Re: [PATCH] init/main: Expose built-in initcalls and blacklist status via debugfs

[Aaron Tomlin]

On Sun, May 10, 2026 at 02:13:01AM -0400, Aaron Tomlin wrote:
> At present, identifying the correct function name to supply to the
> "initcall_blacklist=" kernel command-line parameter requires manual
> inspection of the source code or kernel symbol tables. Furthermore,
> administrators lack a reliable runtime mechanism to verify whether a
> specified built-in module has been successfully blacklisted.
> 
> To resolve this, introduce a new debugfs interface at
> /sys/kernel/debug/modules/builtin_initcalls. This file enumerates all
> built-in modules alongside their corresponding initialisation callbacks
> (e.g., those specified by module_init()) in a simple format:
> "module_name init_callback". If a built-in module has been actively
> blacklisted, the entry is explicitly appended with a " [blacklisted]"
> suffix.
> 
> To achieve this without incurring runtime overhead, the
> ___define_initcall macro is expanded to statically allocate a
> builtin_initcall_record structure (containing the module and function
> name strings) within a dedicated .rodata_builtin_initcall_records
> section.
> 
> This implementation incorporates several critical architectural safety
> measures:
>   - The custom section name deliberately begins with ".rodata" to ensure
>     KASLR (Kernel Address Space Layout Randomisation) relocation tools
>     process and update the string pointers correctly during boot.
>   - The structural alignment is explicitly forced to 8 bytes to prevent
>     the compiler from inserting silent padding that would misalign the
>     linker script boundaries.
>   - The blacklisted_initcalls list head is stripped of its
>     __initdata_or_module annotation, ensuring the list remains safely
>     accessible in memory when queried from user space after early boot.

Hi Sasha, Breno,

I thought this might be of interest to you for review, given your recent
work and interest on the "killswitch" proposal [1].


[1]: https://lore.kernel.org/lkml/20260508195749.1885522-1-sashal@xxxxxxxxxx/

Kind regards,
-- 
Aaron Tomlin

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