Re: [PATCH v3 1/5] string: Add strprefix(), and deprecate str_has_prefix()

[Alejandro Colomar <alx+linux-hardening@kernel.org>]

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Hi Joe,

> Date: 2026-11-10 07:47:55-0800
> From: Joe Perches <joe@perches.com>
>
> On Tue, 2026-11-10 at 16:33 +0100, Alejandro Colomar wrote:
> > As Bartosz reported, str_has_prefix() has a bug: it can't differentiate
> > between an empty prefix matching an empty string from a non-matching
> > prefix.  This is a fundamental design flaw in this API, and not
> > something that can be fixed.
> > 
> > As always, this is a consequence of choosing sizes instead of pointers.
> > Sizes are often the wrong choice.  In this specific case, pointers are
> > more capable of differentiating these cases, as NULL is a special
> > sentinel value that doesn't need to overload a valid value.
> > 
> > Let's implement strprefix(), a function that returns a pointer
> > right after the prefix if found, and a null pointer if not found.
> > 
> > We'll slowly migrate existing code from str_has_prefix() to strprefix().
> 
> Most uses of str_has_prefix could use the existing strstarts.

Hmmm, actually, both str_has_prefix() and strstarts() can be replaced by
strprefix(), which provides functionality for both use cases.  It's the
more generic API for this.  And I expect compilers can optimize the
unused parts if actually unused.


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>

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