Re: [RFC PATCH 2/3] module: move codetag section placement decision to layout_sections()

On Thu, Aug 13, 2026 at 2:34 AM Hao Ge <[email protected]> wrote:
>
> codetag_needs_module_section() is called twice per codetag section, once
> in layout_sections() and once in move_module(), and both depend on
> mem_profiling_support, which changes without a lock. If profiling is
> disabled between the two calls, layout excludes the section (offset 0)
> while move copies it as normal memory to offset 0:
>
>   CPU0 (insmod A)                    CPU1 (insmod B)
>   ----------------                   ----------------
>   layout_sections()
>     needs_section_mem() == true
>     sh_entsize: type, offset = 0
>                                      reserve_module_tags() overflows
>                                        shutdown_mem_profiling()
>                                        mem_profiling_support = false
>   move_module()
>     needs_section_mem() == false
>     offset = sh_entsize & MASK = 0
>     memcpy(mod->mem[type].base + 0, ...)
>        -> overwrites the first section there
>
> Record the decision in layout_sections() in sh_entsize using a
> MOD_MEM_CODETAG type, and have move_module() use that instead of asking
> again.
>
> reserve_module_tags() returns -EAGAIN if profiling was disabled after
> layout, so the loader retries and places the section as normal memory.
>
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Signed-off-by: Hao Ge <[email protected]>
> ---
>  include/linux/module.h | 11 +++++++++++
>  kernel/module/main.c   | 17 ++++++-----------
>  mm/alloc_tag.c         |  8 ++++++++
>  3 files changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 7566815fabbe..a02016528e1d 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -328,6 +328,17 @@ enum mod_mem_type {
>         MOD_INVALID = -1,
>  };
>
> +/*
> + * If CONFIG_CODE_TAGGING is on, modules get a .codetag section.
> + * codetag_needs_module_section() says where it goes: the usual
> + * mod->mem[], or off to the codetag region.
> + *
> + * Mark the codetag-region ones with MOD_MEM_NUM_TYPES.
> + * It's just past the real types, so it doesn't index into mod->mem[]
> + * and for_each_mod_mem_type() skips it.
> + */
> +#define MOD_MEM_CODETAG        MOD_MEM_NUM_TYPES

Ok, it feels a bit hacky but it's probably the simplest way to mark
codetag regions.

> +
>  #define mod_mem_type_is_init(type)     \
>         ((type) == MOD_INIT_TEXT ||     \
>          (type) == MOD_INIT_DATA ||     \
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index ed26f167be84..2337bf604f58 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -1728,11 +1728,8 @@ static void __layout_sections(struct module *mod, struct load_info *info, bool i
>                          * preallocated contiguous memory.
>                          */
>                         if (codetag_needs_module_section(mod, sname, s->sh_size)) {
> -                               /*
> -                                * s->sh_entsize won't be used but populate the
> -                                * type field to avoid confusion.
> -                                */
> -                               s->sh_entsize = ((unsigned long)(type) & SH_ENTSIZE_TYPE_MASK)
> +                               s->sh_entsize = ((unsigned long)MOD_MEM_CODETAG
> +                                                & SH_ENTSIZE_TYPE_MASK)
>                                                 << SH_ENTSIZE_TYPE_SHIFT;
>                                 continue;
>                         }
> @@ -2815,11 +2812,10 @@ static int move_module(struct module *mod, struct load_info *info)
>                         continue;
>
>                 sname = info->secstrings + shdr->sh_name;
> -               /*
> -                * Load codetag sections separately as they might still be used
> -                * after module unload.
> -                */
> -               if (codetag_needs_module_section(mod, sname, shdr->sh_size)) {
> +
> +               enum mod_mem_type type = shdr->sh_entsize >> SH_ENTSIZE_TYPE_SHIFT;
> +
> +               if (type == MOD_MEM_CODETAG) {
>                         dest = codetag_alloc_module_section(mod, sname, shdr->sh_size,
>                                         arch_mod_section_prepend(mod, i), shdr->sh_addralign);
>                         if (WARN_ON(!dest)) {
> @@ -2832,7 +2828,6 @@ static int move_module(struct module *mod, struct load_info *info)
>                         }
>                         codetag_section_found = true;
>                 } else {
> -                       enum mod_mem_type type = shdr->sh_entsize >> SH_ENTSIZE_TYPE_SHIFT;
>                         unsigned long offset = shdr->sh_entsize & SH_ENTSIZE_OFFSET_MASK;
>
>                         dest = mod->mem[type].base + offset;
> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> index 461fa87fbb0b..7481180dadd2 100644
> --- a/mm/alloc_tag.c
> +++ b/mm/alloc_tag.c
> @@ -893,6 +893,14 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
>         if (size < sizeof(struct alloc_tag))
>                 return ERR_PTR(-EINVAL);
>
> +       /*
> +        * Profiling may have been disabled by a concurrent module load.
> +        * Return -EAGAIN so the loader retries with profiling off, laying
> +        * the section out as ordinary module memory.
> +        */
> +       if (!mem_profiling_support)
> +               return ERR_PTR(-EAGAIN);

I think this requires your patch [1] from another patchset to work
correctly, correct? If so, I would suggest sending this patch as part
of that patchset since there is a dependency.

[1] https://lore.kernel.org/all/[email protected]/


> +
>         /*
>          * align is always power of 2, so we can use IS_ALIGNED and ALIGN.
>          * align 0 or 1 means no alignment, to simplify set to 1.
> --
> 2.25.1
>