[PATCH v4 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Hao Ge <hao.ge@linux.dev>]

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.

Skip percpu counter allocation in load_module() when profiling is
disabled, as those counters would never be freed on unload.

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 | 17 +++++++++++++++--
 mm/alloc_tag.c       | 11 ++++++++---
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 46dd8d25a605..c32bb47a573a 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2971,13 +2971,26 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 	 * 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);
+		/*
+		 * -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 = 0;
+		info->sechdrs[info->index.sym].sh_flags &= ~(unsigned long)SHF_ALLOC;
+		info->sechdrs[info->index.str].sh_flags &= ~(unsigned long)SHF_ALLOC;
+		goto retry;
+	}
 
 	/* Module has been copied to its final place now: return it. */
 	mod = (void *)info->sechdrs[info->index.mod].sh_addr;
diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
index af44f90379f2..3eba6331174d 100644
--- a/mm/alloc_tag.c
+++ b/mm/alloc_tag.c
@@ -950,10 +950,12 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
 		int grow_res;
 
 		module_tags.size = offset + size;
-		if (mem_alloc_profiling_enabled() && !tags_addressable()) {
+		if (!tags_addressable()) {
 			shutdown_mem_profiling(true);
-			pr_warn("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
-				mod->name, NR_UNUSED_PAGEFLAG_BITS);
+			pr_warn_once("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
+				     mod->name, NR_UNUSED_PAGEFLAG_BITS);
+			release_module_tags(mod, false);
+			return ERR_PTR(-EAGAIN);
 		}
 
 		grow_res = vm_module_tags_populate();
@@ -975,6 +977,9 @@ static int load_module(struct module *mod, struct codetag *start, struct codetag
 	struct alloc_tag *stop_tag;
 	struct alloc_tag *tag;
 
+	if (!mem_profiling_support)
+		return 0;
+
 	/* percpu counters for core allocations are already statically allocated */
 	if (!mod)
 		return 0;
-- 
2.25.1