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

Chuhong Yuan
2019-07-30T13:47:46Z

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.

> > -Kees
> >
>
> I think with the help of Coccinelle script, all strncmp(str, const, len)
> can be replaced and these problems will be eliminated. :)
>
> Regards,
> Chuhong
>
> > > Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
> > > ---
> > > kernel/cgroup/rdma.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/cgroup/rdma.c b/kernel/cgroup/rdma.c
> > > index ae042c347c64..fd12a227f8e4 100644
> > > --- a/kernel/cgroup/rdma.c
> > > +++ b/kernel/cgroup/rdma.c
> > > @@ -379,7 +379,7 @@ static int parse_resource(char *c, int *intval)
> > > return -EINVAL;
> > > return i;
> > > }
> > > - if (strncmp(value, RDMACG_MAX_STR, len) == 0) {
> > > + if (str_has_prefix(value, RDMACG_MAX_STR)) {
> > > *intval = S32_MAX;
> > > return i;
> > > }
> > > --
> > > 2.20.1
> > >
> >
> > --
> > Kees Cook