pci: Error on PCI capability collisions

Nothing good can happen when we overlap capabilities. This may happen
when plugging in assigned devices or when devices models contain bugs.
Detect the overlap and report it.

Based on qemu-kvm commit by Alex Williamson.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Don Dutile <ddutile@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/pci.c

@@ -1811,6 +1811,25 @@ static uint8_t pci_find_capability_list(PCIDevice *pdev, uint8_t cap_id,
     return next;
 }
 
+static uint8_t pci_find_capability_at_offset(PCIDevice *pdev, uint8_t offset)
+{
+    uint8_t next, prev, found = 0;
+
+    if (!(pdev->used[offset])) {
+        return 0;
+    }
+
+    assert(pdev->config[PCI_STATUS] & PCI_STATUS_CAP_LIST);
+
+    for (prev = PCI_CAPABILITY_LIST; (next = pdev->config[prev]);
+         prev = next + PCI_CAP_LIST_NEXT) {
+        if (next <= offset && next > found) {
+            found = next;
+        }
+    }
+    return found;
+}
+
 /* Patch the PCI vendor and device ids in a PCI rom image if necessary.
    This is needed for an option rom which is used for more than one device. */
 static void pci_patch_ids(PCIDevice *pdev, uint8_t *ptr, int size)
@@ -1952,11 +1971,30 @@ int pci_add_capability(PCIDevice *pdev, uint8_t cap_id,
                        uint8_t offset, uint8_t size)
 {
     uint8_t *config;
+    int i, overlapping_cap;
+
     if (!offset) {
         offset = pci_find_space(pdev, size);
         if (!offset) {
             return -ENOSPC;
         }
+    } else {
+        /* Verify that capabilities don't overlap.  Note: device assignment
+         * depends on this check to verify that the device is not broken.
+         * Should never trigger for emulated devices, but it's helpful
+         * for debugging these. */
+        for (i = offset; i < offset + size; i++) {
+            overlapping_cap = pci_find_capability_at_offset(pdev, i);
+            if (overlapping_cap) {
+                fprintf(stderr, "ERROR: %04x:%02x:%02x.%x "
+                        "Attempt to add PCI capability %x at offset "
+                        "%x overlaps existing capability %x at offset %x\n",
+                        pci_find_domain(pdev->bus), pci_bus_num(pdev->bus),
+                        PCI_SLOT(pdev->devfn), PCI_FUNC(pdev->devfn),
+                        cap_id, offset, overlapping_cap, i);
+                return -EINVAL;
+            }
+        }
     }
 
     config = pdev->config + offset;