Re: [PATCH v2 09/13] lib/fonts: Compare font data for equality with font_data_is_equal()

[Nathan Chancellor <nathan@kernel.org>]

Hi Thomas,

On Mon, Mar 02, 2026 at 03:08:43PM +0100, Thomas Zimmermann wrote:
...
> diff --git a/lib/fonts/fonts.c b/lib/fonts/fonts.c
> index 8c9a6762061c..c9f6328d5dda 100644
> --- a/lib/fonts/fonts.c
> +++ b/lib/fonts/fonts.c
> @@ -12,18 +12,25 @@
>   * for more details.
>   */
>  
> +#include <linux/font.h>
>  #include <linux/module.h>
> -#include <linux/types.h>
>  #include <linux/string.h>
> +#include <linux/types.h>
> +
> +#include <asm/sections.h>
>  #if defined(__mc68000__)
>  #include <asm/setup.h>
>  #endif
> -#include <linux/font.h>
>  
>  /*
>   * Helpers for font_data_t
>   */
>  
> +static bool font_data_is_internal(font_data_t *fd)
> +{
> +	return is_kernel_rodata((unsigned long)fd);
> +}
> +
>  /**
>   * font_data_size - Return size of the font data in bytes
>   * @fd: Font data
> @@ -37,6 +44,32 @@ unsigned int font_data_size(font_data_t *fd)
>  }
>  EXPORT_SYMBOL_GPL(font_data_size);
>  
> +/**
> + * font_data_is_equal - Compares font data for equality
> + * @lhs: Left-hand side font data
> + * @rhs: Right-hand-size font data
> + *
> + * Font data is equal if is constain the same sequence of values. The
> + * helper also use the checksum, if both arguments contain it. Font data
> + * coming from different origins, internal or from user space, is never
> + * equal. Allowing this would break reference counting.
> + *
> + * Returns:
> + * True if the given font data is equal, false otherwise.
> + */
> +bool font_data_is_equal(font_data_t *lhs, font_data_t *rhs)
> +{
> +	if (font_data_is_internal(lhs) != font_data_is_internal(rhs))
> +		return false;

This breaks the build when CONFIG_FONT_SUPPORT is a module.

  $ cat allno.config
  CONFIG_MODULES=y
  CONFIG_DRM=m
  CONFIG_DRM_PANIC=y

  $ make -skj"$(nproc)" ARCH=x86_64 CROSS_COMPILE=x86_64-linux- KCONFIG_ALLCONFIG=1 allnoconfig all
  ERROR: modpost: "__end_rodata" [lib/fonts/font.ko] undefined!
  make[4]: *** [scripts/Makefile.modpost:147: Module.symvers] Error 1
  ...

  $ scripts/config -s FONT_SUPPORT
  m

Cheers,
Nathan

> +	if (font_data_size(lhs) != font_data_size(rhs))
> +		return false;
> +	if (FNTSUM(lhs) && FNTSUM(rhs) && FNTSUM(lhs) != FNTSUM(rhs))
> +		return false;
> +
> +	return !memcmp(lhs, rhs, FNTSIZE(lhs));
> +}
> +EXPORT_SYMBOL_GPL(font_data_is_equal);
> +
>  /*
>   * Font lookup
>   */
> -- 
> 2.53.0
>