Re: [PATCH v1 1/2] string: Add strprefix(), and deprecate str_has_prefix()

[Alejandro Colomar]

Hi Steve,

> Date: 2026-11-05 11:14:23-0500
> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
>
> On Thu, 5 Nov 2026 12:13:03 +0100
> Alejandro Colomar <alx@xxxxxxxxxx> 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().
> > 
> > Reported-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxxxxxxxx>
> 
> Is there a link to the report? Please add that.

Message-Id: <20261104-fwnode-for-each-prop-v1-1-6e28fe1c72c4@xxxxxxxxxxxxxxxx>

or in link format,

Link: <https://lore.kernel.org/linux-hardening/20261104-fwnode-for-each-prop-v1-1-6e28fe1c72c4@xxxxxxxxxxxxxxxx/>

Which one do you prefer?

> 
> >  
> > +/**
> > + * strprefix - string prefix
> > + * @s: The string to be tested
> > + * @prefix: The string to see if @s starts with
> > + *
> 
> Needs a better description. Feel free to use the one for str_has_prefix().

Ok.

> > + * Returns:
> > + * *  @s+strlen(@prefix) if @s starts with @prefix
> > + * *  NULL if @s does not start with @prefix
> > + */
> > +static __always_inline char *strprefix(const char *s, const char *prefix)
> > +{
> > +	size_t len = strlen(prefix);
> > +	return strncmp(s, prefix, len) == 0 ? (char *) s + len : NULL;
> > +}
> > +
> >  /**
> >   * str_has_prefix - Test if a string has a given prefix
> >   * @str: The string to test
> >   * @prefix: The string to see if @str starts with
> >   *
> > + * DEPRECATED: Use strprefix()
> 
> No, it is still useful. Converting trace_event_filter() to strprefix()
> would be very painful, and likely error prone.

What is trace_event_filter()?

	alx@devuan:~/src/linux/linux/master$ grep -rn '\<trace_event_filter\>'
	alx@devuan:~/src/linux/linux/master$ grepc trace_event_filter .
	alx@devuan:~/src/linux/linux/master$ grep -rn trace_event_filter
	kernel/trace/trace_events.c:2833:static const struct file_operations ftrace_event_filter_fops = {
	kernel/trace/trace_events.c:3222:			*fops = &ftrace_event_filter_fops;

If you point me to a specific line of code, I can attempt a rewrite.

> > + *
> >   * A common way to test a prefix of a string is to do:
> >   *  strncmp(str, prefix, sizeof(prefix) - 1)
> >   *
> >   * But this can lead to bugs due to typos, or if prefix is a pointer
> > - * and not a constant. Instead use str_has_prefix().
> > + * and not a constant. Instead use strprefix().
> 
> The above can be replaced with:
> 
>  * This is useful for testing for a prefix and returning the length.
>  * If you only need to test the prefix and perhaps get the pointer to the
>  * position of the string after the prefix, then use strprefix().

Ok, conditional to being convinced that str_has_prefix() is
actually useful in some case.


Have a lovely day!
Alex

> 
> -- Steve
> 
> 
> >   *
> >   * Returns:
> >   * * strlen(@prefix) if @str starts with @prefix
> 

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

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