Re: [RFC PATCH v2 04/74] libfdt: Introduce fdt_get_structured_tag_data()

[David Gibson]

On Wed, Aug 26, 2026 at 11:48:33AM +0200, Herve Codina wrote:
> With structured tag, data len can be encoded in the tag itself (no data,
> 1 cell data, 2 cells) or with a dedicated cell (varlen) added between
> the tag and the data.
> 
> In multiple places, getting the data related to the tag will be needed.
> 
> To avoid code duplication, introduce fdt_get_structured_tag_data() which
> purpose is to get the tag data part and its length based on the tag
> value.

I'd prefer to see this folded into the series introducing structured
tags:  the final result is no more complex than it started, so folding
th patches reduces churn and makes it easier to review.

> 
> Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> ---
>  libfdt/fdt.c             | 80 ++++++++++++++++++++++++++--------------
>  libfdt/libfdt_internal.h |  3 ++
>  2 files changed, 55 insertions(+), 28 deletions(-)
> 
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index 506e0dd3..7941632f 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -167,11 +167,56 @@ const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)
>  	return fdt_offset_ptr_(fdt, offset);
>  }
>  
> +int fdt_get_structured_tag_data(uint32_t tag, const void *fdt, int offset,
> +				uint32_t *data_len)
> +{
> +	int data_offset = offset;
> +	uint32_t len = 0, sum;
> +	const fdt32_t *lenp;
> +
> +	if (!(tag & FDT_TAG_STRUCTURED))
> +		return -FDT_ERR_INTERNAL;
> +
> +	switch (tag & FDT_TAG_DATA_MASK) {
> +	case FDT_TAG_DATA_NONE:
> +		break;
> +	case FDT_TAG_DATA_1CELL:
> +		len = FDT_CELLSIZE;
> +		break;
> +	case FDT_TAG_DATA_2CELLS:
> +		len = 2 * FDT_CELLSIZE;
> +		break;
> +	case FDT_TAG_DATA_VARLEN:
> +		/* Get the length */
> +		lenp = fdt_offset_ptr(fdt, offset, sizeof(*lenp));
> +		if (!can_assume(VALID_DTB) && !lenp)
> +			return -FDT_ERR_BADSTRUCTURE;
> +		len = fdt32_to_cpu(*lenp);
> +
> +		/* Skip the cell encoding the length */
> +		data_offset += sizeof(*lenp);
> +
> +		/* Check for the length value */
> +		sum = len + data_offset;
> +		if (!can_assume(VALID_DTB) &&
> +		    (sum >= INT_MAX || sum < (uint32_t) offset))
> +			return -FDT_ERR_BADSTRUCTURE;
> +
> +		break;
> +	}
> +
> +	if (data_len)
> +		*data_len = len;
> +
> +	return data_offset;
> +}
> +
>  static uint32_t fdt_next_tag_all(const void *fdt, int startoffset, int *nextoffset)
>  {
>  	const fdt32_t *tagp, *lenp;
>  	uint32_t tag, len, sum;
>  	int offset = startoffset;
> +	int tmp_offset;
>  	const char *p;
>  
>  	*nextoffset = -FDT_ERR_TRUNCATED;
> @@ -221,34 +266,13 @@ static uint32_t fdt_next_tag_all(const void *fdt, int startoffset, int *nextoffs
>  		if (!(tag & FDT_TAG_STRUCTURED) || !(tag & FDT_TAG_SKIP_SAFE))
>  			return FDT_END;
>  
> -		switch (tag & FDT_TAG_DATA_MASK) {
> -		case FDT_TAG_DATA_NONE:
> -			break;
> -		case FDT_TAG_DATA_1CELL:
> -			offset += FDT_CELLSIZE;
> -			break;
> -		case FDT_TAG_DATA_2CELLS:
> -			offset += 2 * FDT_CELLSIZE;
> -			break;
> -		case FDT_TAG_DATA_VARLEN:
> -			/* Get the length */
> -			lenp = fdt_offset_ptr(fdt, offset, sizeof(*lenp));
> -			if (!can_assume(VALID_DTB) && !lenp)
> -				return FDT_END; /* premature end */
> -			len = fdt32_to_cpu(*lenp);
> -			/*
> -			 * Skip the cell encoding the length and the
> -			 * following length bytes
> -			 */
> -			len += sizeof(*lenp);
> -			sum = len + offset;
> -			if (!can_assume(VALID_DTB) &&
> -			    (sum >= INT_MAX || sum < (uint32_t) offset))
> -				return FDT_END; /* premature end */
> -
> -			offset += len;
> -			break;
> -		}
> +		tmp_offset = fdt_get_structured_tag_data(tag, fdt, offset, &len);
> +		if (tmp_offset < 0)
> +			return FDT_END; /* premature end */
> +
> +		/* Skip the whole data bloc */
> +		offset = tmp_offset + len;
> +		break;
>  	}
>  
>  	if (!fdt_offset_ptr(fdt, startoffset, offset - startoffset))
> diff --git a/libfdt/libfdt_internal.h b/libfdt/libfdt_internal.h
> index ce128fda..d4154119 100644
> --- a/libfdt/libfdt_internal.h
> +++ b/libfdt/libfdt_internal.h
> @@ -23,6 +23,9 @@ int32_t fdt_ro_probe_(const void *fdt);
>  uint32_t fdt_next_tag_(const void *fdt, int startoffset, int *nextoffset,
>  		       bool *is_unknown);
>  
> +int fdt_get_structured_tag_data(uint32_t tag, const void *fdt, int offset,
> +				uint32_t *data_len);
> +
>  int fdt_check_node_offset_(const void *fdt, int offset);
>  int fdt_check_prop_offset_(const void *fdt, int offset);
>  
> -- 
> 2.55.0
> 
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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