[PATCH v2 5/8] bootm: size the noload lz4 decompression buffer from Content_Size

Add a small static helper bootm_lz4_uncompressed_size() that parses
the lz4 frame header and returns Content_Size when the FLG bit is
set, and wire it into bootm_load_os() alongside gzip and zstd.

The header parse mirrors ulz4fn()'s validation (magic, version==1,
reserved bits, independent-block flag) so the helper does not accept
a frame the decoder itself would reject. Only Content_Size is
extracted; the full validation still runs inside ulz4fn() during the
actual decompression call.

The lz4 command needs the --content-size option to set the FLG bit
that carries the size; frames produced without it fall back to the
existing 8x heuristic.

Signed-off-by: Aristo Chen <[email protected]>
---
 boot/bootm.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/boot/bootm.c b/boot/bootm.c
index e4284fe9844..c5bdc909053 100644
--- a/boot/bootm.c
+++ b/boot/bootm.c
@@ -658,6 +658,36 @@ static ulong bootm_gzip_uncompressed_size(const void *src, ulong len)
 }
 #endif
 
+#if CONFIG_IS_ENABLED(LZ4)
+/*
+ * Return the lz4 frame's Content_Size, or 0 if the buffer is not an
+ * lz4 frame or the frame does not carry the size. The header parse
+ * mirrors ulz4fn()'s validation so we do not accept a stream the
+ * decoder itself would refuse.
+ */
+static ulong bootm_lz4_uncompressed_size(const void *src, ulong len)
+{
+	const u8 *b = src;
+	u8 flg, version, indep_blocks, has_content_size, bd;
+	u64 cs;
+
+	if (len < 4 + 2 || get_unaligned_le32(b) != LZ4F_MAGIC)
+		return 0;
+	flg = b[4];
+	bd = b[5];
+	version = (flg >> 6) & 3;
+	indep_blocks = (flg >> 5) & 1;
+	has_content_size = (flg >> 3) & 1;
+	if (version != 1 || !indep_blocks || (flg & 3) || (bd & 0x8f) ||
+	    !has_content_size)
+		return 0;
+	if (len < 4 + 2 + 8)
+		return 0;
+	cs = get_unaligned_le64(b + 6);
+	return cs > ULONG_MAX ? 0 : (ulong)cs;
+}
+#endif
+
 #if CONFIG_IS_ENABLED(ZSTD)
 /*
  * Return the zstd frame's Frame_Content_Size, or 0 if the header does
@@ -722,6 +752,12 @@ static int bootm_load_os(struct bootm_headers *images, int boot_progress)
 								image_len);
 			break;
 #endif
+#if CONFIG_IS_ENABLED(LZ4)
+		case IH_COMP_LZ4:
+			hdr_size = bootm_lz4_uncompressed_size(image_buf,
+							       image_len);
+			break;
+#endif
 #if CONFIG_IS_ENABLED(ZSTD)
 		case IH_COMP_ZSTD:
 			hdr_size = bootm_zstd_uncompressed_size(image_buf,
-- 
2.43.0