Re: [RFC PATCH v1 07/12] efi: Replace strstarts() by str_has_prefix().

[James Bottomley]

On Sat, 2020-12-05 at 22:20 +0100, Ard Biesheuvel wrote:
> On Sat, 5 Dec 2020 at 22:15, James Bottomley
> <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> > [Rostedt added because this is all his fault]
> > On Sat, 2020-12-05 at 21:57 +0100, Ard Biesheuvel wrote:
> > > On Sat, 5 Dec 2020 at 21:24, James Bottomley
> > > <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> > [...]
> > > > > 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.
> > > > 
> > > > That's your prerogative as a Maintainer ... I was just
> > > > explaining what the original author had in mind when
> > > > str_has_prefix() was created.
> > > > 
> > > 
> > > Sure, I fully understand you are not the one proposing these
> > > changes.
> > > 
> > > But if the pattern in question is so common, couldn't we go one
> > > step further and define something like
> > > 
> > > static inline const u8 *skip_prefix_or_null(const u8 *str, const
> > > u8 *prefix)
> > > {
> > > }
> > > 
> > > which returns a pointer into the original string, or NULL if the
> > > prefix is not present.
> > > 
> > > The current patch as proposed has no benefit whatsoever, but even
> > > the meaningful alternative you are proposing is not actually an
> > > improvement, given that it is not self-explanatory from the name
> > > 'str_has_prefix' what it returns, and so the code becomes more
> > > difficult to understand.
> > 
> > Ah, so this is the kernel maintainer's syndrome: you see an API
> > which isn't quite right for your use case, so you update or change
> > it.  Then you see other use cases for it and suddenly to you it
> > becomes the best thing since sliced bread and with a one ring to
> > rule them all mentality you exhort everyone to use this new API
> > everywhere.  See this comment in the merge commit (495d714ad1400)
> > which comes from the merge cover letter:
> > 
> > >     - Addition of str_has_prefix() and a few use cases. There
> > >       currently is a similar function strstart() that is used in
> > > a
> > >       few places, but only returns a bool and not a length. These
> > >       instances will be removed in the future to use
> > >       str_has_prefix() instead.
> > 
> > Then you forget about it until someone else acts on your somewhat
> > ill considered instruction and actually tries the
> > replacement.  Once someone takes up your cause, the API shows up in
> > dozens of emails and the actual debate about whether or not this is
> > such a good API really begins, with the poor person who picked it
> > up caught in the crossfire.
> > 
> > As maintainers we really should learn to put the cart before the 

s/to put/not to put/

> > horse.
> > 
> 
> I am not disagreeing with any of this, but I simply don't see a point
> in merging patches that apparently result in the exact same machine
> code to be generated, and don't substantially make the code itself
> any better.


Well, I think the pattern

if (strstarts(option, <string>)) {
   ...
   option += strlen(<same string>);

is a bad one because one day <string> may get updated but not <same
string>.  And if <same string> is too far away in the code it might not
even show up in the diff, leading to reviewers not noticing either.  So
I think eliminating the pattern is a definite improvement.

Now whether the improvement is enough that we should churn the code
base to fix it is another question.

James