fmap: ignore areas with zero size

It's impossible for flashrom_layout to represent zero-sized flash
regions but it is possible for a fmap to contain a zero-sized region
which causes the resulting layout to fail layout_sanity_checks(),
preventing use of that fmap.

Because it would very rarely make sense to be able to operate on
zero-sized regions anyway and changing layouts to be able to support
zero-size regions would entail large changes, instead ignore zero-size
regions when present in fmap.

TEST=Warning is now printed when using fmap that contains a zero-sized
     area, and operations on other regions are allowed to proceed.
BUG=b:271933192

Change-Id: Ie20971f779acece7a0b3b8f38796fff128ce689a
Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/73571
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
---
M libflashrom.c
1 file changed, 38 insertions(+), 0 deletions(-)

diff --git a/libflashrom.c b/libflashrom.c
index 2e89fe5..4a59e2a 100644
--- a/libflashrom.c
+++ b/libflashrom.c
@@ -362,6 +362,16 @@
 		return 1;
 
 	for (i = 0, area = fmap->areas; i < fmap->nareas; i++, area++) {
+		if (area->size == 0) {
+			/* Layout regions use inclusive upper and lower bounds,
+			 * so it's impossible to represent a region with zero
+			 * size although it's allowed in fmap. */
+			msg_gwarn("Ignoring zero-size fmap region \"%s\";"
+				  " empty regions are unsupported.\n",
+				  area->name);
+			continue;
+		}
+
 		snprintf(name, sizeof(name), "%s", area->name);
 		if (flashrom_layout_add_region(l, area->offset, area->offset + area->size - 1, name)) {
 			flashrom_layout_release(l);

