * Try to open the output file under the temporary pathname. This
+ * succeeds or fails immediately. To avoid creating a shared-lock DOS
+ * opportunity after we crash, we create the output file with no
+ * group or other permissions, and set the final permissions at the
+ * end (this is one reason why we try to get exclusive control over
+ * the output file instead of the original file). We postpone file
+ * truncation until we have obtained exclusive control over the file
+ * content and temporary pathname. If the open operation fails, we
+ * give up immediately. The caller can retry the call if desirable.
+ *
+ * XXX If we replace the vstream_fopen() call by safe_open(), then we
+ * should replace the stat() call below by lstat().
+ */
+ if ((ep->tmp_fp = vstream_fopen(ep->tmp_path, flags & ~(O_TRUNC),
+ EDIT_FILE_MODE)) == 0) {
+ saved_errno = errno;
+ EDIT_FILE_FREE(ep);
+ errno = saved_errno;
+ return (0);
+ }
+
+ /*
+ * At this point we may have opened an existing output file that was
+ * already locked. Try to lock the open file exclusively. This may
+ * take some time.
+ */
+ if (myflock(vstream_fileno(ep->tmp_fp), INTERNAL_LOCK,
+ MYFLOCK_OP_EXCLUSIVE) < 0)
+ msg_fatal("lock %s: %m", ep->tmp_path);
+
+ /*
+ * At this point we have an exclusive lock, but some other process
+ * may have renamed or removed the output file while we were waiting
+ * for the lock. If that is the case, back out and try again.
+ */
+ if (fstat(vstream_fileno(ep->tmp_fp), &before_lock) < 0)
+ msg_fatal("open %s: %m", ep->tmp_path);
+ if (stat(ep->tmp_path, &after_lock) < 0
+ || before_lock.st_dev != after_lock.st_dev
+ || before_lock.st_ino != after_lock.st_ino
+#ifdef HAS_ST_GEN
+ || before_lock.st_gen != after_lock.st_gen
+#endif
+ /* No need to compare st_rdev or st_nlink here. */
+ ) {
+ continue;
+ }
+
+ /*
+ * At this point we have exclusive control over the output file
+ * content and its temporary pathname (within the rules of the
+ * cooperative protocol). But wait, there is more.
+ *
+ * There are many opportunities for trouble when opening a pre-existing
+ * output file. Here are just a few.
+ *
+ * - Victor observes that a system crash in the middle of the
+ * final-phase rename() operation may result in the output file
+ * having both the temporary pathname and the final pathname. In that
+ * case we must not write to the output file.
+ *
+ * - Wietse observes that crashes may also leave the output file in
+ * other inconsistent states. To avoid permission-related trouble, we
+ * simply refuse to work with an output file that has the wrong
+ * temporary permissions. This won't stop the shared-lock DOS if we
+ * crash after changing the file permissions, though.
+ *
+ * To work around these crash-related problems, remove the temporary
+ * pathname, back out, and try again.
+ */
+ if (!S_ISREG(after_lock.st_mode)
+#ifndef EDIT_FILE_REUSE_AFTER_CRASH
+ || after_lock.st_size > 0
+#endif
+ || after_lock.st_nlink > 1
+ || (after_lock.st_mode & FILE_PERM_MASK) != EDIT_FILE_MODE) {
+ if (unlink(ep->tmp_path) < 0 && errno != ENOENT)
+ msg_fatal("unlink %s: %m", ep->tmp_path);
+ continue;
+ }
+
+ /*
+ * Settle the final details.
+ */
+#ifdef EDIT_FILE_REUSE_AFTER_CRASH
+ if (ftruncate(vstream_fileno(ep->tmp_fp), 0) < 0)
+ msg_fatal("truncate %s: %m", ep->tmp_path);
+#endif
+ return (ep);
+ }
+}
+
+/* edit_file_cleanup - clean up without completing the protocol */
+
+void edit_file_cleanup(EDIT_FILE *ep)
+{
+
+ /*
+ * Don't touch the file after we lose the exclusive lock!
+ */
+ if (unlink(ep->tmp_path) < 0 && errno != ENOENT)
+ msg_fatal("unlink %s: %m", ep->tmp_path);
+ (void) vstream_fclose(ep->tmp_fp);
+ EDIT_FILE_FREE(ep);
+}
+
+/* edit_file_close - rename the file into place and close the file */
+
+int edit_file_close(EDIT_FILE *ep)
+{
+ VSTREAM *fp = ep->tmp_fp;
+ int fd = vstream_fileno(fp);
+ int saved_errno;
+
+ /*
+ * The rename/unlock portion of the protocol is relatively simple. The
+ * only things that really matter here are that we change permissions as
+ * late as possible, and that we rename the file to its final pathname
+ * before we lose the exclusive lock.
+ *
+ * Applications that are concerned about maximal safety should protect the
+ * edit_file_close() call with sigdelay() and sigresume() calls. It is
+ * not safe for us to call these functions directly, because the calls do
+ * not nest. It is also not nice to force every caller to run with
+ * interrupts turned off.
+ */
+ if (vstream_fflush(fp) < 0
+ || fchmod(fd, ep->final_mode) < 0
+#ifdef HAS_FSYNC
+ || fsync(fd) < 0
+#endif
+ || rename(ep->tmp_path, ep->final_path) < 0) {
+ saved_errno = errno;
+ edit_file_cleanup(ep);
+ errno = saved_errno;
+ return (VSTREAM_EOF);
+ } else {
+ (void) vstream_fclose(ep->tmp_fp);
+ EDIT_FILE_FREE(ep);
+ return (0);
+ }
+}
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