[Tarantool-patches] [PATCH luajit][v2] LJ_GC64: Fix lua_concat().

[Maxim Kokryashkin]

Hi, Sergey!
Thanks for the patch!
LGTM
 
 
--
Best regards,
Maxim Kokryashkin
 
  
>Пятница, 6 октября 2023, 17:16 +03:00 от Sergey Bronnikov via Tarantool-patches <tarantool-patches at dev.tarantool.org>:
> 
>From: Sergey Bronnikov < sergeyb at tarantool.org >
>
>Reported by Mathias Westerdahl.
>
>(cherry picked from commit 633f265f67f322cbe2c5fd11d3e46d968ac220f7)
>
>Lua 5.1 Reference Manual [1] defines a function `lua_concat`, that:
> 
>> void lua_concat (lua_State *L, int n);
>>
>> Concatenates the n values at the top of the stack, pops them, and leaves
>> the result at the top.
>Without the patch `lua_concat()` behaved incorrectly with userdata with
>defined `__concat` metamethod. The problem is GC64-specific.
>
>Assuming we have three literals and a userdata with defined "__concat"
>metamethod on top of the Lua stack:
>
>1 [string]
>2 [string]
>3 [string]
>4 [string]
>5 [userdata] <--- top
>
>On attempt to concatenate *two* items on top of the Lua stack,
>`lua_concat()` concatenates *four* items and leaves result on the top:
>
>1 [string]
>2 [string][string][string][userdata] <--- top
>
>The problem is in incorrect calculation of `n` counter in the loop in
>implementation of function `lua_concat`. Without the fix `n` is equal to
>3 at the end of the first iteration and therefore it goes to the next
>iteration of concatenation. In the fixed implementation of
>`lua_concat()` `n` is equal to 1 at the end of the first loop iteration,
>decremented in a loop postcondition and breaks the loop.
>
>The patch fixes incorrect behaviour.
>
>1.  https://www.lua.org/manual/5.1/manual.html
>
>Sergey Bronnikov:
>* added the description and the test for the problem
>
>Part of tarantool/tarantool#8825
>---
>PR:  https://github.com/tarantool/tarantool/pull/9176
>Branch:  https://github.com/tarantool/luajit/commits/ligurio/lj-881-fix-concat
>Issues:
>-  https://github.com/LuaJIT/LuaJIT/issues/881
>-  https://github.com/tarantool/tarantool/issues/8825
>
>Changes:
>- fixed typos
>- updated assertions (bool asserts replaced with int assertions)
>- replaced userdata with struct with userdata with integer per Max's request
>that's all, folks
>
> src/lj_api.c | 2 +-
> .../lj-881-fix-lua-concat.test.c | 95 +++++++++++++++++++
> 2 files changed, 96 insertions(+), 1 deletion(-)
> create mode 100644 test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
>
>diff --git a/src/lj_api.c b/src/lj_api.c
>index 05e02029..3bacad33 100644
>--- a/src/lj_api.c
>+++ b/src/lj_api.c
>@@ -801,7 +801,7 @@ LUA_API void lua_concat(lua_State *L, int n)
>  L->top -= n;
>  break;
>       }
>- n -= (int)(L->top - top);
>+ n -= (int)(L->top - (top - 2*LJ_FR2));
>       L->top = top+2;
>       jit_secure_call(L, top, 1+1);
>       L->top -= 1+LJ_FR2;
>diff --git a/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c b/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
>new file mode 100644
>index 00000000..1b1f146f
>--- /dev/null
>+++ b/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
>@@ -0,0 +1,95 @@
>+#include <string.h>
>+
>+#include "lua.h"
>+#include "lauxlib.h"
>+
>+#include "test.h"
>+#include "utils.h"
>+
>+/*
>+ * This test demonstrates LuaJIT's incorrect behaviour, when
>+ * calling `lua_concat()` with userdata with __concat metamethod.
>+ * See  https://github.com/LuaJIT/LuaJIT/issues/881 for details.
>+ */
>+
>+#define MT_NAME "int"
>+
>+static int INFO_concat(lua_State *L)
>+{
>+ const char *s = luaL_checkstring(L, 1);
>+ int *n = (int *)luaL_checkudata(L, 2, MT_NAME);
>+ /* Do non-default concatenation. */
>+ lua_pushfstring(L, "%s + %d", s, *n);
>+ return 1;
>+}
>+
>+static const luaL_Reg mt[] =
>+{
>+ {"__concat", INFO_concat},
>+ {NULL, NULL}
>+};
>+
>+static int lua_concat_testcase(void *test_state)
>+{
>+ /* Setup. */
>+ lua_State *L = test_state;
>+ const int top = 4;
>+
>+ /* Create metatable and put it to the Lua registry. */
>+ luaL_newmetatable(L, MT_NAME);
>+ /* Fill metatable. */
>+ luaL_register(L, 0, mt);
>+ lua_pop(L, 1);
>+
>+ assert_int_equal(lua_gettop(L), 0);
>+
>+ lua_pushliteral(L, "C");
>+ lua_pushliteral(L, "B");
>+ lua_pushliteral(L, "A");
>+
>+ int *n = (int *)lua_newuserdata(L, sizeof(int));
>+ *n = 100;
>+
>+ luaL_getmetatable(L, MT_NAME);
>+ lua_setmetatable(L, -2);
>+
>+ assert_int_equal(lua_gettop(L), top);
>+
>+ /* Test body. */
>+
>+ /*
>+ * void lua_concat (lua_State *L, int n);
>+ *
>+ * Concatenates the n values at the top of the stack,
>+ * pops them, and leaves the result at the top. If n is 1,
>+ * the result is the single value on the stack; if n is 0,
>+ * the result is the empty string [1].
>+ *
>+ * 1.  https://www.lua.org/manual/5.1/manual.html
>+ */
>+
>+ /* Concatenate two elements on top. */
>+ lua_concat(L, 2);
>+
>+ const char *str = lua_tostring(L, -1);
>+ assert_int_equal(lua_gettop(L), top - 2 + 1);
>+ const char expected_str[] = "A + 100";
>+ assert_str_equal(str, expected_str, strlen(expected_str));
>+
>+ /* Teardown. */
>+ lua_settop(L, 0);
>+
>+ return TEST_EXIT_SUCCESS;
>+}
>+
>+int main(void)
>+{
>+ lua_State *L = utils_lua_init();
>+ const struct test_unit tgroup[] = {
>+ test_unit_def(lua_concat_testcase),
>+ };
>+ const int test_result = test_run_group(tgroup, L);
>+ utils_lua_close(L);
>+
>+ return test_result;
>+}
>--
>2.34.1
 
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