linux-kernel - Re: [PATCH v2 3/3] module: Make .static_call

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Message-ID: <CABCJKudevAT5PK09Gu_j_m-oaULumwXab4EkPdv=ZdSu6-PpJQ@mail.gmail.com>
Date: Wed, 12 Mar 2025 16:21:13 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Petr Pavlu <petr.pavlu@...e.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>, Luis Chamberlain <mcgrof@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Jason Baron <jbaron@...mai.com>, Daniel Gomez <da.gomez@...sung.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ard Biesheuvel <ardb@...nel.org>, linux-modules@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] module: Make .static_call_sites read-only after init

Hi Petr,

On Wed, Mar 12, 2025 at 5:05 AM Petr Pavlu <petr.pavlu@...e.com> wrote:
>
> 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.

That's a fair point. Perhaps it makes sense to keep these sections
separate for consistency, and look into cleaning this up later if
needed.

> 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.

You can use the preprocessor to add the symbols only if the relevant
kernel config is present, similarly to how STATIC_CALL_DATA is defined
in include/asm-generic/vmlinux.lds.h.

In any case, the code looks correct to me. For the series:

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami