Re: [PATCH v2] bisect--helper: plug strvec leak

> Jeff King <peff@peff.net> writes:
>
>> On Fri, Oct 07, 2022 at 05:08:42PM +0200, René Scharfe wrote:
>>
>>> diff --git a/builtin/bisect--helper.c b/builtin/bisect--helper.c
>>> index 501245fac9..28ef7ec2a4 100644
>>> --- a/builtin/bisect--helper.c
>>> +++ b/builtin/bisect--helper.c
>>> @@ -765,11 +765,10 @@ static enum bisect_error bisect_start(struct bisect_terms *terms, const char **a
>>>  		strbuf_read_file(&start_head, git_path_bisect_start(), 0);
>>>  		strbuf_trim(&start_head);
>>>  		if (!no_checkout) {
>>> -			struct strvec argv = STRVEC_INIT;
>>> +			const char *argv[] = { "checkout", start_head.buf,
>>> +					       "--", NULL };
>>> 
>>> -			strvec_pushl(&argv, "checkout", start_head.buf,
>>> -				     "--", NULL);
>>> -			if (run_command_v_opt(argv.v, RUN_GIT_CMD)) {
>>> +			if (run_command_v_opt(argv, RUN_GIT_CMD)) {
>>
>> This is OK with me, but note that one thing we lose is compiler
>> protection that we remembered the trailing NULL pointer in the argv
>> array (whereas strvec_pushl() has an attribute that makes sure the last
>> argument is a NULL).
>
> The first parameter to run_command_v_opt() must be a NULL terminated
> array of strings.  argv.v[] after strvec_push*() is such a NULL
> terminated array, and is suitable to be passed to the function.
>
> That much human programmers would know.
>
> But does the compiler know that run_command_v_opt() requires a NULL
> terminated array of strings, and does it know to check that argv.v[]
> came from strvec_pushl() without any annotation in the first place?
>
> For such a check to happen, I think we need to tell the compiler
> with some annotation that the first parameter to run_command_v_opt()
> is supposed to be a NULL terminated char *[] array.
>
> In the code before the patch, strvec_pushl() is annotated to require
> the last arg to be NULL, but without following data/control flow, it
> may not be clear to the compiler that argv.v[] will be NULL terminated
> after the function returns.  If the compiler is sufficiently clever
> to figure it out, with the knowledge that run_command_v_opt() must
> be given a NULL terminated array of strings, we do have compiler
> protection to make the run_command_v_opt() safe.
>
> But if the code tells the compiler that run_command_v_opt() must be
> given a NULL terminated array of strings, it is obvious that the
> array passed by the code after the patch, i.e. argv[], is NULL
> terminated, and the compiler would be happy, as well.
>
>> Probably not that big a deal in practice. It would be nice if there was
>> a way to annotate this for the compiler, but I don't think there's any
>> attribute for "this pointer-to-pointer parameter should have a NULL at
>> the end".
>
> But the code before this patch is safe only for strvec_pushl() call,
> not run_command_v_opt() call, so we are not losing anything, I would
> think.