[Skiboot] [PATCH] libflash/libffs: Zero checksum words

[Cyril Bur]

On writing ffs entries to flash libffs doesn't zero checksum words
before calculating the checksum across the entire structure. This causes
an inaccurate calculation of the checksum as it may calculate a checksum
on non-zero checksum bytes.

This patch solves this by zeroing the entire structure which is to be
written to the flash before calculating the checksum across the struct.

Fixes: 602dee45 libflash/libffs: Rework libffs
Signed-off-by: Cyril Bur <cyril.bur at au1.ibm.com>
---
 libflash/libffs.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/libflash/libffs.c b/libflash/libffs.c
index 763e061c..ab6f9328 100644
--- a/libflash/libffs.c
+++ b/libflash/libffs.c
@@ -144,6 +144,14 @@ static int ffs_entry_to_flash(struct ffs_hdr *hdr,
 	if (!ent)
 		return FFS_ERR_PART_NOT_FOUND;
 
+	/*
+	 * So that the checksum gets calculated correctly at least the
+	 * dst->checksum must be zero before calling ffs_entry_checksum()
+	 * memset()ting the entire struct to zero is probably wise as it
+	 * appears the reserved fields are always zero.
+	 */
+	memset(dst, 0, sizeof(*dst));
+
 	memcpy(dst->name, src->name, sizeof(dst->name));
 	dst->name[FFS_PART_NAME_MAX] = '\0';
 	dst->base = cpu_to_be32(src->base / hdr->block_size);
@@ -623,6 +631,14 @@ int ffs_hdr_finalise(struct blocklevel_device *bl, struct ffs_hdr *hdr)
 	if (!real_hdr)
 		return FLASH_ERR_MALLOC_FAILED;
 
+	/*
+	 * So that the checksum gets calculated correctly at least the
+	 * real_hdr->checksum must be zero before calling ffs_hdr_checksum()
+	 * memset()ting the entire struct to zero is probably wise as it
+	 * appears the reserved fields are always zero.
+	 */
+	memset(real_hdr, 0, sizeof(*real_hdr));
+
 	real_hdr->magic = cpu_to_be32(FFS_MAGIC);
 	real_hdr->version = cpu_to_be32(hdr->version);
 	real_hdr->size = cpu_to_be32(hdr->size / hdr->block_size);
-- 
2.13.1