Re: [PATCH v5 2/7] string: Warn if the prefix is the empty string

[Alejandro Colomar <alx@kernel.org>]

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Hi Andy,

> Date: 2026-11-13 16:25:35+0200
> From: Andy Shevchenko <andriy.shevchenko@intel.com>
>
> On Fri, Nov 13, 2026 at 02:48:01PM +0100, Alejandro Colomar wrote:
> 
> ...
> 
> > > [Severity: Low]
> > > Does moving str_has_prefix() out-of-line into lib/string.c bypass
> > > CONFIG_FORTIFY_SOURCE and prevent compile-time constant folding of
> > > strlen(prefix)?
> > 
> > Preventing the compile-time constant folding of strlen(3) is indeed
> > a performance degradation, but that's necessary to be able to
> > WARN_ON_ONCE(), so there isn't anything we can do.
> > 
> > > lib/string.c is built with __NO_FORTIFY and its top-of-file comment notes
> > > that it is reserved for routines that have architecture-specific
> > > alternatives in <asm/string.h> or get overloaded by FORTIFY_SOURCE, while
> > > other helpers belong in lib/string_helpers.c.
> > 
> > Do we want to move it to "lib/string_helpers.c"?  I don't know enough
> > about the organization in these files, so just let me know what I should
> > do.
> 
> string library (generic) is for broad use, so it should really be clean from
> kernel things. string_helpers is exactly for some custom abstractions we use
> in kernel. Now you should answer these questions (or more than that):
> - Are there or potentially be users that needs this API earlier at the boot?
> - Is this API is detached from the rest of string library semantically?
>   I.o.w. can we group semantically coupled APIs and consider them as a bundle?
> - Due to the above, ask the first Q against semantically grouped APIs.
> 
> My personal feeling that what you are trying to do is no go for generic string
> as is. If you confirm that these APIs are not used / needed at early stages at
> boot, they can be moved completely to string_helpers.

Thanks!  I guess I'll just drop the addition of WARN_ON_ONCE().  Let's
leave the function as is, and document its problems.  It's not worth the
effort.  After all, I think it's a bad function, and should not be used
in new code.


Have a lovely day!
Alex

> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
> 

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

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