Re: [PATCH v5 7/7] string: Implement strstarts() with strprefix()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] string: strstarts() via strprefix() prevents optimizing out
  pointer offset
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commit 8cdc2e9c96584305ea94313f2618d17d8e28f303
Author: Alejandro Colomar <alx@kernel.org>

string: Implement strstarts() with strprefix()

This commit changes strstarts() to call strprefix() instead of calling
strncmp() and strlen() directly.

> diff --git a/include/linux/string.h b/include/linux/string.h
> index 8c6a79d320474..8ca0deda61606 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -595,7 +595,7 @@ static inline const char *strprefix(const char *s, const char *prefix)
>   */
>  static inline bool strstarts(const char *str, const char *prefix)
>  {
> -	return strncmp(str, prefix, strlen(prefix)) == 0;
> +	return strprefix(str, prefix);
>  }

[Severity: Low]
Can the compiler actually optimize away the s + len pointer arithmetic in
strprefix() here?

strprefix() returns strncmp(s, prefix, len) == 0 ? s + len : NULL, and
converting that result to bool requires proving that s + len is non-NULL
whenever strncmp() returns 0.  Because the kernel builds with
-fno-delete-null-pointer-checks and -fno-strict-overflow, the compiler
cannot assume that a runtime pointer is non-NULL or that s + len does not
wrap to NULL, so callers of strstarts() end up keeping str and len live
across strncmp() and emitting the extra addition and NULL check.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1794574496.git.alx@kernel.org?part=7