xen/dt-overlay: support phandle-based targeting in overlay_get_nodes_info

overlay_get_nodes_info() is called before fdt_overlay_apply() to extract
target paths from the overlay. This fails for overlays using phandle-based
targeting (target = <&label>) because DTC compiles these as unresolved
fixups (target = <0xffffffff>), causing fdt_overlay_target_offset() to
return -FDT_ERR_BADPHANDLE. Prior to this change users were forced to
manually modify the dtbo (even for hwdom) to switch from target to
target-phandle by manually inspecting also the host DTB.

Introduce overlay_get_target_path() which directly handles the two
targeting cases that occur before fixup resolution:
 - target-path: the string property is returned directly.
 - target = <&label>: the label is found in the overlay's __fixups__
   node, then resolved to a path via the base DTB's __symbols__ node.

Libfdt fdt_for_each_property_offset() violates MISRA R20.7. Despite
libfdt being excluded from the analysis, this causes ECLAIR scan to
report a regression because the violation introduced by a macro is
reported at the call site. Deviate R20.7 for libfdt.h.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Nicola Vetrini <nicola.vetrini@bugseng.com>

automation/eclair_analysis/ECLAIR/deviations.ecl

@@ -629,8 +629,10 @@ of the IS_ENABLED or STATIC_IF/STATIC_IF_NOT macros, so it always receives a lit
 not in scope for compliance are allowed, as that is imported code."
 -file_tag+={gnu_efi_include, "^xen/include/efi/.*$"}
 -file_tag+={acpi_cpu_idle, "^xen/arch/x86/acpi/cpu_idle\\.c$"}
+-file_tag+={libfdt, "^xen/include/xen/libfdt/libfdt\\.h$"}
 -config=MC3A2.R20.7,reports+={safe, "any_area(any_loc(file(gnu_efi_include)||any_exp(macro(^NextMemoryDescriptor$))))"}
 -config=MC3A2.R20.7,reports+={safe, "any_area(any_loc(file(acpi_cpu_idle)))"}
+-config=MC3A2.R20.7,reports+={safe, "any_area(any_loc(any_exp(file(libfdt))))"}
 -doc_end
 
 -doc_begin="To avoid compromising readability, the macros alternative_(v)?call[0-9] are allowed

xen/common/device-tree/dt-overlay.c

@@ -286,6 +286,63 @@ static unsigned int overlay_node_count(const void *overlay_fdt)
     return num_overlay_nodes;
 }
 
+/*
+ * Resolve the target path for an overlay fragment.
+ *
+ * This is called before fdt_overlay_apply(), so phandle-based targets
+ * (target = <&label>) are still unresolved (compiled as 0xffffffff by DTC).
+ * Handle the two cases that actually occur:
+ *  - target-path property: the path string is used directly,
+ *  - target = <&label>: the label is looked up in the overlay's __fixups__
+ *    node, then resolved to a path via the base DTB's __symbols__ node.
+ *
+ * Returns a pointer into the FDT on success, NULL on failure.
+ */
+static const char *overlay_get_target_path(const void *fdt, const void *fdto,
+                                           int fragment)
+{
+    const char *path, *fragment_name;
+    int fixups_off, symbols_off, property;
+    int fragment_name_len;
+
+    /* Try target-path first (string-based targeting) */
+    path = fdt_getprop(fdto, fragment, "target-path", NULL);
+    if ( path )
+        return path;
+
+    /* Phandle-based target: resolve via __fixups__ and __symbols__ */
+    fixups_off = fdt_path_offset(fdto, "/__fixups__");
+    if ( fixups_off < 0 )
+        return NULL;
+
+    symbols_off = fdt_path_offset(fdt, "/__symbols__");
+    if ( symbols_off < 0 )
+        return NULL;
+
+    fragment_name = fdt_get_name(fdto, fragment, &fragment_name_len);
+    if ( !fragment_name )
+        return NULL;
+
+    fdt_for_each_property_offset(property, fdto, fixups_off)
+    {
+        const char *val, *label, *p;
+        int val_len;
+
+        val = fdt_getprop_by_offset(fdto, property, &label, &val_len);
+        if ( !val || !val_len || (val[val_len - 1] != '\0') )
+            continue;
+
+        /* Match entries of the form "/<fragment_name>:target:0" */
+        for ( p = val; p < (val + val_len); p += (strlen(p) + 1) )
+            if ( p[0] == '/' &&
+                 !strncmp(p + 1, fragment_name, fragment_name_len) &&
+                 !strcmp(p + 1 + fragment_name_len, ":target:0") )
+                return fdt_getprop(fdt, symbols_off, label, NULL);
+    }
+
+    return NULL;
+}
+
 /*
  * overlay_get_nodes_info gets full name with path for all the nodes which
  * are in one level of __overlay__ tag. This is useful when checking node for
@@ -298,7 +355,6 @@ static int overlay_get_nodes_info(const void *fdto, char **nodes_full_path)
 
     fdt_for_each_subnode(fragment, fdto, 0)
     {
-        int target;
         int overlay;
         int subnode;
         const char *target_path;
@@ -307,11 +363,8 @@ static int overlay_get_nodes_info(const void *fdto, char **nodes_full_path)
         if ( overlay < 0 )
             continue;
 
-        target = fdt_overlay_target_offset(device_tree_flattened, fdto,
+        target_path = overlay_get_target_path(device_tree_flattened, fdto,
-                                           fragment, &target_path);
+                                              fragment);
-        if ( target < 0 )
-            return target;
-
         if ( target_path == NULL )
             return -EINVAL;