Re: [PATCH 7/7] exec: Implement kernel_execve

[Eric W. Biederman]

Christoph Hellwig <hch@xxxxxxxxxxxxx> writes:

>> +static int count_strings_kernel(const char *const *argv)
>> +{
>> +	int i;
>> +
>> +	if (!argv)
>> +		return 0;
>> +
>> +	for (i = 0; argv[i]; ++i) {
>> +		if (i >= MAX_ARG_STRINGS)
>> +			return -E2BIG;
>> +		if (fatal_signal_pending(current))
>> +			return -ERESTARTNOHAND;
>> +		cond_resched();
>
> I don't think we need a fatal_signal_pending and cond_resched() is
> needed in each step given that we don't actually do anything.

If we have a MAX_ARG_STRINGS sized argv passed in, that is 2^31
iterations of the loop.  A processor at 2Ghz performs roughly 2^31
cycles per second.  So this loop has the potential to run for an entire
second.  That is long enough to need fatal_signal_pending() and
cond_resched checks.

In practice I don't think we have any argv arrays anywhere near that big
passed in from the kernel.  However removing the logic that accounts for
long running loops is best handled as a separate change so that people
will analyze the patch based on that criterian, and so that in the
highly unlikely even something goes wrong people have a nice commit
to bisect things to.

Eric