git/tempfile.c
Jeff King 6c003d6ffb reopen_tempfile(): truncate opened file
We provide a reopen_tempfile() function, which is in turn
used by reopen_lockfile().  The idea is that a caller may
want to rewrite the tempfile without letting go of the lock.
And that's what our one caller does: after running
add--interactive, "commit -p" will update the cache-tree
extension of the index and write out the result, all while
holding the lock.

However, because we open the file with only the O_WRONLY
flag, the existing index content is left in place, and we
overwrite it starting at position 0. If the new index after
updating the cache-tree is smaller than the original, those
final bytes are not overwritten and remain in the file. This
results in a corrupt index, since those cruft bytes are
interpreted as part of the trailing hash (or even as an
extension, if there are enough bytes).

This bug actually pre-dates reopen_tempfile(); the original
code from 9c4d6c0297 (cache-tree: Write updated cache-tree
after commit, 2014-07-13) has the same bug, and those lines
were eventually refactored into the tempfile module. Nobody
noticed until now for two reasons:

 - the bug can only be triggered in interactive mode
   ("commit -p" or "commit -i")

 - the size of the index must shrink after updating the
   cache-tree, which implies a non-trivial deletion. Notice
   that the included test actually has to create a 2-deep
   hierarchy. A single level is not enough to actually cause
   shrinkage.

The fix is to truncate the file before writing out the
second index. We can do that at the caller by using
ftruncate(). But we shouldn't have to do that. There is no
other place in Git where we want to open a file and
overwrite bytes, making reopen_tempfile() a confusing and
error-prone interface. Let's pass O_TRUNC there, which gives
callers the same state they had after initially opening the
file or lock.

It's possible that we could later add a caller that wants
something else (e.g., to open with O_APPEND). But this is
the only caller we've had in the history of the codebase.
Let's punt on doing anything more clever until another one
comes along.

Reported-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-09-05 09:46:16 -07:00

321 lines
8.3 KiB
C

/*
* State diagram and cleanup
* -------------------------
*
* If the program exits while a temporary file is active, we want to
* make sure that we remove it. This is done by remembering the active
* temporary files in a linked list, `tempfile_list`. An `atexit(3)`
* handler and a signal handler are registered, to clean up any active
* temporary files.
*
* Because the signal handler can run at any time, `tempfile_list` and
* the `tempfile` objects that comprise it must be kept in
* self-consistent states at all times.
*
* The possible states of a `tempfile` object are as follows:
*
* - Uninitialized. In this state the object's `on_list` field must be
* zero but the rest of its contents need not be initialized. As
* soon as the object is used in any way, it is irrevocably
* registered in `tempfile_list`, and `on_list` is set.
*
* - Active, file open (after `create_tempfile()` or
* `reopen_tempfile()`). In this state:
*
* - the temporary file exists
* - `active` is set
* - `filename` holds the filename of the temporary file
* - `fd` holds a file descriptor open for writing to it
* - `fp` holds a pointer to an open `FILE` object if and only if
* `fdopen_tempfile()` has been called on the object
* - `owner` holds the PID of the process that created the file
*
* - Active, file closed (after `close_tempfile_gently()`). Same
* as the previous state, except that the temporary file is closed,
* `fd` is -1, and `fp` is `NULL`.
*
* - Inactive (after `delete_tempfile()`, `rename_tempfile()`, or a
* failed attempt to create a temporary file). In this state:
*
* - `active` is unset
* - `filename` is empty (usually, though there are transitory
* states in which this condition doesn't hold). Client code should
* *not* rely on the filename being empty in this state.
* - `fd` is -1 and `fp` is `NULL`
* - the object is removed from `tempfile_list` (but could be used again)
*
* A temporary file is owned by the process that created it. The
* `tempfile` has an `owner` field that records the owner's PID. This
* field is used to prevent a forked process from deleting a temporary
* file created by its parent.
*/
#include "cache.h"
#include "tempfile.h"
#include "sigchain.h"
static VOLATILE_LIST_HEAD(tempfile_list);
static void remove_tempfiles(int in_signal_handler)
{
pid_t me = getpid();
volatile struct volatile_list_head *pos;
list_for_each(pos, &tempfile_list) {
struct tempfile *p = list_entry(pos, struct tempfile, list);
if (!is_tempfile_active(p) || p->owner != me)
continue;
if (p->fd >= 0)
close(p->fd);
if (in_signal_handler)
unlink(p->filename.buf);
else
unlink_or_warn(p->filename.buf);
p->active = 0;
}
}
static void remove_tempfiles_on_exit(void)
{
remove_tempfiles(0);
}
static void remove_tempfiles_on_signal(int signo)
{
remove_tempfiles(1);
sigchain_pop(signo);
raise(signo);
}
static struct tempfile *new_tempfile(void)
{
struct tempfile *tempfile = xmalloc(sizeof(*tempfile));
tempfile->fd = -1;
tempfile->fp = NULL;
tempfile->active = 0;
tempfile->owner = 0;
INIT_LIST_HEAD(&tempfile->list);
strbuf_init(&tempfile->filename, 0);
return tempfile;
}
static void activate_tempfile(struct tempfile *tempfile)
{
static int initialized;
if (is_tempfile_active(tempfile))
BUG("activate_tempfile called for active object");
if (!initialized) {
sigchain_push_common(remove_tempfiles_on_signal);
atexit(remove_tempfiles_on_exit);
initialized = 1;
}
volatile_list_add(&tempfile->list, &tempfile_list);
tempfile->owner = getpid();
tempfile->active = 1;
}
static void deactivate_tempfile(struct tempfile *tempfile)
{
tempfile->active = 0;
strbuf_release(&tempfile->filename);
volatile_list_del(&tempfile->list);
free(tempfile);
}
/* Make sure errno contains a meaningful value on error */
struct tempfile *create_tempfile(const char *path)
{
struct tempfile *tempfile = new_tempfile();
strbuf_add_absolute_path(&tempfile->filename, path);
tempfile->fd = open(tempfile->filename.buf,
O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, 0666);
if (O_CLOEXEC && tempfile->fd < 0 && errno == EINVAL)
/* Try again w/o O_CLOEXEC: the kernel might not support it */
tempfile->fd = open(tempfile->filename.buf,
O_RDWR | O_CREAT | O_EXCL, 0666);
if (tempfile->fd < 0) {
deactivate_tempfile(tempfile);
return NULL;
}
activate_tempfile(tempfile);
if (adjust_shared_perm(tempfile->filename.buf)) {
int save_errno = errno;
error("cannot fix permission bits on %s", tempfile->filename.buf);
delete_tempfile(&tempfile);
errno = save_errno;
return NULL;
}
return tempfile;
}
struct tempfile *register_tempfile(const char *path)
{
struct tempfile *tempfile = new_tempfile();
strbuf_add_absolute_path(&tempfile->filename, path);
activate_tempfile(tempfile);
return tempfile;
}
struct tempfile *mks_tempfile_sm(const char *filename_template, int suffixlen, int mode)
{
struct tempfile *tempfile = new_tempfile();
strbuf_add_absolute_path(&tempfile->filename, filename_template);
tempfile->fd = git_mkstemps_mode(tempfile->filename.buf, suffixlen, mode);
if (tempfile->fd < 0) {
deactivate_tempfile(tempfile);
return NULL;
}
activate_tempfile(tempfile);
return tempfile;
}
struct tempfile *mks_tempfile_tsm(const char *filename_template, int suffixlen, int mode)
{
struct tempfile *tempfile = new_tempfile();
const char *tmpdir;
tmpdir = getenv("TMPDIR");
if (!tmpdir)
tmpdir = "/tmp";
strbuf_addf(&tempfile->filename, "%s/%s", tmpdir, filename_template);
tempfile->fd = git_mkstemps_mode(tempfile->filename.buf, suffixlen, mode);
if (tempfile->fd < 0) {
deactivate_tempfile(tempfile);
return NULL;
}
activate_tempfile(tempfile);
return tempfile;
}
struct tempfile *xmks_tempfile_m(const char *filename_template, int mode)
{
struct tempfile *tempfile;
struct strbuf full_template = STRBUF_INIT;
strbuf_add_absolute_path(&full_template, filename_template);
tempfile = mks_tempfile_m(full_template.buf, mode);
if (!tempfile)
die_errno("Unable to create temporary file '%s'",
full_template.buf);
strbuf_release(&full_template);
return tempfile;
}
FILE *fdopen_tempfile(struct tempfile *tempfile, const char *mode)
{
if (!is_tempfile_active(tempfile))
BUG("fdopen_tempfile() called for inactive object");
if (tempfile->fp)
BUG("fdopen_tempfile() called for open object");
tempfile->fp = fdopen(tempfile->fd, mode);
return tempfile->fp;
}
const char *get_tempfile_path(struct tempfile *tempfile)
{
if (!is_tempfile_active(tempfile))
BUG("get_tempfile_path() called for inactive object");
return tempfile->filename.buf;
}
int get_tempfile_fd(struct tempfile *tempfile)
{
if (!is_tempfile_active(tempfile))
BUG("get_tempfile_fd() called for inactive object");
return tempfile->fd;
}
FILE *get_tempfile_fp(struct tempfile *tempfile)
{
if (!is_tempfile_active(tempfile))
BUG("get_tempfile_fp() called for inactive object");
return tempfile->fp;
}
int close_tempfile_gently(struct tempfile *tempfile)
{
int fd;
FILE *fp;
int err;
if (!is_tempfile_active(tempfile) || tempfile->fd < 0)
return 0;
fd = tempfile->fd;
fp = tempfile->fp;
tempfile->fd = -1;
if (fp) {
tempfile->fp = NULL;
if (ferror(fp)) {
err = -1;
if (!fclose(fp))
errno = EIO;
} else {
err = fclose(fp);
}
} else {
err = close(fd);
}
return err ? -1 : 0;
}
int reopen_tempfile(struct tempfile *tempfile)
{
if (!is_tempfile_active(tempfile))
BUG("reopen_tempfile called for an inactive object");
if (0 <= tempfile->fd)
BUG("reopen_tempfile called for an open object");
tempfile->fd = open(tempfile->filename.buf, O_WRONLY|O_TRUNC);
return tempfile->fd;
}
int rename_tempfile(struct tempfile **tempfile_p, const char *path)
{
struct tempfile *tempfile = *tempfile_p;
if (!is_tempfile_active(tempfile))
BUG("rename_tempfile called for inactive object");
if (close_tempfile_gently(tempfile)) {
delete_tempfile(tempfile_p);
return -1;
}
if (rename(tempfile->filename.buf, path)) {
int save_errno = errno;
delete_tempfile(tempfile_p);
errno = save_errno;
return -1;
}
deactivate_tempfile(tempfile);
*tempfile_p = NULL;
return 0;
}
void delete_tempfile(struct tempfile **tempfile_p)
{
struct tempfile *tempfile = *tempfile_p;
if (!is_tempfile_active(tempfile))
return;
close_tempfile_gently(tempfile);
unlink_or_warn(tempfile->filename.buf);
deactivate_tempfile(tempfile);
*tempfile_p = NULL;
}