vcs-svn: guard against overflow when computing preimage length

Signed integer overflow produces undefined behavior in C and off_t is
a signed type.  For predictable behavior, add some checks to protect
in advance against overflow.

On 32-bit systems ftell as called by buffer_tmpfile_prepare_to_read
is likely to fail with EOVERFLOW when reading the corresponding
postimage, and this patch does not fix that.  So it's more of a
futureproofing measure than a complete fix.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

vcs-svn/fast_export.c

@@ -166,6 +166,7 @@ static int ends_with(const char *s, size_t len, const char *suffix)
 static int parse_cat_response_line(const char *header, off_t *len)
 {
 	size_t headerlen = strlen(header);
+	uintmax_t n;
 	const char *type;
 	const char *end;
 
@@ -174,14 +175,25 @@ static int parse_cat_response_line(const char *header, off_t *len)
 	type = memmem(header, headerlen, " blob ", strlen(" blob "));
 	if (!type)
 		return error("cat-blob header has wrong object type: %s", header);
-	*len = strtoumax(type + strlen(" blob "), (char **) &end, 10);
+	n = strtoumax(type + strlen(" blob "), (char **) &end, 10);
 	if (end == type + strlen(" blob "))
 		return error("cat-blob header does not contain length: %s", header);
+	if (memchr(type + strlen(" blob "), '-', end - type - strlen(" blob ")))
+		return error("cat-blob header contains negative length: %s", header);
+	if (n == UINTMAX_MAX || n > maximum_signed_value_of_type(off_t))
+		return error("blob too large for current definition of off_t");
+	*len = n;
 	if (*end)
 		return error("cat-blob header contains garbage after length: %s", header);
 	return 0;
 }
 
+static void check_preimage_overflow(off_t a, off_t b)
+{
+	if (signed_add_overflows(a, b))
+		die("blob too large for current definition of off_t");
+}
+
 static long apply_delta(off_t len, struct line_buffer *input,
 			const char *old_data, uint32_t old_mode)
 {
@@ -204,6 +216,7 @@ static long apply_delta(off_t len, struct line_buffer *input,
 	}
 	if (old_mode == REPO_MODE_LNK) {
 		strbuf_addstr(&preimage.buf, "link ");
+		check_preimage_overflow(preimage_len, strlen("link "));
 		preimage_len += strlen("link ");
 	}
 	if (svndiff0_apply(input, len, &preimage, out))