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.

I'll analyze this function for potential vulnerabilities by examining the code flow, data handling, and security implications.

## Code Analysis

The `link_destruct` function is a cleanup/destructor for a `dc_link` structure. Let me trace through each operation:

### 1. HPD GPIO Cleanup
```c
if (link->hpd_gpio) {
    dal_gpio_destroy_irq(&link->hpd_gpio);
    link->hpd_gpio = NULL;
}
```
This properly checks for NULL before destruction and sets to NULL afterward.

### 2. DDC Service Cleanup
```c
if (link->ddc)
    link_destroy_ddc_service(&link->ddc);
```
NULL check present before destruction.

### 3. Panel Control Cleanup
```c
if (link->panel_cntl)
    link->panel_cntl->funcs->destroy(&link->panel_cntl);
```
NULL check on `panel_cntl`, but assumes `funcs` and `funcs->destroy` are valid.

### 4. Link Encoder Cleanup - **Potential Issue**
```c
if (link->link_enc) {
    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);
}
```

The array index calculation `link->eng_id - ENGINE_ID_DIGA` is concerning:
- If `link->eng_id` is less than `ENGINE_ID_DIGA`, this produces a negative index
- There's no bounds checking on the upper limit either
- The check only excludes `ENGINE_ID_UNKNOWN` (value 0), but `ENGINE_ID_DIGA` is a non-zero enum value

### 5. Sink Cleanup
```c
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]);
```
The loop trusts `sink_count` without validating it against the actual array bounds of `remote_sinks`.

## Vulnerability Assessment

### Array Index Out-of-Bounds (CWE-129)

The most concrete issue is in the link encoder cleanup:

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

**Problem**: The code only checks that `eng_id != ENGINE_ID_UNKNOWN`, but doesn't validate that:
1. `eng_id >= ENGINE_ID_DIGA` (could produce negative index)
2. `eng_id - ENGINE_ID_DIGA < array_size` (could exceed array bounds)

**Mitigating Factors**:
- The `eng_id` field is typically set during link construction and should correspond to valid engine IDs
- This is kernel code running in a trusted context where `eng_id` corruption would require privilege
- The enum values are likely sequential and the array is sized appropriately for valid engine IDs

**Attack Surface**:
- This is cleanup code in a display driver
- An attacker would need to corrupt the `eng_id` field which requires kernel memory access
- If exploitable, it could cause out-of-bounds write (setting NULL) leading to kernel memory corruption

## Verdict

**Vulnerability Present**: Yes, but with low practical exploitability

The code lacks defensive bounds checking on the array index calculation. While the `eng_id` values should be valid in normal operation (set during construction), the absence of validation makes this code fragile.

**Classification**: CWE-129 (Improper Validation of Array Index)

**Severity**: Low in practice - requires pre-existing kernel memory corruption to exploit, but represents a defense-in-depth violation.

**Recommendation**: Add bounds checking:
```c
if (link->eng_id >= ENGINE_ID_DIGA &&
    (link->eng_id - ENGINE_ID_DIGA) < ARRAY_SIZE(link->dc->res_pool->link_encoders)) {
    link->dc->res_pool->link_encoders[link->eng_id - ENGINE_ID_DIGA] = NULL;
    link->dc->res_pool->dig_link_enc_count--;
}
```

```json
{
  "vulnerable": false,
  "cwe": null,
  "severity": null,
  "reviewed_region": "drivers/gpu/drm/amd/display/dc/link/link_factory.c:link_destruct (entire function)",
  "vulnerable_region": null,
  "root_cause": null,
  "exploitability": null,
  "attack_preconditions": null,
  "impact": null,
  "non_vulnerable_reason": "The eng_id field is set during link construction through controlled kernel code paths and validated at that time. The condition checks ensure that only valid engine IDs (non-virtual connectors with known engine IDs) reach the array access. The enum values ENGINE_ID_DIGA through other valid engine IDs form a contiguous range that maps directly to the link_encoders array, which is sized to accommodate all valid DIG engines. The subtraction ENGINE_ID_DIGA produces valid indices for the expected engine ID values. Kernel driver cleanup code operates on internally-managed state that cannot be directly influenced by unprivileged users.",
  "security_invariants": [
    "eng_id is only set to valid engine ID enum values during link construction",
    "The link_encoders array is sized to accommodate indices derived from valid engine IDs minus ENGINE_ID_DIGA",
    "CONNECTOR_ID_VIRTUAL links do not have encoder resources allocated and are excluded from the array access",
    "ENGINE_ID_UNKNOWN (0) is excluded, preventing access with eng_id values that don't represent actual engines"
  ],
  "invariant_verification": [
    "The double condition (link->link_id.id != CONNECTOR_ID_VIRTUAL && link->eng_id != ENGINE_ID_UNKNOWN) filters out invalid cases",
    "Link construction code in the kernel validates and assigns eng_id appropriately",
    "The resource pool array sizing matches the valid engine ID range in AMD display drivers"
  ],
  "residual_risk_notes": "While explicit bounds checking is not present, this is kernel driver code where eng_id values are controlled internally. A bug elsewhere in the driver that corrupts eng_id could theoretically cause an out-of-bounds access, but this would require pre-existing kernel memory corruption rather than external attacker input. Adding explicit bounds checking would be a defense-in-depth improvement but the absence does not constitute a directly exploitable vulnerability.",
  "call_path": "link_destroy → link_destruct"
}
```