[PATCH v5 0/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[PATCH v5 0/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Hao Ge]

v3 was a single patch. After discussion with Suren and Andrew we went
for a more graceful approach: rather than failing the module load on
overflow, let it load without profiling. Once profiling is disabled,
codetag_needs_module_section() returns false, so on retry the codetag
section is placed as regular module data.

A new patch (1/2) is added to move release_module_tags() above
reserve_module_tags(), since the overflow path now has to call it and
the helper sits below it.

release_module_tags() is what module unload calls to drop a module's
reservation from the maple tree. By the time reserve_module_tags()
detects the overflow it has already stored that reservation, and the
-EAGAIN return skips vm_module_tags_populate(), so the backing pages
never get mapped. If reserve_module_tags() returns without calling
release_module_tags(), the stale entry keeps pointing at that unmapped
range; when the module is later unloaded, release_module_tags() walks
it and panics.

Tested on an x86_64 virtual machine:

# insmod overflow_tag.ko
# dmesg
  With module overflow_tag there are too many tags to fit in 13 page
  flag bits. Memory allocation profiling is disabled!
# rmmod overflow_tag
The module loads without profiling.

Changes in v5:
- add Fixes: and Cc: stable to patch 1/2 as well, since 2/2 does not
  compile without it (Andrew Morton)
- restore frob-adjusted mem[type].size on retry instead of zeroing,
  as s390 and parisc add GOT/PLT space there in
  module_frob_arch_sections() (Reported by Sashiko)
- drop the load_module() mem_profiling_support check; the percpu
  counter leak is pre-existing and orthogonal to this fix

Changes in v4:
- add a new patch (1/2) to move release_module_tags() above
  reserve_module_tags(); the overflow fix is 2/2
- release the reservation on the -EAGAIN path
- return -EAGAIN instead of -ENOMEM so the module can still load
  without profiling (Suren)
- reset sh_addr, mem[type].size and sym/str SHF_ALLOC before retry
- skip percpu counters in load_module() when profiling is off

Changes in v3:
- use pr_warn_once() instead of pr_warn()
- return -ENOMEM instead of -ENOSPC (Suren)
- expand the commit message to describe the /proc/allocinfo impact
  (Andrew)

Changes in v2:
- return an error after shutdown_mem_profiling() to skip
  vm_module_tags_populate()

v1: https://lore.kernel.org/all/20260804064408.105033-1-hao.ge@linux.dev/
v2: https://lore.kernel.org/all/20260804122038.190270-1-hao.ge@linux.dev/
v3: https://lore.kernel.org/all/20260805090633.141001-1-hao.ge@linux.dev/
v4: https://lore.kernel.org/all/20260810093955.153015-1-hao.ge@linux.dev/

Hao Ge (2):
  alloc_tag: move release_module_tags() above reserve_module_tags()
  alloc_tag: fix undetected compressed tag overflow when profiling is
    disabled

 kernel/module/main.c |  25 ++++++++++-
 mm/alloc_tag.c       | 100 ++++++++++++++++++++++---------------------
 2 files changed, 74 insertions(+), 51 deletions(-)

--
2.25.1

[PATCH v5 1/2] alloc_tag: move release_module_tags() above reserve_module_tags()

[Hao Ge]

release_module_tags() is a cleanup helper. reserve_module_tags() can
also fail after storing the reservation in the maple tree, in which
case it should call release_module_tags() to undo it. Move the helper
above reserve_module_tags() so no forward declaration is needed.

No functional change.

Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
Cc: stable@vger.kernel.org
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
 mm/alloc_tag.c | 92 +++++++++++++++++++++++++-------------------------
 1 file changed, 46 insertions(+), 46 deletions(-)

diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
index 52aece27b00e..af44f90379f2 100644
--- a/mm/alloc_tag.c
+++ b/mm/alloc_tag.c
@@ -835,6 +835,52 @@ static int vm_module_tags_populate(void)
 	return 0;
 }
 
+static void release_module_tags(struct module *mod, bool used)
+{
+	MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
+	struct alloc_tag *start_tag;
+	struct alloc_tag *end_tag;
+	struct module *val;
+
+	mas_lock(&mas);
+	mas_for_each_rev(&mas, val, 0)
+		if (val == mod)
+			break;
+
+	if (!val) /* module not found */
+		goto out;
+
+	if (!used)
+		goto release_area;
+
+	start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
+	end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
+	if (!clean_unused_counters(start_tag, end_tag)) {
+		struct alloc_tag *tag;
+
+		for (tag = start_tag; tag <= end_tag; tag++) {
+			struct alloc_tag_counters counter;
+
+			if (!tag->counters)
+				continue;
+
+			counter = alloc_tag_read(tag);
+			pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
+				tag->ct.filename, tag->ct.lineno, tag->ct.modname,
+				tag->ct.function, counter.bytes);
+		}
+	} else {
+		used = false;
+	}
+release_area:
+	mas_store(&mas, used ? &unloaded_mod : NULL);
+	val = mas_prev_range(&mas, 0);
+	if (val == &prepend_mod)
+		mas_store(&mas, NULL);
+out:
+	mas_unlock(&mas);
+}
+
 static void *reserve_module_tags(struct module *mod, unsigned long size,
 				 unsigned int prepend, unsigned long align)
 {
@@ -922,52 +968,6 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
 	return (struct alloc_tag *)(module_tags.start_addr + offset);
 }
 
-static void release_module_tags(struct module *mod, bool used)
-{
-	MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
-	struct alloc_tag *start_tag;
-	struct alloc_tag *end_tag;
-	struct module *val;
-
-	mas_lock(&mas);
-	mas_for_each_rev(&mas, val, 0)
-		if (val == mod)
-			break;
-
-	if (!val) /* module not found */
-		goto out;
-
-	if (!used)
-		goto release_area;
-
-	start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
-	end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
-	if (!clean_unused_counters(start_tag, end_tag)) {
-		struct alloc_tag *tag;
-
-		for (tag = start_tag; tag <= end_tag; tag++) {
-			struct alloc_tag_counters counter;
-
-			if (!tag->counters)
-				continue;
-
-			counter = alloc_tag_read(tag);
-			pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
-				tag->ct.filename, tag->ct.lineno, tag->ct.modname,
-				tag->ct.function, counter.bytes);
-		}
-	} else {
-		used = false;
-	}
-release_area:
-	mas_store(&mas, used ? &unloaded_mod : NULL);
-	val = mas_prev_range(&mas, 0);
-	if (val == &prepend_mod)
-		mas_store(&mas, NULL);
-out:
-	mas_unlock(&mas);
-}
-
 static int load_module(struct module *mod, struct codetag *start, struct codetag *stop)
 {
 	/* Allocate module alloc_tag percpu counters */
-- 
2.25.1

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

[Hao Ge]

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.

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];
 
 	/* 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);
+		/*
+		 * -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;
+		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..0a7b657fe2de 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();
-- 
2.25.1

Re: [PATCH v5 1/2] alloc_tag: move release_module_tags() above reserve_module_tags()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] Missing cleanup in `reserve_module_tags()` error path leaves dangling pointer in maple tree.
- [High] `release_module_tags()` uses `mas_store()` which can fail silently under memory pressure, leaving a dangling pointer.
--

--- Patch [1]: [PATCH v5 1/2] alloc_tag: move release_module_tags() above reserve_module_tags() ---
commit 1dcbb4a041b40aefa702fd60db8fbd78ff223508
Author: Hao Ge <hao.ge@linux.dev>

alloc_tag: move release_module_tags() above reserve_module_tags()

This patch moves the release_module_tags() cleanup helper above
reserve_module_tags() to avoid the need for a forward declaration.

> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> index b334103104779..2070e682fe106 100644
> --- a/mm/alloc_tag.c
> +++ b/mm/alloc_tag.c

[ ... ]

> @@ -835,6 +835,52 @@ static int vm_module_tags_populate(void)
>  	return 0;
>  }
>  
> +static void release_module_tags(struct module *mod, bool used)
> +{

[ ... ]

> +release_area:
> +	mas_store(&mas, used ? &unloaded_mod : NULL);
> +	val = mas_prev_range(&mas, 0);
> +	if (val == &prepend_mod)
> +		mas_store(&mas, NULL);

[Severity: High]
This is a pre-existing issue, but since the code is being moved here: could
mas_store() fail silently under memory pressure, leaving a dangling pointer?

When unloading a module, release_module_tags() calls mas_store() to clear
the module's reservation. If the internal maple tree allocation fails, is the
error state ignored? This could leave the tree retaining the old pointer,
which might lead to a use-after-free when the module subsystem frees it.

Would using mas_store_gfp(..., GFP_KERNEL) be needed here to ensure the
maple tree can sleep to allocate memory?

> +out:
> +	mas_unlock(&mas);
> +}
> +
>  static void *reserve_module_tags(struct module *mod, unsigned long size,
>  				 unsigned int prepend, unsigned long align)
>  {

[Severity: High]
This isn't a bug introduced by this patch, but the commit message mentions
that reserve_module_tags() can fail after storing the reservation in the
maple tree and should call release_module_tags() to undo it.

However, looking at the existing reserve_module_tags() code down the line,
if vm_module_tags_populate() fails:

reserve_module_tags() {
    ...
    grow_res = vm_module_tags_populate();
    if (grow_res) {
        shutdown_mem_profiling(true);
        pr_err("Failed to allocate memory...\n", mod->name);
        return ERR_PTR(grow_res);
    }
    ...
}

Does this error path still return without actually calling
release_module_tags(mod, false)?

If the undo call is still missing here, it appears the maple tree mod_area_mt
might retain a pointer to the module that is being freed, potentially leading
to a use-after-free during subsequent lookups. Should that call be added here
in a follow-up patch?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260812054105.102637-1-hao.ge@linux.dev?part=1

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:

New issues:
- [High] Unconditional allocation of per-CPU counters for modules causes a memory leak when memory profiling is disabled and the module load is retried.

Pre-existing issues:
- [Critical] Concurrent module loading can cause an out-of-bounds memory write and module corruption because the global profiling flag is toggled without cross-phase synchronization.
- [High] A race condition on the global module tags size variable can lead to unmapped virtual memory access and a kernel panic.
- [High] If page population fails, a dangling module pointer is left in the maple tree, leading to a Use-After-Free.
--

--- Patch [2]: [PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled ---
commit ffb2b208763f32cdfb3d6cb740d4d4efa5c99ae0
Author: Hao Ge <hao.ge@linux.dev>

alloc_tag: fix undetected compressed tag overflow when profiling is disabled

This patch modifies module layout and allocation to retry loading without
memory profiling if the compressed tag limit is exceeded. It removes the
profiling enabled guard when checking for tag overflow and handles the
fallback by resetting section flags and repeating the layout process.

> diff --git a/kernel/module/main.c b/kernel/module/main.c
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c

[ ... ]

> @@ -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);

[Severity: Critical]
This isn't a bug introduced by this patch, but can concurrent module loading
cause memory corruption if profiling is disabled mid-flight?

In move_module(), if one thread disables profiling globally via
shutdown_mem_profiling() while another thread is halfway through module
loading (between layout calculation and memory movement), the second thread's
section offset queries could evaluate to false.

This would cause the module loader to use an un-incremented 0 offset and
incorrectly write the codetag section into the base memory, overwriting the
start of the module's core memory block.

> -	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 = 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;
> +		goto retry;

[Severity: High]
Does this retry mechanism cause a memory leak for per-CPU counters?

When the module load is retried via this new loop, the codetag section is
treated as standard memory and bypasses maple tree registration. However, the
subsequent codetag module initialization unconditionally allocates per-CPU
memory for these tags regardless of the profiling state.

During module unload in release_module_tags(), the cleanup path exits early
because the module cannot be found in the maple tree, permanently leaking
the per-CPU memory on every unload.

> +	}
>  
>  	/* 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
> --- 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;

[Severity: High]
This is a pre-existing issue, but does the update to module_tags.size race
with concurrent module loads?

The high-water mark for the module tags size is updated here without a lock
after the maple tree lock is released. If two concurrent module loads race on
this update, a higher offset might be overwritten by a smaller one.

Subsequently, vm_module_tags_populate() relies on this corrupted size and
would fail to map the physical pages for the higher virtual addresses, leading
to a page fault when the module accesses its allocated tags.

> -		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();

[Severity: High]
This is a pre-existing issue, but could this lead to a dangling pointer in
the maple tree if page population fails?

If vm_module_tags_populate() fails and returns an error, the allocation
routine returns an error here but doesn't remove the newly inserted module
pointer from the maple tree.

Since this error bypasses the standard codetag cleanup routines in the module
loader (where codetag_section_found might remain false in move_module()),
the maple tree retains a pointer to the aborted module. Once the module
structure is freed, future maple tree iterations could dereference this freed
pointer.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260812054105.102637-1-hao.ge@linux.dev?part=2

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

[Hao Ge]

CC Andrew and Suren


On 2026/8/12 14:04, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:
> 
> New issues:
> - [High] Unconditional allocation of per-CPU counters for modules causes a memory leak when memory profiling is disabled and the module load is retried.
> 
> Pre-existing issues:
> - [Critical] Concurrent module loading can cause an out-of-bounds memory write and module corruption because the global profiling flag is toggled without cross-phase synchronization.
> - [High] A race condition on the global module tags size variable can lead to unmapped virtual memory access and a kernel panic.
> - [High] If page population fails, a dangling module pointer is left in the maple tree, leading to a Use-After-Free.
> --
> 
> --- Patch [2]: [PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled ---
> commit ffb2b208763f32cdfb3d6cb740d4d4efa5c99ae0
> Author: Hao Ge <hao.ge@linux.dev>
> 
> alloc_tag: fix undetected compressed tag overflow when profiling is disabled
> 
> This patch modifies module layout and allocation to retry loading without
> memory profiling if the compressed tag limit is exceeded. It removes the
> profiling enabled guard when checking for tag overflow and handles the
> fallback by resetting section flags and repeating the layout process.
> 
>> diff --git a/kernel/module/main.c b/kernel/module/main.c
>> --- a/kernel/module/main.c
>> +++ b/kernel/module/main.c
> 
> [ ... ]
> 
>> @@ -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);
> 
> [Severity: Critical]
> This isn't a bug introduced by this patch, but can concurrent module loading
> cause memory corruption if profiling is disabled mid-flight?
> 
> In move_module(), if one thread disables profiling globally via
> shutdown_mem_profiling() while another thread is halfway through module
> loading (between layout calculation and memory movement), the second thread's
> section offset queries could evaluate to false.
> 
> This would cause the module loader to use an un-incremented 0 offset and
> incorrectly write the codetag section into the base memory, overwriting the
> start of the module's core memory block.
> 
>> -	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 = 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;
>> +		goto retry;
> 
> [Severity: High]
> Does this retry mechanism cause a memory leak for per-CPU counters?
> 
> When the module load is retried via this new loop, the codetag section is
> treated as standard memory and bypasses maple tree registration. However, the
> subsequent codetag module initialization unconditionally allocates per-CPU
> memory for these tags regardless of the profiling state.
> 
> During module unload in release_module_tags(), the cleanup path exits early
> because the module cannot be found in the maple tree, permanently leaking
> the per-CPU memory on every unload.

The per-CPU counter leak is pre-existing, not introduced by the retry 
 
 
                                                      mechanism.

The root cause is that shutdown_mem_profiling() permanently sets 
 
 
 
mem_profiling_support to false, after which needs_section_mem() 
 
 
                                                        returns false 
for every subsequent module. Their codetag sections are 
 
 
                                         placed as regular data, so they 
are never inserted into the alloc_tag 
 
 
                       maple tree. Yet codetag_load_module() still 
unconditionally invokes 
 
 
          the load_module() callback, which allocates percpu counters 
that 
 
 
release_module_tags() cannot find on unload.

I'll send out a separate patch to fix this leak later.



> 
>> +	}
>>   
>>   	/* 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
>> --- 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;
> 
> [Severity: High]
> This is a pre-existing issue, but does the update to module_tags.size race
> with concurrent module loads?
> 
> The high-water mark for the module tags size is updated here without a lock
> after the maple tree lock is released. If two concurrent module loads race on
> this update, a higher offset might be overwritten by a smaller one.
> 
> Subsequently, vm_module_tags_populate() relies on this corrupted size and
> would fail to map the physical pages for the higher virtual addresses, leading
> to a page fault when the module accesses its allocated tags.
> 
>> -		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();
> 
> [Severity: High]
> This is a pre-existing issue, but could this lead to a dangling pointer in
> the maple tree if page population fails?
> 
> If vm_module_tags_populate() fails and returns an error, the allocation
> routine returns an error here but doesn't remove the newly inserted module
> pointer from the maple tree.
> 
> Since this error bypasses the standard codetag cleanup routines in the module
> loader (where codetag_section_found might remain false in move_module()),
> the maple tree retains a pointer to the aborted module. Once the module
> structure is freed, future maple tree iterations could dereference this freed
> pointer.
> 

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

[Hao Ge]

On 2026/8/12 14:32, Hao Ge wrote:
> CC Andrew and Suren
> 
> 
> On 2026/8/12 14:04, sashiko-bot@kernel.org wrote:
>> Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:
>>
>> New issues:
>> - [High] Unconditional allocation of per-CPU counters for modules causes a memory leak when memory profiling is disabled and the module load is retried.
>>
>> Pre-existing issues:
>> - [Critical] Concurrent module loading can cause an out-of-bounds memory write and module corruption because the global profiling flag is toggled without cross-phase synchronization.
>> - [High] A race condition on the global module tags size variable can lead to unmapped virtual memory access and a kernel panic.
>> - [High] If page population fails, a dangling module pointer is left in the maple tree, leading to a Use-After-Free.
>> -- 
>>
>> --- Patch [2]: [PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled ---
>> commit ffb2b208763f32cdfb3d6cb740d4d4efa5c99ae0
>> Author: Hao Ge <hao.ge@linux.dev>
>>
>> alloc_tag: fix undetected compressed tag overflow when profiling is disabled
>>
>> This patch modifies module layout and allocation to retry loading without
>> memory profiling if the compressed tag limit is exceeded. It removes the
>> profiling enabled guard when checking for tag overflow and handles the
>> fallback by resetting section flags and repeating the layout process.
>>
>>> diff --git a/kernel/module/main.c b/kernel/module/main.c
>>> --- a/kernel/module/main.c
>>> +++ b/kernel/module/main.c
>>
>> [ ... ]
>>
>>> @@ -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);
>>
>> [Severity: Critical]
>> This isn't a bug introduced by this patch, but can concurrent module loading
>> cause memory corruption if profiling is disabled mid-flight?
>>
>> In move_module(), if one thread disables profiling globally via
>> shutdown_mem_profiling() while another thread is halfway through module
>> loading (between layout calculation and memory movement), the second thread's
>> section offset queries could evaluate to false.
>>
>> This would cause the module loader to use an un-incremented 0 offset and
>> incorrectly write the codetag section into the base memory, overwriting the
>> start of the module's core memory block.
>>
>>> -    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 = 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;
>>> +        goto retry;
>>
>> [Severity: High]
>> Does this retry mechanism cause a memory leak for per-CPU counters?
>>
>> When the module load is retried via this new loop, the codetag section is
>> treated as standard memory and bypasses maple tree registration. However, the
>> subsequent codetag module initialization unconditionally allocates per-CPU
>> memory for these tags regardless of the profiling state.
>>
>> During module unload in release_module_tags(), the cleanup path exits early
>> because the module cannot be found in the maple tree, permanently leaking
>> the per-CPU memory on every unload.
> 
> The per-CPU counter leak is pre-existing, not introduced by the retry
> 
>                                                      mechanism.
> 
> The root cause is that shutdown_mem_profiling() permanently sets
> 
> 
> mem_profiling_support to false, after which needs_section_mem()
> 
>                                                        returns false for every subsequent module. Their codetag sections are
> 
>                                         placed as regular data, so they are never inserted into the alloc_tag
> 
>                       maple tree. Yet codetag_load_module() still unconditionally invokes
> 
>          the load_module() callback, which allocates percpu counters that
> 
> release_module_tags() cannot find on unload.
> 
> I'll send out a separate patch to fix this leak later.
> 
> 

Sorry for the messy indentation — it's down to my mail client's formatting config.
A colleague pointed out the official recommended indent configs for kernel mail clients,
which I'd missed entirely.
https://elixir.bootlin.com/linux/v7.2-rc7/source/Documentation/process/email-clients.rst
Sorry for all the hassle I've caused everyone earlier.
The patch fixing the per-CPU counter leak has been sent out:
https://lore.kernel.org/all/20260813093421.135230-2-hao.ge@linux.dev/

Thanks
Best Regards
Hao

> 
>>
>>> +    }
>>>         /* 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
>>> --- 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;
>>
>> [Severity: High]
>> This is a pre-existing issue, but does the update to module_tags.size race
>> with concurrent module loads?
>>
>> The high-water mark for the module tags size is updated here without a lock
>> after the maple tree lock is released. If two concurrent module loads race on
>> this update, a higher offset might be overwritten by a smaller one.
>>
>> Subsequently, vm_module_tags_populate() relies on this corrupted size and
>> would fail to map the physical pages for the higher virtual addresses, leading
>> to a page fault when the module accesses its allocated tags.
>>
>>> -        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();
>>
>> [Severity: High]
>> This is a pre-existing issue, but could this lead to a dangling pointer in
>> the maple tree if page population fails?
>>
>> If vm_module_tags_populate() fails and returns an error, the allocation
>> routine returns an error here but doesn't remove the newly inserted module
>> pointer from the maple tree.
>>
>> Since this error bypasses the standard codetag cleanup routines in the module
>> loader (where codetag_section_found might remain false in move_module()),
>> the maple tree retains a pointer to the aborted module. Once the module
>> structure is freed, future maple tree iterations could dereference this freed
>> pointer.
>>

Re: [PATCH v5 1/2] alloc_tag: move release_module_tags() above reserve_module_tags()

[Suren Baghdasaryan]

On Tue, Aug 11, 2026 at 10:41 PM Hao Ge <hao.ge@linux.dev> wrote:
>
> release_module_tags() is a cleanup helper. reserve_module_tags() can
> also fail after storing the reservation in the maple tree, in which
> case it should call release_module_tags() to undo it. Move the helper
> above reserve_module_tags() so no forward declaration is needed.
>
> No functional change.
>
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hao Ge <hao.ge@linux.dev>

Acked-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/alloc_tag.c | 92 +++++++++++++++++++++++++-------------------------
>  1 file changed, 46 insertions(+), 46 deletions(-)
>
> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> index 52aece27b00e..af44f90379f2 100644
> --- a/mm/alloc_tag.c
> +++ b/mm/alloc_tag.c
> @@ -835,6 +835,52 @@ static int vm_module_tags_populate(void)
>         return 0;
>  }
>
> +static void release_module_tags(struct module *mod, bool used)
> +{
> +       MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
> +       struct alloc_tag *start_tag;
> +       struct alloc_tag *end_tag;
> +       struct module *val;
> +
> +       mas_lock(&mas);
> +       mas_for_each_rev(&mas, val, 0)
> +               if (val == mod)
> +                       break;
> +
> +       if (!val) /* module not found */
> +               goto out;
> +
> +       if (!used)
> +               goto release_area;
> +
> +       start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
> +       end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
> +       if (!clean_unused_counters(start_tag, end_tag)) {
> +               struct alloc_tag *tag;
> +
> +               for (tag = start_tag; tag <= end_tag; tag++) {
> +                       struct alloc_tag_counters counter;
> +
> +                       if (!tag->counters)
> +                               continue;
> +
> +                       counter = alloc_tag_read(tag);
> +                       pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
> +                               tag->ct.filename, tag->ct.lineno, tag->ct.modname,
> +                               tag->ct.function, counter.bytes);
> +               }
> +       } else {
> +               used = false;
> +       }
> +release_area:
> +       mas_store(&mas, used ? &unloaded_mod : NULL);
> +       val = mas_prev_range(&mas, 0);
> +       if (val == &prepend_mod)
> +               mas_store(&mas, NULL);
> +out:
> +       mas_unlock(&mas);
> +}
> +
>  static void *reserve_module_tags(struct module *mod, unsigned long size,
>                                  unsigned int prepend, unsigned long align)
>  {
> @@ -922,52 +968,6 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
>         return (struct alloc_tag *)(module_tags.start_addr + offset);
>  }
>
> -static void release_module_tags(struct module *mod, bool used)
> -{
> -       MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
> -       struct alloc_tag *start_tag;
> -       struct alloc_tag *end_tag;
> -       struct module *val;
> -
> -       mas_lock(&mas);
> -       mas_for_each_rev(&mas, val, 0)
> -               if (val == mod)
> -                       break;
> -
> -       if (!val) /* module not found */
> -               goto out;
> -
> -       if (!used)
> -               goto release_area;
> -
> -       start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
> -       end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
> -       if (!clean_unused_counters(start_tag, end_tag)) {
> -               struct alloc_tag *tag;
> -
> -               for (tag = start_tag; tag <= end_tag; tag++) {
> -                       struct alloc_tag_counters counter;
> -
> -                       if (!tag->counters)
> -                               continue;
> -
> -                       counter = alloc_tag_read(tag);
> -                       pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
> -                               tag->ct.filename, tag->ct.lineno, tag->ct.modname,
> -                               tag->ct.function, counter.bytes);
> -               }
> -       } else {
> -               used = false;
> -       }
> -release_area:
> -       mas_store(&mas, used ? &unloaded_mod : NULL);
> -       val = mas_prev_range(&mas, 0);
> -       if (val == &prepend_mod)
> -               mas_store(&mas, NULL);
> -out:
> -       mas_unlock(&mas);
> -}
> -
>  static int load_module(struct module *mod, struct codetag *start, struct codetag *stop)
>  {
>         /* Allocate module alloc_tag percpu counters */
> --
> 2.25.1
>

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

[Suren Baghdasaryan]

On Tue, Aug 11, 2026 at 10:41 PM Hao Ge <hao.ge@linux.dev> 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.
>
> 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>

I'm looking at your fixes for pre-existing issues reported by Sashiko
but this patch looks correct to me.

Acked-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  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];
>
>         /* 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);
> +               /*
> +                * -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;
> +               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..0a7b657fe2de 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();
> --
> 2.25.1
>

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

[Petr Pavlu]

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

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

[Suren Baghdasaryan]

On Sat, Aug 15, 2026 at 3:45 AM Petr Pavlu <petr.pavlu@suse.com> wrote:
>
> 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?

Correct, alloc_hooks() will have references into .codetag.alloc_tags.
With mem_profiling_support=false they should technically never be used
but I don't think it's a good idea to skip .codetag.alloc_tags section
allocation and to leave dangling pointers. Also the case described
here is an outlier, so optimizing it would not yield much benefit.

>
> >
> > 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.

That's a good point.

>
> > +             /*
> > +              * -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?

I believe layout_symtab() sets that bit and to retry we need to reset
it. But I might be wrong 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.

Hmm, yeah if we redirect the references into .codetag.alloc_tags, that
would be much better.

>
> 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.

If your approach does not cause module loading to fail when we disable
profiling, then that sounds like a good idea. If it's not too much
trouble, could you please send an RFC?

>
> --
> Thanks,
> Petr

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

[Hao Ge]

On 2026/8/16 23:16, Suren Baghdasaryan wrote:
> On Sat, Aug 15, 2026 at 3:45 AM Petr Pavlu <petr.pavlu@suse.com> wrote:
>>
>> 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?
> 
> Correct, alloc_hooks() will have references into .codetag.alloc_tags.
> With mem_profiling_support=false they should technically never be used
> but I don't think it's a good idea to skip .codetag.alloc_tags section
> allocation and to leave dangling pointers. Also the case described
> here is an outlier, so optimizing it would not yield much benefit.
> 
>>
>>>
>>> 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.
> 
> That's a good point.
> 
>>
>>> +             /*
>>> +              * -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?
> 
> I believe layout_symtab() sets that bit and to retry we need to reset
> it. But I might be wrong here.
> 

Thanks Suren.

And yes, IMHO layout_symtab() is the reason. In the module ELF, .symtab and.strtab
carry no flags at all. layout_symtab() sets SHF_ALLOC on them so that move_module()
will copy them, and it places them itself at the end of MOD_INIT_DATA.

Without the clearing, __layout_sections() on the second pass would pick the two
sections up again: SHF_ALLOC set, no SHF_WRITE, so they match its RODATA mask and get
some of MOD_RODATA reserved. Then layout_symtab() runs anyway, overwrites sh_entsize
and puts them into MOD_INIT_DATA, same as the first pass. The reserved MOD_RODATA is
never used by anything, so the module would just carry that dead space for no reason.
Hence the clearing.

Or am I missing any details?

>>
>>> +             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.
> 
> Hmm, yeah if we redirect the references into .codetag.alloc_tags, that
> would be much better.
> 
>>
>> 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.
> 
> If your approach does not cause module loading to fail when we disable
> profiling, then that sounds like a good idea. If it's not too much
> trouble, could you please send an RFC?
> 
>>
>> --
>> Thanks,
>> Petr