[PATCH 1/1] libfdisk: write mbr only when useful

[Philippe Reynes <philippe.reynes@xxxxxxxxxxxxxx>]

The MBR is critical as it doesn't have
a backup, so we only write it when it
is really useful (ie: the current MBR
is different than the new one).

Signed-off-by: Philippe Reynes <philippe.reynes@xxxxxxxxxxxxxx>
---
 libfdisk/src/gpt.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/libfdisk/src/gpt.c b/libfdisk/src/gpt.c
index 72370a19a..08b0a81dd 100644
--- a/libfdisk/src/gpt.c
+++ b/libfdisk/src/gpt.c
@@ -2024,6 +2024,19 @@ static int gpt_set_partition(struct fdisk_context *cxt, size_t n,
 	return rc;
 }
 
+static int gpt_read(struct fdisk_context *cxt, off_t offset, void *buf, size_t count)
+{
+	if (offset != lseek(cxt->dev_fd, offset, SEEK_SET))
+		return -errno;
+
+	if (read_all(cxt->dev_fd, buf, count))
+		return -errno;
+
+	DBG(GPT, ul_debug("  read OK [offset=%zu, size=%zu]",
+				(size_t) offset, count));
+	return 0;
+}
+
 static int gpt_write(struct fdisk_context *cxt, off_t offset, void *buf, size_t count)
 {
 	if (offset != lseek(cxt->dev_fd, offset, SEEK_SET))
@@ -2079,6 +2092,8 @@ static int gpt_write_header(struct fdisk_context *cxt,
 static int gpt_write_pmbr(struct fdisk_context *cxt)
 {
 	struct gpt_legacy_mbr *pmbr;
+	struct gpt_legacy_mbr *current;
+	int rc;
 
 	assert(cxt);
 	assert(cxt->firstsector);
@@ -2107,6 +2122,25 @@ static int gpt_write_pmbr(struct fdisk_context *cxt)
 		pmbr->partition_record[0].size_in_lba =
 			cpu_to_le32((uint32_t) (cxt->total_sectors - 1ULL));
 
+	current = malloc(sizeof(*current));
+	if (!current)
+		goto do_write;
+
+	rc = gpt_read(cxt, GPT_PMBR_LBA * cxt->sector_size,
+		      current, cxt->sector_size);
+	if (rc)
+		goto do_write;
+
+	rc = memcmp(pmbr, current, sizeof(*current));
+
+	free(current);
+
+	if (!rc) {
+		DBG(GPT, ul_debug("Same MBR on disk => don't write it"));
+		return 0;
+	}
+
+ do_write:
 	/* pMBR covers the first sector (LBA) of the disk */
 	return gpt_write(cxt, GPT_PMBR_LBA * cxt->sector_size,
 			 pmbr, cxt->sector_size);
-- 
2.25.1