Re: [RFC PATCH v6 2/2] module: allocate codetag sections before the regular module layout

[Petr Pavlu <petr.pavlu@suse.com>]

On 8/31/26 9:21 AM, Hao Ge wrote:
> Whether a codetag section goes to the codetag region is decided by
> layout_sections() and asked again in move_module(). A concurrent
> load can shut profiling down in between, and move_module() then
> copies the section to offset 0 of its regular destination,
> overwriting whatever is there.
> 
> Decide and allocate in one pass, before the layout. Allocation
> errors fail the load. On a tag area overflow profiling is already
> disabled, so -EAGAIN makes the section fall back to regular module
> data and the module still loads.
> 
> The overflow and populate failure paths of reserve_module_tags() now
> release their reservation instead of leaking the maple tree entry.
> When profiling was toggled off, the overflow check did not run, a
> module could load with more tags than the page flags can address,
> and re-enabling profiling then silently corrupted /proc/allocinfo.
> The check no longer depends on mem_alloc_profiling_enabled().
> 
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Based-on-a-patch-by: Petr Pavlu <petr.pavlu@suse.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> ---
> Changes against Petr's prototype:
> - allocate_codetag_sections() returns an error instead of void, and
>   only -EAGAIN falls back to a regular section. Any other error now
>   fails the load. The prototype fell back on everything, which can
>   leave live tags in module memory.
> - reserve_module_tags() releases its reservation when populate fails
>   too, that path used to leak the maple tree entry.
> - codetag_free_module_sections() on the move_module() error path uses
>   info->mod, the local mod is assigned only after a successful move.
> - The percpu section is marked only when index.pcpu != 0, otherwise
>   sechdrs[0] gets marked.
> - Dropped the SHF_ALLOC check, .codetag.* sections always have it.
> ---
>  include/linux/module.h   |   2 +
>  kernel/module/internal.h |   4 ++
>  kernel/module/main.c     | 120 ++++++++++++++++++++-------------------
>  mm/alloc_tag.c           |   9 ++-
>  4 files changed, 75 insertions(+), 60 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 7566815fabbe..33548daa31a3 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -325,6 +325,8 @@ enum mod_mem_type {
>  	MOD_INIT_RODATA,
>  
>  	MOD_MEM_NUM_TYPES,
> +
> +	MOD_STANDALONE = -2,
>  	MOD_INVALID = -1,
>  };
>  

It might be better to split this patch into two: the first to introduce
MOD_STANDALONE and use it only for the percpu section, and the second
with all the codetag-related changes.

The introduction of SH_ENTSIZE_STANDALONE should also allow us to clean
up the current resetting of SHF_ALLOC for the percpu section, and now
also for codetag sections. The problem is that find_sec(".data..percpu")
can currently return different results depending on whether it is called
before layout_and_allocate() or later. In addition, apply_relocations()
needs a special case for SH_ENTSIZE_STANDALONE, where it could otherwise
just test SHF_ALLOC.

Instead of resetting SHF_ALLOC for percpu/codetag sections, both
__layout_sections() and move_module() can check for
SH_ENTSIZE_STANDALONE to determine whether a section is handled
specially and should be skipped.

I think it would be useful to include this change in the first patch
introducing MOD_STANDALONE, but I'm also ok with the current version.
I can send a separate patch later to make more use of
SH_ENTSIZE_STANDALONE in this way.

> @@ -2966,18 +2967,23 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>  	 */
>  	module_mark_ro_after_init(info->hdr, info->sechdrs, info->secstrings);
>  
> -	/*
> -	 * Determine total sizes, and put offsets in sh_entsize.  For now
> -	 * this is done generically; there doesn't appear to be any
> -	 * special cases for the architectures.
> -	 */
> +	/* Allow codetag sections to be allocated separately first. */
> +	err = allocate_codetag_sections(info);
> +	if (err) {
> +		codetag_free_module_sections(info->mod);

The usual convention is that functions clean up after themselves on
error. That means this codetag_free_module_sections() call should be
done ideally by allocate_codetag_sections().

-- 
Thanks,
Petr