Re: [PATCH v2 3/3] module: Make .static_call_sites read-only after init

[Petr Pavlu]

On 3/7/25 01:12, Sami Tolvanen wrote:
> On Thu, Mar 06, 2025 at 06:28:58PM +0100, Christophe Leroy wrote:
>> Le 06/03/2025 à 14:13, Petr Pavlu a écrit :
>>> Section .static_call_sites holds data structures that need to be sorted and
>>> processed only at module load time. This initial processing happens in
>>> static_call_add_module(), which is invoked as a callback to the
>>> MODULE_STATE_COMING notification from prepare_coming_module().
>>>
>>> The section is never modified afterwards. Make it therefore read-only after
>>> module initialization to avoid any (non-)accidental modifications.
>>
>> Maybe this suggestion is stupid, I didn't investigate the feasability but:
>> why don't we group everything that is ro_after_init in a single section just
>> like we do in vmlinux ? That would avoid having to add every new possible
>> section in the C code.
>>
>> Like we have in asm-generic/vmlinux.lds.h:
>>
>> #define RO_AFTER_INIT_DATA						\
>> 	. = ALIGN(8);							\
>> 	__start_ro_after_init = .;					\
>> 	*(.data..ro_after_init)						\
>> 	JUMP_TABLE_DATA							\
>> 	STATIC_CALL_DATA						\
>> 	__end_ro_after_init = .;
> 
> I like this idea. Grouping the sections in the module linker script
> feels cleaner than having an array of section names in the code. To be
> fair, I think this code predates v5.10, where scripts/module.lds.S was
> first added.

I agree in principle. I like that the information about ro_after_init
sections for vmlinux and modules would be in the same source form, in
linker scripts. This could eventually allow us to share the definition
of ro_after_init sections between vmlinux and modules.

The problem is however how to find the location of the __jump_table and
static_call_sites data. In vmlinux, as a final binary, they are
annotated with start and end symbols. In modules, as relocatable files,
the approach is to rely on them being separate sections, see function
find_module_sections().

I could add start+end symbols for __jump_table and static_call_sites
data in scripts/module.lds.S and use them by the module loader, but this
would create an inconsistency in how various data is looked up. Another
problem is that I can't find a way to tell the linker to add these
symbols only if the specific data is actually present.

-- 
Cheers,
Petr