Re: [Tarantool-patches] [PATCH luajit] Prevent sanitizer warning in os.time().

[Evgeniy Temirgaleev via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi, Sergey! Thanks for the patch!

Please, see my comment.

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Best regards,
Evgeniy Temirgaleev

> 
> From: Sergey Bronnikov <estetus@gmail.com>
> To: tarantool-patches@dev.tarantool.org, Sergey Kaplun <skaplun@tarantool.org
> >, e.temirgaleev@tarantool.org
> Date: Friday, June 5, 2026 6:19 PM +03:00
> From: Mike Pall <mike>
> 
> Reported by Sergey Bronnikov.
> 
> (cherry picked from commit 86d414f5cae062b06998ec66b0696a47d4f6a0f0)
> 
> The function `lj_cf_os_time()` calculates `tm_mon` and `tm_year`
> values using `get_field()` and when the helper function returns
> a negative value the resulted values may be negative as well. This
> is an undefined behaviour (signed integer overflow). The patch fixes
> that by adding a cast for the resulted value to returned by
> `get_field()`.
> 
> Sergey Bronnikov:
> * added the description and the test for the problem
> 
> Part of tarantool/tarantool#12480
> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/ligurio/lj-1454-ub-os-time
> 
> 
> Related issues:
> 
> * https://github.com/tarantool/tarantool/issues/12480
> * https://github.com/LuaJIT/LuaJIT/issues/1454
> 
> src/lib_os.c | 4 ++--
> test/tarantool-tests/lj-1454-os-time.test.lua | 19 +++++++++++++++++++
> 2 files changed, 21 insertions(+), 2 deletions(-)
> create mode 100644 test/tarantool-tests/lj-1454-os-time.test.lua
> 
> diff --git a/src/lib_os.c b/src/lib_os.c
> index ffbc3fdc..0feb0d47 100644
> --- a/src/lib_os.c
> +++ b/src/lib_os.c
> @@ -239,8 +239,8 @@ LJLIB_CF(os_time)
> ts.tm_min = getfield(L, "min", 0);
> ts.tm_hour = getfield(L, "hour", 12);
> ts.tm_mday = getfield(L, "day", -1);
> - ts.tm_mon = getfield(L, "month", -1) - 1;
> - ts.tm_year = getfield(L, "year", -1) - 1900;
> + ts.tm_mon = (int)((unsigned int)getfield(L, "month", -1) - 1u);
> + ts.tm_year = (int)((unsigned int)getfield(L, "year", -1) - 1900u);
> 

We just suppress signed integer-overflow warning implicitly by casting (and get an unsigned integer-overflow warning instead — try `luajit -e "os.time({ day = 0, month = 0, year = 0 })"` with clang and -fsanitize=integer).

The ignored warning is meaningful in a case, when we get a valid input for mktime() and it returns a valid output for it args, but that result is invalid for args of os.time() due to integer overflow.

I think, the range checks for Lua os.time() args are needed to provide predictable error response. At least, a check for year arg to prevent the invalid result of os.time().

Where are some example calls below (Ubuntu 24, amd64) with some debug printing.

*Not patched version — warning + invalid result*
/src/src/luajit -e "print(os.time({ day = 1, month = 1, year = -2^31 }),'\n')"
lib_os.c:244:42: runtime error: signed integer overflow: -2147483648 - 1900 cannot be represented in type 'int'
lj_cf_os_time: (1) <- ts.tm_mday=1 ts.tm_mon=0 ts.tm_year=2147481748
lj_cf_os_time: (2) -> ts.tm_mday=1 ts.tm_mon=0 ts.tm_year=2147481748 t=67767976233576000 errno=0
6.7767976233576e+16

*Patched version — invalid result only*
/src/src/luajit -e "print(os.time({ day = 1, month = 1, year = -2^31 }),'\n')"
lj_cf_os_time: (1) <- getfield(month)=1 getfield(year)=-2147483648 (u)getfield(month)=0 (u)getfield(year)=2147481748 ts.tm_mday=1 ts.tm_mon=0 ts.tm_year=2147481748
lj_cf_os_time: (2) -> ts.tm_mday=1 ts.tm_mon=0 ts.tm_year=2147481748 t=67767976233576000 errno=0
6.7767976233576e+16

Some more calls for example: the mktime() may ignore invalid tm_mday, tm_mon values of struct tm and update them in a «no error» case.

*Not patched version*

/src/src/luajit -e "print(os.time({ day = -2^31, month = -2^31, year = -2^31+1900 }),'\n')"
lib_os.c:243:42: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
lj_cf_os_time: (1) ts.tm_mday=-2147483648 ts.tm_mon=2147483647 ts.tm_year=-2147483648
lj_cf_os_time: (2) ts.tm_mday=20 ts.tm_mon=0 ts.tm_year=-1974406288 t=-62306246666232000 errno=0
-6.2306246666232e+16

/src/src/luajit -e "print(os.time({ day = -2^31, month = -2^31, year = -2^31+1899 }),'\n')"
lib_os.c:243:42: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
lib_os.c:244:42: runtime error: signed integer overflow: -2147481749 - 1900 cannot be represented in type 'int'
lj_cf_os_time: (1) ts.tm_mday=-2147483648 ts.tm_mon=2147483647 ts.tm_year=2147483647
lj_cf_os_time: (2) ts.tm_mday=-2147483648 ts.tm_mon=2147483647 ts.tm_year=2147483647 t=-1 errno=75
nil

*Patched version*

/src/src/luajit -e "print(os.time({ day = -2^31, month = -2^31, year = -2^31+1900 }),'\n')"
lj_cf_os_time: (1) <- getfield(month)=-2147483648 getfield(year)=-2147481748 (u)getfield(month)=2147483647 (u)getfield(year)=2147483648 ts.tm_mday=-2147483648 ts.tm_mon=2147483647 ts.tm_year=-2147483648
lj_cf_os_time: (2) -> ts.tm_mday=20 ts.tm_mon=0 ts.tm_year=-1974406288 t=-62306246666232000 errno=0
-6.2306246666232e+16

/src/src/luajit -e "print(os.time({ day = -2^31, month = -2^31, year = -2^31+1899 }),'\n')"
lj_cf_os_time: (1) <- getfield(month)=-2147483648 getfield(year)=-2147481749 (u)getfield(month)=2147483647 (u)getfield(year)=2147483647 ts.tm_mday=-2147483648 ts.tm_mon=2147483647 ts.tm_year=2147483647
lj_cf_os_time: (2) -> ts.tm_mday=-2147483648 ts.tm_mon=2147483647 ts.tm_year=2147483647 t=-1 errno=75
nil

> 
> ts.tm_isdst = getboolfield(L, "isdst");
> t = mktime(&ts);
> }
> diff --git a/test/tarantool-tests/lj-1454-os-time.test.lua
> b/test/tarantool-tests/lj-1454-os-time.test.lua
> new file mode 100644
> index 00000000..2a48750c
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1454-os-time.test.lua
> @@ -0,0 +1,19 @@
> +local tap = require('tap')
> +
> +-- The test file to demonstrate UBSan warning for `os.time()` with
> +-- a huge indices value for month and/or year.
> +-- See also: https://github.com/LuaJIT/LuaJIT/issues/1454.
> +local test = tap.test('lj-1454-os-time')
> +
> +test:plan(1)
> +
> +local INT_MIN = -2 ^ 31
> +
> +local cur_time = os.time({
> + day = 1,
> + month = INT_MIN,
> + year = INT_MIN,
> +})
> +test:is(cur_time, nil, 'os.time() with INT_MIN')
> +
> +test:done(true)
> --
> 2.43.0
>

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