[PATCH] lib: decompress_unlz4: drop redundant -4 in wrapper call to unlz4

[Johannes Schneider <johannes.schneider@leica-geosystems.com>]

decompress_unlz4() passes in_len - 4 to unlz4(), as if the caller had
already stripped the LZ4 legacy frame magic from the buffer. But unlz4()
reads the magic itself from buf[0..3] and, in the buffer-input path,
explicitly accounts for it via:

	if (!fill) {
		inp += 4;
		size -= 4;
	}

So the magic is subtracted twice from the running size counter. After
the last chunk is consumed, the counter underflows to -4 instead of 0,
which trips the "data corrupted" check at the end of the chunk loop:

	if (!fill) {
		size -= chunksize;
		if (size == 0)
			break;
		else if (size < 0) {
			error("data corrupted");
			goto exit_2;
		}
		inp += chunksize;
	}

The bug is benign in practice — by the time the size check fires, all
chunks have already been successfully decompressed and flushed, and ret
holds the 0 returned by the last lz4_decompress_unknownoutputsize()
call. unlz4() returns 0 and the caller treats the operation as
successful, but a spurious "data corrupted" line is printed on every
LZ4 decompression. With pr_fmt set by image-fit.c, FIT-image kernel
boots show:

	ERROR: FIT: data corrupted

right between the configuration line and the cmdline-append line.

Pass in_len through unchanged. unlz4() handles the magic itself in both
the input and fill paths.

Fill-mode callers (e.g. uncompress_fd_to_fd) are unaffected: they pass
in_len=0, and unlz4() overwrites the initial size on the first fill()
call.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
---
 lib/decompress_unlz4.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/decompress_unlz4.c b/lib/decompress_unlz4.c
index 2bb30e71bb..17bb4e5de7 100644
--- a/lib/decompress_unlz4.c
+++ b/lib/decompress_unlz4.c
@@ -209,6 +209,6 @@ STATIC int decompress_unlz4(unsigned char *buf, long in_len,
 			      void(*error)(char *x)
 	)
 {
-	return unlz4(buf, in_len - 4, fill, flush, output, posp, error);
+	return unlz4(buf, in_len, fill, flush, output, posp, error);
 }
 #define decompress decompress_unlz4

base-commit: 6c70fb327d486376c1f2e37dfff2212cb9eebb1b
-- 
2.43.0