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

On Thu, Apr 09, 2026 at 01:54:22PM +0200, Herve Codina wrote:
> 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 <[email protected]>
> Reviewed-by: Luca Ceresoli <[email protected]>
> ---

Reviewed-by: Frank Li <[email protected]>

>  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
>