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

Chuhong Yuan
2019-07-30T06:39:40Z

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))) {
>
> -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