Re: [PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

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

On 8/12/26 7:41 AM, Hao Ge wrote:
> In reserve_module_tags(), the tag overflow check is gated on
> mem_alloc_profiling_enabled():
> 
>     if (mem_alloc_profiling_enabled() && !tags_addressable())
> 
> If profiling is toggled off at runtime and a module is loaded whose
> tags exceed the compressed-mode limit, shutdown_mem_profiling() is
> skipped. vm_module_tags_populate() still maps memory for the tags and
> the module loads successfully, but the total tag count now exceeds what
> NR_UNUSED_PAGEFLAG_BITS can address.
> 
> Once profiling is re-enabled, ref_to_idx() computes each tag's index
> as its position in the alloc_tag array. update_page_tag_ref() masks
> it to alloc_tag_ref_mask before storing in page->flags. Indices
> beyond the mask are truncated and idx_to_ref() resolves them to wrong
> tags.
> 
> This silently corrupts /proc/allocinfo: allocated pages get attributed
> to the wrong call sites, so the statistics it reports are wrong.
> 
> mem_alloc_profiling_enabled() and mem_profiling_compressed are
> independent. Once compressed mode is established at boot, it stays
> active regardless of runtime toggles of mem_profiling.
> 
> Remove the mem_alloc_profiling_enabled() guard. On overflow, shut down
> profiling, release the reservation, and return -EAGAIN so that
> layout_and_allocate() retries with profiling disabled: codetag sections
> are then placed as regular module data and the module loads without
> profiling rather than being rejected entirely.

When the described overflow occurs, why should codetag sections be
placed as regular module data? Will the codetag support use them in any
way, or do they simply waste space? Is the issue that alloc_hooks()
creates relocations pointing into .codetag.alloc_tags?

> 
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Cc: stable@vger.kernel.org
> Suggested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> ---
>  kernel/module/main.c | 25 +++++++++++++++++++++++--
>  mm/alloc_tag.c       |  8 +++++---
>  2 files changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 46dd8d25a605..ed26f167be84 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -2944,6 +2944,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>  {
>  	struct module *mod;
>  	int err;
> +	unsigned long frob_size[MOD_MEM_NUM_TYPES];

frob_size is used to store values of module_memory::size, which has type
`unsigned int`. The types should match.

>  
>  	/* Allow arches to frob section contents and sizes.  */
>  	err = module_frob_arch_sections(info->hdr, info->sechdrs,
> @@ -2966,18 +2967,38 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>  	 */
>  	module_mark_ro_after_init(info->hdr, info->sechdrs, info->secstrings);
>  
> +	/*
> +	 * Save the sizes reserved by module_frob_arch_sections() so they can
> +	 * be restored if we retry below.
> +	 */
> +	for_each_mod_mem_type(type)
> +		frob_size[type] = info->mod->mem[type].size;
> +
>  	/*
>  	 * 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.
>  	 */
> +retry:
>  	layout_sections(info->mod, info);
>  	layout_symtab(info->mod, info);
>  
>  	/* Allocate and move to the final place */
>  	err = move_module(info->mod, info);
> -	if (err)
> -		return ERR_PTR(err);
> +	if (err) {
> +		if (err != -EAGAIN)
> +			return ERR_PTR(err);

The move_module() logic is non-trivial. -EAGAIN could be returned by
other code, now or in the future.

> +		/*
> +		 * -EAGAIN means profiling was disabled but the module
> +		 * can still load without it. Reset state and retry.
> +		 */
> +		rewrite_section_headers(info, flags);
> +		for_each_mod_mem_type(type)
> +			info->mod->mem[type].size = frob_size[type];
> +		info->sechdrs[info->index.sym].sh_flags &= ~(unsigned long)SHF_ALLOC;
> +		info->sechdrs[info->index.str].sh_flags &= ~(unsigned long)SHF_ALLOC;

Why is it necessary to reset SHF_ALLOC for .symtab and .strtab here?

> +		goto retry;
> +	}
>  
>  	/* Module has been copied to its final place now: return it. */
>  	mod = (void *)info->sechdrs[info->index.mod].sh_addr;

I'm not sure this is the best approach. It's complex logic for what
appears to be an edge case related to a debugging facility. It will have
the usual problem of error paths not getting enough testing and breaking
subtly over time.

An alternative could be to reset SHF_ALLOC on the codetag section to
remove it from further processing and have relocations that point to
this section resolve to something else. It seems that alloc_hooks_tag()
could tolerate this, since it only needs to reference the associated
alloc_tag when mem_alloc_profiling_enabled() is true and that gets
disabled by reserve_module_tags() on the overflow.

It is also not an ideal approach, but I feel it could be less intrusive
to the module loader. I can put together a prototype if needed.

-- 
Thanks,
Petr