[PATCH v2 06/10] flattree: Handle unknown tags

Herve Codina
2026-04-09T11:55:01Z

The structured tag value definition introduced recently gives the
ability to ignore unknown tags without any error when they are read.

Handle those structured tag.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
flattree.c | 65 ++++++++++++++++++++--
tests/run_tests.sh | 5 ++
tests/unknown_tags_can_skip.dtb.dts.expect | 19 +++++++
3 files changed, 84 insertions(+), 5 deletions(-)
create mode 100644 tests/unknown_tags_can_skip.dtb.dts.expect

diff --git a/flattree.c b/flattree.c
index f3b698c..88dbfa7 100644
--- a/flattree.c
+++ b/flattree.c
@@ -579,7 +579,8 @@ static void flat_read_chunk(struct inbuf *inb, void *p, int len)
if ((inb->ptr + len) > inb->limit)
die("Premature end of data parsing flat device tree\n");

- memcpy(p, inb->ptr, len);
+ if (p)
+ memcpy(p, inb->ptr, len);

inb->ptr += len;
}
@@ -604,6 +605,61 @@ static void flat_realign(struct inbuf *inb, int align)
die("Premature end of data parsing flat device tree\n");
}

+static bool flat_skip_unknown_tag(struct inbuf *inb, uint32_t tag)
+{
+ uint32_t lng;
+
+ if (!(tag & FDT_TAG_STRUCTURED) || !(tag & FDT_TAG_SKIP_SAFE))
+ return false;
+
+ switch (tag & FDT_TAG_DATA_MASK) {
+ case FDT_TAG_DATA_NONE:
+ break;
+
+ case FDT_TAG_DATA_1CELL:
+ flat_read_word(inb);
+ break;
+
+ case FDT_TAG_DATA_2CELLS:
+ flat_read_word(inb);
+ flat_read_word(inb);
+ break;
+
+ case FDT_TAG_DATA_VARLEN:
+ /* Get the length */
+ lng = flat_read_word(inb);
+
+ /* Skip the following length bytes */
+ flat_read_chunk(inb, NULL, lng);
+
+ flat_realign(inb, sizeof(uint32_t));
+ break;
+ }
+
+ return true;
+}
+
+static uint32_t flat_read_tag(struct inbuf *inb)
+{
+ uint32_t tag;
+
+ do {
+ tag = flat_read_word(inb);
+ switch (tag) {
+ case FDT_BEGIN_NODE:
+ case FDT_END_NODE:
+ case FDT_PROP:
+ case FDT_NOP:
+ case FDT_END:
+ return tag;
+ default:
+ break;
+ }
+ } while (flat_skip_unknown_tag(inb, tag));
+
+ die("Cannot skip unknown tag 0x%08x\n", tag);
+}
+
static const char *flat_read_string(struct inbuf *inb)
{
int len = 0;
@@ -750,7 +806,7 @@ static struct node *unflatten_tree(struct inbuf *dtbuf,
struct property *prop;
struct node *child;

- val = flat_read_word(dtbuf);
+ val = flat_read_tag(dtbuf);
switch (val) {
case FDT_PROP:
if (node->children)
@@ -905,14 +961,13 @@ struct dt_info *dt_from_blob(const char *fname)

reservelist = flat_read_mem_reserve(&memresvbuf);

- val = flat_read_word(&dtbuf);
-
+ val = flat_read_tag(&dtbuf);
if (val != FDT_BEGIN_NODE)
die("Device tree blob doesn't begin with FDT_BEGIN_NODE (begins with 0x%08x)\n", val);

tree = unflatten_tree(&dtbuf, &strbuf, "", flags);

- val = flat_read_word(&dtbuf);
+ val = flat_read_tag(&dtbuf);
if (val != FDT_END)
die("Device tree blob doesn't end with FDT_END\n");

diff --git a/tests/run_tests.sh b/tests/run_tests.sh
index f2855dd..d147011 100755
--- a/tests/run_tests.sh
+++ b/tests/run_tests.sh
@@ -881,6 +881,11 @@ dtc_tests () {

# Tests for overlay/plugin generation
dtc_overlay_tests
+
+ # Tests with "unknown tags"
+ run_dtc_test -I dtb -O dts -o unknown_tags_can_skip.dtb.dts unknown_tags_can_skip.dtb
+ base_run_test check_diff unknown_tags_can_skip.dtb.dts "$SRCDIR/unknown_tags_can_skip.dtb.dts.expect"
+ run_wrap_error_test $DTC -I dtb -O dts -o unknown_tags_no_skip.dtb.dts unknown_tags_no_skip.dtb
}

cmp_tests () {
diff --git a/tests/unknown_tags_can_skip.dtb.dts.expect b/tests/unknown_tags_can_skip.dtb.dts.expect
new file mode 100644
index 0000000..a11ee57
--- /dev/null
+++ b/tests/unknown_tags_can_skip.dtb.dts.expect
@@ -0,0 +1,19 @@
+/dts-v1/;
+
+/ {
+ prop-int = <0x3201>;
+ prop-str = "abcd";
+
+ subnode1 {
+ prop-int = <0x6401 0x6402>;
+ };
+
+ subnode2 {
+ prop-int1 = <0x64020 0x64021>;
+ prop-int2 = <0x32022>;
+
+ subsubnode {
+ prop-int = <0x64023 0x64024>;
+ };
+ };
+};
--
2.53.0