linux-kernel - Re: [RFC PATCH v1 07/12] efi: Replace strstarts() by str_has

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Date:   Sat, 5 Dec 2020 20:36:02 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     laniel_francis@...vacyrequired.com,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 07/12] efi: Replace strstarts() by str_has_prefix().

On Fri, 4 Dec 2020 at 19:02, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
>
> On Fri, 2020-12-04 at 18:07 +0100, Ard Biesheuvel wrote:
> > On Fri, 4 Dec 2020 at 18:06, <laniel_francis@...vacyrequired.com>
> > wrote:
> > > From: Francis Laniel <laniel_francis@...vacyrequired.com>
> > >
> > > The two functions indicates if a string begins with a given prefix.
> > > The only difference is that strstarts() returns a bool while
> > > str_has_prefix()
> > > returns the length of the prefix if the string begins with it or 0
> > > otherwise.
> > >
> >
> > Why?
>
> I think I can answer that.  If the conversion were done properly (which
> it's not) you could get rid of the double strings in the code which are
> error prone if you update one and forget another.  This gives a good
> example: 3d739c1f6156 ("tracing: Use the return of str_has_prefix() to
> remove open coded numbers"). so in your code you'd replace things like
>
>     if (strstarts(option, "rgb")) {
>         option += strlen("rgb");
>         ...
>
> with
>
>     len = str_has_prefix(option, "rgb");
>     if (len) {
>         option += len
>         ...
>
> Obviously you also have cases where strstart is used as a boolean with
> no need to know the length ... I think there's no value to converting
> those.
>

This will lead to worse code being generated. strlen() is evaluated at
build time by the compiler if the argument is a string literal, so
your 'before' version gets turned into 'option += 3', whereas the
latter needs to use a runtime variable.

So I don't object to using str_has_prefix() in new code in this way,
but I really don't see the point of touching existing code.