Audit the following candidate code to determine whether it presents a real vulnerability.
This sample may come from a noisy retrieval or transformation stage, so reason from the supplied code and context rather than assuming the label.

Repository: git.kernel.org/pub/scm/linux/kernel/git/stable/linux
Language: C
File: drivers/gpu/drm/amd/display/dc/link/link_factory.c
Function: link_destruct

Call path: link_destroy (drivers/gpu/drm/amd/display/dc/link/link_factory.c) → link_destruct (drivers/gpu/drm/amd/display/dc/link/link_factory.c)

### Primary Function

```c
static void link_destruct(struct dc_link *link)
{
	int i;

	if (link->hpd_gpio) {
		dal_gpio_destroy_irq(&link->hpd_gpio);
		link->hpd_gpio = NULL;
	}

	if (link->ddc)
		link_destroy_ddc_service(&link->ddc);

	if (link->panel_cntl)
		link->panel_cntl->funcs->destroy(&link->panel_cntl);

	if (link->link_enc) {
		/* Update link encoder resource tracking variables. These are used for
		 * the dynamic assignment of link encoders to streams. Virtual links
		 * are not assigned encoder resources on creation.
		 */
		if (link->link_id.id != CONNECTOR_ID_VIRTUAL && link->eng_id != ENGINE_ID_UNKNOWN) {
			link->dc->res_pool->link_encoders[link->eng_id - ENGINE_ID_DIGA] = NULL;
			link->dc->res_pool->dig_link_enc_count--;
		}
		link->link_enc->funcs->destroy(&link->link_enc);
	}

	if (link->local_sink)
		dc_sink_release(link->local_sink);

	for (i = 0; i < link->sink_count; ++i)
		dc_sink_release(link->remote_sinks[i]);
}
```

### Cross-File Context

[ENGINE_ID_UNKNOWN — constant — drivers/gpu/drm/amd/display/dc/dc_types.h (referenced)]
ENGINE_ID_UNKNOWN → 0  (drivers/gpu/drm/amd/display/dc/dc_types.h (referenced))

[ENGINE_ID_DIGA — constant — drivers/gpu/drm/amd/display/dc/dc_types.h (referenced)]
ENGINE_ID_DIGA → ENGINE_ID_DIGA is a non-zero enum value in enum engine_id  (drivers/gpu/drm/amd/display/dc/dc_types.h (referenced))

[CONNECTOR_ID_VIRTUAL — constant — drivers/gpu/drm/amd/display/dc/dc_types.h (referenced)]
CONNECTOR_ID_VIRTUAL → CONNECTOR_ID_VIRTUAL is a enum value for virtual connectors  (drivers/gpu/drm/amd/display/dc/dc_types.h (referenced))

Control flow: The function sequentially checks and destroys optional sub-resources: hpd_gpio, ddc, panel_cntl, and link_enc. Within the link_enc block, it conditionally updates a resource tracking array and counter based on link and engine identifiers before destroying the encoder. Finally, it releases the local sink and iterates through the remote_sinks array to release each entry.

Data flow: Input is a pointer to a dc_link structure. Fields are read to determine cleanup actions. link->eng_id is subtracted by ENGINE_ID_DIGA to compute an index into the link_encoders array. link->sink_count controls the iteration limit for the remote_sinks array. All pointers are validated before dereferencing or passing to destroy/release functions.

Verdict: Not Vulnerable

Reviewed region:
[drivers/gpu/drm/amd/display/dc/link/link_factory.c]
```c
static void link_destruct(struct dc_link *link)
{
	int i;

	if (link->hpd_gpio) {
		dal_gpio_destroy_irq(&link->hpd_gpio);
		link->hpd_gpio = NULL;
	}

	if (link->ddc)
		link_destroy_ddc_service(&link->ddc);

	if (link->panel_cntl)
		link->panel_cntl->funcs->destroy(&link->panel_cntl);

	if (link->link_enc) {
		/* Update link encoder resource tracking variables. These are used for
		 * the dynamic assignment of link encoders to streams. Virtual links
		 * are not assigned encoder resources on creation.
		 */
		if (link->link_id.id != CONNECTOR_ID_VIRTUAL && link->eng_id != ENGINE_ID_UNKNOWN) {
			link->dc->res_pool->link_encoders[link->eng_id - ENGINE_ID_DIGA] = NULL;
			link->dc->res_pool->dig_link_enc_count--;
		}
		link->link_enc->funcs->destroy(&link->link_enc);
	}

	if (link->local_sink)
		dc_sink_release(link->local_sink);

	for (i = 0; i < link->sink_count; ++i)
		dc_sink_release(link->remote_sinks[i]);
}
```

Why it is not vulnerable: Array access for link_encoders is guarded by a check ensuring link->eng_id is not ENGINE_ID_UNKNOWN, and relies on initialization code guaranteeing eng_id contains a valid enum value that maps correctly to the array bounds after subtraction. The remote_sinks loop is strictly bounded by sink_count. All pointer dereferences are protected by null checks.

Security invariants:
- link->eng_id must hold a valid enum value corresponding to the allocated link encoder slots.
- link->sink_count must accurately reflect the number of valid entries in link->remote_sinks.
- All sub-resource pointers must be either NULL or valid addresses before destruction functions are called.

Invariant verification:
- Array index bounds validation for link_encoders: holds=true. Evidence: The access link->dc->res_pool->link_encoders[link->eng_id - ENGINE_ID_DIGA] is guarded by link->eng_id != ENGINE_ID_UNKNOWN. The index calculation relies on link->eng_id being initialized to a valid enum value within the expected range.
- Loop bounds validation for remote_sinks: holds=true. Evidence: The loop for (i = 0; i < link->sink_count; ++i) strictly uses sink_count as the termination condition, ensuring access only to initialized entries in link->remote_sinks.
- Null pointer dereference prevention: holds=true. Evidence: Each optional pointer (link->hpd_gpio, link->ddc, link->panel_cntl, link->link_enc, link->local_sink) is explicitly checked for non-null before any dereference or function call.

Residual risk notes: The array index calculation assumes link->eng_id remains uncorrupted and within the valid enum range up to the point of destruction. While standard for cleanup routines, the absence of an explicit runtime upper-bound check means safety depends entirely on the integrity of the initialization and state-management paths.

**Verdict: no real vulnerability demonstrated from the supplied code/context**

### What the function does
`link_destruct()` tears down resources owned by a `struct dc_link`:

- destroys `hpd_gpio` IRQ and nulls the pointer
- destroys the DDC service
- destroys panel control
- if a link encoder exists:
  - updates encoder tracking in `dc->res_pool`
  - destroys the encoder
- releases the local sink
- releases each remote sink

### Security-relevant observations

#### 1. Potentially risky array write
The most obvious candidate is:

```c
link->dc->res_pool->link_encoders[link->eng_id - ENGINE_ID_DIGA] = NULL;
link->dc->res_pool->dig_link_enc_count--;
```

This would only be dangerous if `link->eng_id` could be an invalid enum value such that `link->eng_id - ENGINE_ID_DIGA` is out of bounds.

**Observed guards in the code:**
```c
if (link->link_id.id != CONNECTOR_ID_VIRTUAL &&
    link->eng_id != ENGINE_ID_UNKNOWN) {
```

So the code excludes:
- virtual links
- the explicit “unknown” engine id (`0`)

However, from the provided snippet alone, there is **no explicit range check** proving that `eng_id >= ENGINE_ID_DIGA` and is within the size of `link_encoders`.

### Why this is not enough to call it a real vulnerability
To classify this as a real out-of-bounds write, we would need evidence that:
- `link->eng_id` is attacker-influenced or can become invalid in reachable execution,
- or construction paths can set inconsistent `eng_id` values,
- or `link_encoders` indexing is not guaranteed by design.

That evidence is **not present** in the supplied context.

This code is in a kernel-internal GPU driver teardown path. The fields involved (`eng_id`, `link_enc`, `dc->res_pool`) appear to be internal driver state, not direct user-controlled input. The comment also indicates a resource-tracking invariant:

```c
/* Update link encoder resource tracking variables. These are used for
 * the dynamic assignment of link encoders to streams. Virtual links
 * are not assigned encoder resources on creation.
 */
```

That strongly suggests:
- non-virtual links with `link->link_enc != NULL` are expected to have a valid assigned encoder,
- `eng_id` is expected to correspond to that encoder assignment.

So while the lack of a defensive bounds check is worth noting as a robustness concern, the provided code does **not** establish a reachable memory corruption bug.

---

#### 2. Destruction via function pointers
There are calls like:

```c
link->panel_cntl->funcs->destroy(&link->panel_cntl);
link->link_enc->funcs->destroy(&link->link_enc);
```

These rely on valid internal object initialization. There are null checks on the owning pointers (`panel_cntl`, `link_enc`) but not on `funcs` or `destroy`.

Again, this is normal kernel object-oriented C style. Nothing shown indicates these function pointers can be corrupted or controlled by an attacker through this path. Without evidence of prior memory corruption or bad initialization, this is **not** a standalone vulnerability.

---

#### 3. Loop over `remote_sinks`
```c
for (i = 0; i < link->sink_count; ++i)
    dc_sink_release(link->remote_sinks[i]);
```

This would be unsafe if `sink_count` could exceed the size of `remote_sinks`, but the structure definition is not provided. There is no evidence here that `sink_count` is untrusted or unchecked elsewhere. As with `eng_id`, this is internal bookkeeping, and no exploitability is shown from the supplied material.

---

### Control-flow / data-flow summary
- Inputs are internal `dc_link` fields.
- The function mostly performs conditional destruction/release of already-allocated objects.
- The only memory write based on a computed index is the `link_encoders[...] = NULL` update.
- The code guards against virtual links and unknown engine IDs.
- The missing explicit bounds check on `eng_id` is a potential defensive-programming gap, but no reachable invalid state is demonstrated.

### Conclusion
Based on the supplied code and context, **this is not a confirmed vulnerability**.

### Residual uncertainty
If other code can create a `dc_link` where:
- `link->link_enc != NULL`, but
- `link->eng_id` is invalid or inconsistent with `res_pool->link_encoders`,

then the array write could become an out-of-bounds access. But that would depend on invariants and construction paths not included here. From the present evidence, this remains **speculative**, not a demonstrated security issue.

Destructor for dc_link-owned resources: destroys GPIO IRQ, DDC service, panel control, link encoder, and releases local/remote sinks. Also clears link encoder tracking in dc->res_pool for non-virtual links with known engine IDs.

No real vulnerability is demonstrated from the supplied code. The potentially risky operation is an indexed write into res_pool->link_encoders using link->eng_id - ENGINE_ID_DIGA, but the snippet alone does not show attacker control over eng_id or a reachable invalid state violating the driver's resource-assignment invariants.

A dc_link object would need to reach link_destruct with internally inconsistent state, such as link->link_enc being non-NULL while link->eng_id is outside the valid encoder range.

An attacker would need some separate capability to corrupt or misinitialize internal kernel driver state, since no direct user-controlled input path is shown here.

From the supplied code alone, no concrete security impact can be established. If external code violated invariants, the indexed write or sink-release loop could become unsafe, but that is speculative based on the provided context.

The function operates on internal driver-owned objects during teardown and includes basic guards for optional resources. For the encoder-tracking update, it excludes virtual links and ENGINE_ID_UNKNOWN before indexing. The surrounding comment indicates a design invariant: only non-virtual links assigned an encoder should participate in encoder-resource tracking. No provided evidence shows that eng_id, sink_count, function pointers, or other fields are attacker-controlled or can be invalid on this path. Therefore the code does not, on the shown evidence, establish a reachable memory-safety or control-flow vulnerability.

If link->link_enc is non-NULL for a non-virtual link, link->eng_id is expected to represent a valid assigned encoder tracked in dc->res_pool.

Virtual links are not assigned encoder resources on creation and are excluded from the resource-tracking update.

ENGINE_ID_UNKNOWN is excluded before computing the encoder array index.

Destructor targets (hpd_gpio, ddc, panel_cntl, link_enc, sinks) are treated as owned internal resources and only acted on when the corresponding pointers/counts indicate presence.

if (link->link_enc) { if (link->link_id.id != CONNECTOR_ID_VIRTUAL && link->eng_id != ENGINE_ID_UNKNOWN) { ... } ... }

for (i = 0; i < link->sink_count; ++i) ...

The resource-tracking comment explicitly states virtual links are not assigned encoder resources on creation.

ENGINE_ID_UNKNOWN is defined as 0; ENGINE_ID_DIGA is a non-zero enum value.

No explicit bounds check proves link->eng_id - ENGINE_ID_DIGA is within link_encoders bounds.

No explicit validation of sink_count against remote_sinks capacity is shown.

No explicit NULL checks for panel_cntl->funcs/destroy or link_enc->funcs/destroy are shown.

These are robustness gaps only in the supplied view. The snippet does not show a reachable path where these invariants are attacker-breakable.

If other code can construct or corrupt dc_link with invalid eng_id while link_enc remains set, the encoder-tracking write could become out-of-bounds.

If sink_count can exceed remote_sinks capacity due to unrelated bugs, the sink-release loop could read beyond the array.

Such conditions are not established by the provided code and context, so they remain speculative.