odb_mkstemp: write filename into strbuf

The odb_mkstemp() function expects the caller to provide a
fixed buffer to write the resulting tempfile name into. But
it creates the template using snprintf without checking the
return value. This means we could silently truncate the
filename.

In practice, it's unlikely that the truncation would end in
the template-pattern that mkstemp needs to open the file. So
we'd probably end up failing either way, unless the path was
specially crafted.

The simplest fix would be to notice the truncation and die.
However, we can observe that most callers immediately
xstrdup() the result anyway. So instead, let's switch to
using a strbuf, which is easier for them (and isn't a big
deal for the other 2 callers, who can just strbuf_release
when they're done with it).

Note that many of the callers used static buffers, but this
was purely to avoid putting a large buffer on the stack. We
never passed the static buffers out of the function, so
there's no complicated memory handling we need to change.

Signed-off-by: Jeff King <peff@peff.net>

builtin/index-pack.c

@@ -307,10 +307,10 @@ static const char *open_pack_file(const char *pack_name)
 	if (from_stdin) {
 		input_fd = 0;
 		if (!pack_name) {
-			static char tmp_file[PATH_MAX];
-			output_fd = odb_mkstemp(tmp_file, sizeof(tmp_file),
+			struct strbuf tmp_file = STRBUF_INIT;
+			output_fd = odb_mkstemp(&tmp_file,
 						"pack/tmp_pack_XXXXXX");
-			pack_name = xstrdup(tmp_file);
+			pack_name = strbuf_detach(&tmp_file, NULL);
 		} else {
 			output_fd = open(pack_name, O_CREAT|O_EXCL|O_RDWR, 0600);
 			if (output_fd < 0)

cache.h

@@ -1679,7 +1679,7 @@ extern void pack_report(void);
  * usual "XXXXXX" trailer, and the resulting filename is written into the
  * "template" buffer. Returns the open descriptor.
  */
-extern int odb_mkstemp(char *template, size_t limit, const char *pattern);
+extern int odb_mkstemp(struct strbuf *template, const char *pattern);
 
 /*
  * Generate the filename to be used for a pack file with checksum "sha1" and

environment.c

@@ -274,7 +274,7 @@ char *get_object_directory(void)
 	return git_object_dir;
 }
 
-int odb_mkstemp(char *template, size_t limit, const char *pattern)
+int odb_mkstemp(struct strbuf *template, const char *pattern)
 {
 	int fd;
 	/*
@@ -282,18 +282,18 @@ int odb_mkstemp(char *template, size_t limit, const char *pattern)
 	 * restrictive except to remove write permission.
 	 */
 	int mode = 0444;
-	snprintf(template, limit, "%s/%s",
-		 get_object_directory(), pattern);
-	fd = git_mkstemp_mode(template, mode);
+	strbuf_reset(template);
+	strbuf_addf(template, "%s/%s", get_object_directory(), pattern);
+	fd = git_mkstemp_mode(template->buf, mode);
 	if (0 <= fd)
 		return fd;
 
 	/* slow path */
 	/* some mkstemp implementations erase template on failure */
-	snprintf(template, limit, "%s/%s",
-		 get_object_directory(), pattern);
-	safe_create_leading_directories(template);
-	return xmkstemp_mode(template, mode);
+	strbuf_reset(template);
+	strbuf_addf(template, "%s/%s", get_object_directory(), pattern);
+	safe_create_leading_directories(template->buf);
+	return xmkstemp_mode(template->buf, mode);
 }
 
 int odb_pack_keep(const char *name)

fast-import.c

@@ -890,14 +890,15 @@ static struct tree_content *dup_tree_content(struct tree_content *s)
 
 static void start_packfile(void)
 {
-	static char tmp_file[PATH_MAX];
+	struct strbuf tmp_file = STRBUF_INIT;
 	struct packed_git *p;
 	struct pack_header hdr;
 	int pack_fd;
 
-	pack_fd = odb_mkstemp(tmp_file, sizeof(tmp_file),
-			      "pack/tmp_pack_XXXXXX");
-	FLEX_ALLOC_STR(p, pack_name, tmp_file);
+	pack_fd = odb_mkstemp(&tmp_file, "pack/tmp_pack_XXXXXX");
+	FLEX_ALLOC_STR(p, pack_name, tmp_file.buf);
+	strbuf_release(&tmp_file);
+
 	p->pack_fd = pack_fd;
 	p->do_not_close = 1;
 	pack_file = sha1fd(pack_fd, p->pack_name);

pack-bitmap-write.c

@@ -508,16 +508,16 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 			  const char *filename,
 			  uint16_t options)
 {
-	static char tmp_file[PATH_MAX];
 	static uint16_t default_version = 1;
 	static uint16_t flags = BITMAP_OPT_FULL_DAG;
+	struct strbuf tmp_file = STRBUF_INIT;
 	struct sha1file *f;
 
 	struct bitmap_disk_header header;
 
-	int fd = odb_mkstemp(tmp_file, sizeof(tmp_file), "pack/tmp_bitmap_XXXXXX");
+	int fd = odb_mkstemp(&tmp_file, "pack/tmp_bitmap_XXXXXX");
 
-	f = sha1fd(fd, tmp_file);
+	f = sha1fd(fd, tmp_file.buf);
 
 	memcpy(header.magic, BITMAP_IDX_SIGNATURE, sizeof(BITMAP_IDX_SIGNATURE));
 	header.version = htons(default_version);
@@ -537,9 +537,11 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 
 	sha1close(f, NULL, CSUM_FSYNC);
 
-	if (adjust_shared_perm(tmp_file))
+	if (adjust_shared_perm(tmp_file.buf))
 		die_errno("unable to make temporary bitmap file readable");
 
-	if (rename(tmp_file, filename))
+	if (rename(tmp_file.buf, filename))
 		die_errno("unable to rename temporary bitmap file to '%s'", filename);
+
+	strbuf_release(&tmp_file);
 }

pack-write.c

@@ -71,9 +71,9 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 		f = sha1fd_check(index_name);
 	} else {
 		if (!index_name) {
-			static char tmp_file[PATH_MAX];
-			fd = odb_mkstemp(tmp_file, sizeof(tmp_file), "pack/tmp_idx_XXXXXX");
-			index_name = xstrdup(tmp_file);
+			struct strbuf tmp_file = STRBUF_INIT;
+			fd = odb_mkstemp(&tmp_file, "pack/tmp_idx_XXXXXX");
+			index_name = strbuf_detach(&tmp_file, NULL);
 		} else {
 			unlink(index_name);
 			fd = open(index_name, O_CREAT|O_EXCL|O_WRONLY, 0600);
@@ -329,11 +329,11 @@ int encode_in_pack_object_header(unsigned char *hdr, int hdr_len,
 
 struct sha1file *create_tmp_packfile(char **pack_tmp_name)
 {
-	char tmpname[PATH_MAX];
+	struct strbuf tmpname = STRBUF_INIT;
 	int fd;
 
-	fd = odb_mkstemp(tmpname, sizeof(tmpname), "pack/tmp_pack_XXXXXX");
-	*pack_tmp_name = xstrdup(tmpname);
+	fd = odb_mkstemp(&tmpname, "pack/tmp_pack_XXXXXX");
+	*pack_tmp_name = strbuf_detach(&tmpname, NULL);
 	return sha1fd(fd, *pack_tmp_name);
 }