Re: [PATCH v7 2/4] module: introduce SH_ENTSIZE_STANDALONE for separately allocated sections

[Petr Pavlu]

On 9/2/26 10:18 AM, Hao Ge wrote:
> SHF_ALLOC means, per the ELF spec, that a section occupies memory
> during process execution. Some module sections occupy memory
> outside the regular module layout, for example the percpu section
> with its per-CPU allocations. The loader currently excludes such
> a section from the layout by clearing its SHF_ALLOC, which
> overloads the flag with a loader-internal meaning.
> apply_relocations() needs a special case for the section, and
> find_sec(".data..percpu") returns different results before and
> after layout_and_allocate().
> 
> Introduce SH_ENTSIZE_STANDALONE to mark sections with a separate
> allocation. The percpu section is its first user. layout_sections()
> and move_module() skip marked sections, and apply_relocations() goes
> back to testing only SHF_ALLOC.
> 
> No functional change intended.
> 
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Based-on-a-patch-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
> Link: https://lore.kernel.org/all/499bb60c-c6e3-43a3-bd92-95a0567ece5e@xxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> ---
> [...]
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index c32f1d370b73..078dae188aba 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -1618,14 +1618,8 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
>  		if (infosec >= info->hdr->e_shnum)
>  			continue;
>  
> -		/*
> -		 * Don't bother with non-allocated sections.
> -		 * An exception is the percpu section, which has separate allocations
> -		 * for individual CPUs. We relocate the percpu section in the initial
> -		 * ELF template and subsequently copy it to the per-CPU destinations.
> -		 */
> -		if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC) &&
> -		    (!infosec || infosec != info->index.pcpu))
> +		/* Don't bother with non-allocated sections. */
> +		if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC))
>  			continue;
>  
>  		if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)

Please keep a comment about how .data..percpu is handled. This is not
obvious and should be described explicitly.

Something like the following:

		/*
		 * Don't bother with non-allocated sections.
		 *
		 * Note that .data..percpu has separate allocations for
		 * individual CPUs. We relocate the section in the
		 * initial ELF template and subsequently copy it to the
		 * per-CPU destinations.
		 */

-- 
Thanks,
Petr