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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 opportunies 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 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);
}
}