[Tarantool-patches] [PATCH luajit 4/6] FFI: Fix dangling reference to CType.

[Maxim Kokryashkin]

Hi, Sergey!
Thanks for the patch!
Please consider my comments below.
On Mon, Oct 23, 2023 at 12:22:04PM +0300, Sergey Kaplun wrote:
> From: Mike Pall <mike>
> 
> (cherry-picked from commit ae533e3a6c009b5df79b11cd5787d249202fa69c)
> 
> During the conversion of a cdata function object to some cdata value in
> `lj_cconv_ct_tv()`, reallocation of `cts->tab` in `lj_ctype_intern()`
> may occur. In that case, the reference to the `CType` object becomes
> invalid. This patch saves the `CTypeID` of the given function and gets
> its `CType` again after possible reallocation.
> 
> Sergey Kaplun:
> * added the description and the test for the problem
> 
> Part of tarantool/tarantool#9145
> ---
>  src/lj_cconv.c                                |  2 +
>  .../fix-dangling-reference-to-ctype.test.lua  | 59 +++++++++++++++++++
>  2 files changed, 61 insertions(+)
>  create mode 100644 test/tarantool-tests/fix-dangling-reference-to-ctype.test.lua
> 
> diff --git a/src/lj_cconv.c b/src/lj_cconv.c
> index 37c88852..94ca93bb 100644
> --- a/src/lj_cconv.c
> +++ b/src/lj_cconv.c
<snipped>

> diff --git a/test/tarantool-tests/fix-dangling-reference-to-ctype.test.lua b/test/tarantool-tests/fix-dangling-reference-to-ctype.test.lua
> new file mode 100644
> index 00000000..c0e2c07b
> --- /dev/null
> +++ b/test/tarantool-tests/fix-dangling-reference-to-ctype.test.lua
> @@ -0,0 +1,59 @@
> +local tap = require('tap')
> +local ffi = require('ffi')
> +local test = tap.test('fix-dangling-reference-to-ctype'):skipcond({
> +  -- luacheck: no global
> +  ['Impossible to predict the value of cts->top'] = _TARANTOOL,
> +})
> +
> +test:plan(1)
> +
> +-- This test demonstrates LuaJIT's incorrect behaviour when the
> +-- reallocation of `cts->tab` strikes during the conversion of a
> +-- TValue (cdata function pointer) to a C type.
> +-- The test fails under ASAN.
Let's change the last sentence to 'Before the patch the test fails only
under ASAN' because now it is a bit misleading. 
> +
> +-- XXX: Just some C functions to be casted. There is no need to
> +-- declare their prototypes correctly.
> +ffi.cdef[[
> +  int malloc(void);
> +  int fprintf(void);
> +  int printf(void);
> +  int memset(void);
> +  int memcpy(void);
> +  int memmove(void);
> +  int getppid(void);
> +]]
> +
> +-- XXX: structure to set `cts->top` to 110.
> +local _ = ffi.new('struct {int a; long b; float c; double d;}', 0)
> +
> +-- Anchor table to prevent cdata objects from being collected.
> +local anchor = {}
> +-- Each call to this function grows `cts->top` by 3.
Please drop a comment, referring to a point in sources, so the size
of the growth becomes obvious.

> +local function save_new_func(func)
> +  anchor[#anchor + 1] = ffi.cast('void (*)(void)', func)
> +end
> +
> +save_new_func(ffi.C.malloc)  -- `cts->top` = 110
> +save_new_func(ffi.C.fprintf) -- `cts->top` = 113
> +save_new_func(ffi.C.printf)  -- `cts->top` = 116
> +save_new_func(ffi.C.memset)  -- `cts->top` = 119
> +save_new_func(ffi.C.memcpy)  -- `cts->top` = 122

Is it possible to bring us to this value of `cts->top`
with a structure?
> +
> +-- Assertions to check the `cts->top` value and step between
> +-- calls.
> +assert(ffi.typeinfo(122), 'cts->top >= 122')
> +assert(not ffi.typeinfo(123), 'cts->top < 123')
> +
> +save_new_func(ffi.C.memmove) -- `cts->top` = 125
> +
> +assert(ffi.typeinfo(125), 'cts->top >= 125')
> +assert(not ffi.typeinfo(126), 'cts->top < 126')
> +
> +-- Last call to grow `cts->top` up to 128, so this causes
> +-- `cts->tab` reallocation.
> +save_new_func(ffi.C.getppid) -- `cts->top` = 128

Should we add an extra assertion after reallocation?
> +
> +test:ok(true, 'no heap-use-after-free in lj_cconv_ct_tv')
> +
> +test:done(true)
> -- 
> 2.42.0
>