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

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Date:   Thu, 10 Dec 2020 13:14:22 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        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 Sat, Dec 05, 2020 at 08:36:02PM +0100, Ard Biesheuvel wrote:
> 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.

The EFI stub is -ffreestanding, so you actually get multiple calls to
strlen() in any case. I could have used strncmp() directly with sizeof()
to avoid that, but the strstarts()/strlen() was slightly more readable
and the performance of this code doesn't really matter.

I wasn't aware of str_has_prefix() at the time. It does seem useful to
eliminate the duplication of the string literal, I like the
skip_prefix() API suggestion, maybe even

	bool str_skip_prefix(const char **s, const char *pfx)
	{
		size_t len = str_has_prefix(*s, pfx);
		*s += len;
		return !!len;
	}
	...
	if (str_skip_prefix(&option, prefix)) { ... }

to avoid the intermediate 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.