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[Qemu-devel] [PATCH v3 56/58] json: Improve safety of qobject_from_jsonf
From: |
Markus Armbruster |
Subject: |
[Qemu-devel] [PATCH v3 56/58] json: Improve safety of qobject_from_jsonf_nofail() & friends |
Date: |
Thu, 23 Aug 2018 18:40:23 +0200 |
The JSON parser optionally supports interpolation. This is used to
build QObjects by parsing string templates. The templates are C
literals, so parse errors (such as invalid interpolation
specifications) are actually programming errors. Consequently, the
functions providing parsing with interpolation
(qobject_from_jsonf_nofail(), qobject_from_vjsonf_nofail(),
qdict_from_jsonf_nofail(), qdict_from_vjsonf_nofail()) pass
&error_abort to the parser.
However, there's another, more dangerous kind of programming error:
since we use va_arg() to get the value to interpolate, behavior is
undefined when the variable argument isn't consistent with the
interpolation specification.
The same problem exists with printf()-like functions, and the solution
is to have the compiler check consistency. This is what
GCC_FMT_ATTR() is about.
To enable this type checking for interpolation as well, we carefully
chose our interpolation specifications to match printf conversion
specifications, and decorate functions parsing templates with
GCC_FMT_ATTR().
Note that this only protects against undefined behavior due to type
errors. It can't protect against use of invalid interpolation
specifications that happen to be valid printf conversion
specifications.
However, there's still a gaping hole in the type checking: GCC
recognizes '%' as start of printf conversion specification anywhere in
the template, but the parser recognizes it only outside JSON strings.
For instance, if someone were to pass a "{ '%s': %d }" template, GCC
would require a char * and an int argument, but the parser would
va_arg() only an int argument, resulting in undefined behavior.
Avoid undefined behavior by catching the programming error at run
time: have the parser recognize and reject '%' in JSON strings.
Signed-off-by: Markus Armbruster <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
---
qobject/json-parser.c | 12 ++++++++++--
tests/check-qjson.c | 17 +++++++----------
2 files changed, 17 insertions(+), 12 deletions(-)
diff --git a/qobject/json-parser.c b/qobject/json-parser.c
index 273f448a52..63e9229f1c 100644
--- a/qobject/json-parser.c
+++ b/qobject/json-parser.c
@@ -144,7 +144,8 @@ static QString *parse_string(JSONParserContext *ctxt,
JSONToken *token)
while (*ptr != quote) {
assert(*ptr);
- if (*ptr == '\\') {
+ switch (*ptr) {
+ case '\\':
beg = ptr++;
switch (*ptr++) {
case '"':
@@ -205,7 +206,14 @@ static QString *parse_string(JSONParserContext *ctxt,
JSONToken *token)
parse_error(ctxt, token, "invalid escape sequence in string");
goto out;
}
- } else {
+ break;
+ case '%':
+ if (ctxt->ap) {
+ parse_error(ctxt, token, "can't interpolate into string");
+ goto out;
+ }
+ /* fall through */
+ default:
cp = mod_utf8_codepoint(ptr, 6, &end);
if (cp < 0) {
parse_error(ctxt, token, "invalid UTF-8 sequence in string");
diff --git a/tests/check-qjson.c b/tests/check-qjson.c
index 936258ddd4..a1854573de 100644
--- a/tests/check-qjson.c
+++ b/tests/check-qjson.c
@@ -1037,16 +1037,13 @@ static void interpolation_unknown(void)
static void interpolation_string(void)
{
- QLitObject decoded = QLIT_QLIST(((QLitObject[]){
- QLIT_QSTR("%s"),
- QLIT_QSTR("eins"),
- {}}));
- QObject *qobj;
-
- /* Dangerous misfeature: % is silently ignored in strings */
- qobj = qobject_from_jsonf_nofail("['%s', %s]", "eins", "zwei");
- g_assert(qlit_equal_qobject(&decoded, qobj));
- qobject_unref(qobj);
+ if (g_test_subprocess()) {
+ qobject_from_jsonf_nofail("['%s', %s]", "eins", "zwei");
+ }
+ g_test_trap_subprocess(NULL, 0, 0);
+ g_test_trap_assert_failed();
+ g_test_trap_assert_stderr("*Unexpected error*"
+ "can't interpolate into string*");
}
static void simple_dict(void)
--
2.17.1
- [Qemu-devel] [PATCH v3 43/58] qjson: Fix qobject_from_json() & friends for multiple values, (continued)
- [Qemu-devel] [PATCH v3 43/58] qjson: Fix qobject_from_json() & friends for multiple values, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 48/58] json: Enforce token count and size limits more tightly, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 38/58] json: Treat unwanted interpolation as lexical error, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 57/58] json: Support %% in JSON strings when interpolating, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 53/58] json: Clean up headers, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 54/58] tests/drive_del-test: Fix harmless JSON interpolation bug, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 58/58] json: Update references to RFC 7159 to RFC 8259, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 52/58] qobject: Drop superfluous includes of qemu-common.h, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 50/58] json: Unbox tokens queue in JSONMessageParser, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 56/58] json: Improve safety of qobject_from_jsonf_nofail() & friends,
Markus Armbruster <=
- [Qemu-devel] [PATCH v3 45/58] json: Fix streamer not to ignore trailing unterminated structures, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 55/58] json: Keep interpolation state in JSONParserContext, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 40/58] json: Leave rejecting invalid interpolation to parser, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 41/58] json: Replace %I64d, %I64u by %PRId64, %PRIu64, Markus Armbruster, 2018/08/23