Re: [PATCH 01/12] rdmacg: Replace strncmp with str_has_prefix

Michael Ellerman
2019-08-02T12:16:00Z

Chuhong Yuan <hslester96@gmail.com> writes:
> Chuhong Yuan <hslester96@gmail.com> 于2019年7月30日周二 下午2:39写道：
>> Kees Cook <keescook@chromium.org> 于2019年7月30日周二 下午12:26写道：
>> > On Mon, Jul 29, 2019 at 11:13:46PM +0800, Chuhong Yuan wrote:
>> > > strncmp(str, const, len) is error-prone.
>> > > We had better use newly introduced
>> > > str_has_prefix() instead of it.
>> >
>> > Wait, stop. :) After Laura called my attention to your conversion series,
>> > mpe pointed out that str_has_prefix() is almost redundant to strstarts()
>> > (from 2009), and the latter has many more users. Let's fix strstarts()
>> > match str_has_prefix()'s return behavior (all the existing callers are
>> > doing boolean tests, so the change in return value won't matter), and
>> > then we can continue with this replacement. (And add some documentation
>> > to Documenation/process/deprecated.rst along with a checkpatch.pl test
>> > maybe too?)
>> >
>>
>> Thanks for your advice!
>> Does that mean replacing strstarts()'s implementation with
>> str_has_prefix()'s and then use strstarts() to substitute
>> strncmp?
>>
>> I am not very clear about how to add the test into checkpatch.pl.
>> Should I write a check for this pattern or directly add strncmp into
>> deprecated_apis?
>>
>> > Actually I'd focus first on the actually broken cases first (sizeof()
>> > without the "-1", etc):
>> >
>> > $ git grep strncmp.*sizeof | grep -v -- '-' | wc -l
>> > 17
>> >
>> > I expect the "copy/paste" changes could just be a Coccinelle script that
>> > Linus could run to fix all the cases (and should be added to the kernel
>> > source's list of Coccinelle scripts). Especially since the bulk of the
>> > usage pattern are doing literals like this:
>> >
>>
>> Actually I am using a Coccinelle script to detect the cases and
>> have found 800+ places of strncmp(str, const, len).
>> But the script still needs some improvement since it has false
>> negatives and only focuses on detecting, not replacement.
>> I can upload it after improvement.
>> In which form should I upload it? In a patch's description or put it
>> in coccinelle scripts?
>>
>> > arch/alpha/kernel/setup.c: if (strncmp(p, "mem=", 4) == 0) {
>> >
>> > $ git grep -E 'strncmp.*(sizeof|, *[0-9]*)' | wc -l
>> > 2565
>> >
>> > And some cases are weirdly backwards:
>> >
>> > tools/perf/util/callchain.c: if (!strncmp(tok, "none", strlen(tok))) {
>
> I find there are cases of this pattern are not wrong.
> One example is kernel/irq/debugfs.c: if (!strncmp(buf, "trigger", size)) {
>
> Thus I do not know whether I should include these cases in my script.

That case isn't looking for a prefix AFAICS, so you should skip it.

I think Kees regexp was just slightly wrong, it should be:

git grep -E 'strncmp.*(sizeof|, *[0-9]+)'

ie. either literal "sizeof" or *at least one* digit.

cheers