Re: [Tarantool-patches] [PATCH luajit] FFI: Shrink container of packed bitfield.

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hello, Sergey,

thanks for the patch! LGTM

Sergey

On 7/20/26 15:59, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by Huang Haiyang.
>
> (cherry picked from commit e4c7d8b38040518d42599eef8ddb5e67aa967a9c)
>
> The bitfield of packed structure uses the original type size, which
> leads to heap-buffer-overflow access on conversions.
>
> This patch fixes it by adjusting the size of the bitfield container when
> necessary.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#12880
> ---
>
> Branch:https://github.com/tarantool/luajit/tree/skaplun/lj-1451-ffi-packed-bitfield
> Related issues:
> *https://github.com/LuaJIT/LuaJIT/issues/1451
> *https://github.com/tarantool/tarantool/issues/12880
>
>   src/lj_cparse.c                               |  2 ++
>   .../lj-1451-ffi-packed-bitfield.test.lua      | 28 +++++++++++++++++++
>   2 files changed, 30 insertions(+)
>   create mode 100644 test/tarantool-tests/lj-1451-ffi-packed-bitfield.test.lua
>
> diff --git a/src/lj_cparse.c b/src/lj_cparse.c
> index ff23b44b..1b3ce7ec 100644
> --- a/src/lj_cparse.c
> +++ b/src/lj_cparse.c
> @@ -1341,6 +1341,8 @@ static void cp_struct_layout(CPState *cp, CTypeID sid, CTInfo sattr)
>   		     CTALIGN(lj_fls(sz));
>   	  ct->size = (bofs >> 3);  /* Store field offset. */
>   	} else {
> +	  if (csz > amask+1 && bsz <= amask+1)
> +	    csz = amask+1;  /* Shrink container of packed bitfield. */
>   	  ct->info = CTINFO(CT_BITFIELD,
>   	    (info & (CTF_QUAL|CTF_UNSIGNED|CTF_BOOL)) +
>   	    (csz << (CTSHIFT_BITCSZ-3)) + (bsz << CTSHIFT_BITBSZ));
> diff --git a/test/tarantool-tests/lj-1451-ffi-packed-bitfield.test.lua b/test/tarantool-tests/lj-1451-ffi-packed-bitfield.test.lua
> new file mode 100644
> index 00000000..07e53f76
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1451-ffi-packed-bitfield.test.lua
> @@ -0,0 +1,28 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate LuaJIT's incorrect behaviour of the
> +-- `#pragma` pack directive for bitfields in structures.
> +-- See also:https://github.com/LuaJIT/LuaJIT/issues/1451.
> +
> +local test = tap.test('lj-1451-ffi-packed-bitfield')
> +
> +local ffi = require('ffi')
> +
> +ffi.cdef[[
> +#pragma pack(push, 2)
> +typedef struct {
> +  unsigned intbitfield:1;
> +} packed_struct;
> +#pragma pack(pop)
> +]]
> +
> +test:plan(2)
> +
> +local packed = ffi.new('packed_struct')
> +
> +test:is(packed.bitfield, 0, 'correct 0-initialization')
> +
> +packed.bitfield = 1
> +test:is(packed.bitfield, 1, 'bitfield set correctly')
> +
> +test:done(true)

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