From 07d1d6bdf9d6615e03f0b814cab473c9fa15ea17 Mon Sep 17 00:00:00 2001
From: kasey <489222+kasey@users.noreply.github.com>
Date: Mon, 29 Dec 2025 15:35:46 -0500
Subject: [PATCH] Fix validation bug in --backfill-oldest-slot (#16173)

**What type of PR is this?**

Bug fix


**What does this PR do? Why is it needed?**

Validation of `--backfill-oldest-slot` fails for values > 1056767,
because the validation code is comparing the slot/32 to
`MIN_EPOCHS_FOR_BLOCK_REQUESTS` (33024), instead of comparing it to
`current_epoch - MIN_EPOCHS_FOR_BLOCK_REQUESTS`.

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

---------

Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>
---
 beacon-chain/das/needs.go            |  4 ++--
 beacon-chain/das/needs_test.go       | 34 +++++++++++++++++++++-------
 changelog/kasey_fix-backfill-flag.md |  2 ++
 3 files changed, 30 insertions(+), 10 deletions(-)
 create mode 100644 changelog/kasey_fix-backfill-flag.md

diff --git a/beacon-chain/das/needs.go b/beacon-chain/das/needs.go
index bd7b53c077..425165ace3 100644
--- a/beacon-chain/das/needs.go
+++ b/beacon-chain/das/needs.go
@@ -67,9 +67,9 @@ func NewSyncNeeds(current CurrentSlotter, oldestSlotFlagPtr *primitives.Slot, bl
 
 	// Override spec minimum block retention with user-provided flag only if it is lower than the spec minimum.
 	sn.blockRetention = primitives.Epoch(params.BeaconConfig().MinEpochsForBlockRequests)
+
 	if oldestSlotFlagPtr != nil {
-		oldestEpoch := slots.ToEpoch(*oldestSlotFlagPtr)
-		if oldestEpoch < sn.blockRetention {
+		if *oldestSlotFlagPtr <= syncEpochOffset(current(), sn.blockRetention) {
 			sn.validOldestSlotPtr = oldestSlotFlagPtr
 		} else {
 			log.WithField("backfill-oldest-slot", *oldestSlotFlagPtr).
diff --git a/beacon-chain/das/needs_test.go b/beacon-chain/das/needs_test.go
index 6986f7933a..6b9e863633 100644
--- a/beacon-chain/das/needs_test.go
+++ b/beacon-chain/das/needs_test.go
@@ -128,6 +128,9 @@ func TestSyncNeedsInitialize(t *testing.T) {
 	slotsPerEpoch := params.BeaconConfig().SlotsPerEpoch
 	minBlobEpochs := params.BeaconConfig().MinEpochsForBlobsSidecarsRequest
 	minColEpochs := params.BeaconConfig().MinEpochsForDataColumnSidecarsRequest
+	denebSlot := slots.UnsafeEpochStart(params.BeaconConfig().DenebForkEpoch)
+	fuluSlot := slots.UnsafeEpochStart(params.BeaconConfig().FuluForkEpoch)
+	minSlots := slots.UnsafeEpochStart(primitives.Epoch(params.BeaconConfig().MinEpochsForBlockRequests))
 
 	currentSlot := primitives.Slot(10000)
 	currentFunc := func() primitives.Slot { return currentSlot }
@@ -141,6 +144,7 @@ func TestSyncNeedsInitialize(t *testing.T) {
 		expectedCol       primitives.Epoch
 		name              string
 		input             SyncNeeds
+		current           func() primitives.Slot
 	}{
 		{
 			name:              "basic initialization with no flags",
@@ -174,13 +178,13 @@ func TestSyncNeedsInitialize(t *testing.T) {
 		{
 			name:              "valid oldestSlotFlagPtr (earlier than spec minimum)",
 			blobRetentionFlag: 0,
-			oldestSlotFlagPtr: func() *primitives.Slot {
-				slot := primitives.Slot(10)
-				return &slot
-			}(),
+			oldestSlotFlagPtr: &denebSlot,
 			expectValidOldest: true,
 			expectedBlob:      minBlobEpochs,
 			expectedCol:       minColEpochs,
+			current: func() primitives.Slot {
+				return fuluSlot + minSlots
+			},
 		},
 		{
 			name:              "invalid oldestSlotFlagPtr (later than spec minimum)",
@@ -210,6 +214,9 @@ func TestSyncNeedsInitialize(t *testing.T) {
 		{
 			name:              "both blob retention flag and oldest slot set",
 			blobRetentionFlag: minBlobEpochs + 5,
+			current: func() primitives.Slot {
+				return fuluSlot + minSlots
+			},
 			oldestSlotFlagPtr: func() *primitives.Slot {
 				slot := primitives.Slot(100)
 				return &slot
@@ -232,16 +239,27 @@ func TestSyncNeedsInitialize(t *testing.T) {
 			expectedBlob:      5000,
 			expectedCol:       5000,
 		},
+		{
+			name:              "regression for deneb start",
+			blobRetentionFlag: 8212500,
+			expectValidOldest: true,
+			oldestSlotFlagPtr: &denebSlot,
+			current: func() primitives.Slot {
+				return fuluSlot + minSlots
+			},
+			expectedBlob: 8212500,
+			expectedCol:  8212500,
+		},
 	}
 
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
-			result, err := NewSyncNeeds(currentFunc, tc.oldestSlotFlagPtr, tc.blobRetentionFlag)
+			if tc.current == nil {
+				tc.current = currentFunc
+			}
+			result, err := NewSyncNeeds(tc.current, tc.oldestSlotFlagPtr, tc.blobRetentionFlag)
 			require.NoError(t, err)
 
-			// Check that current, deneb, fulu are set correctly
-			require.Equal(t, currentSlot, result.current())
-
 			// Check retention calculations
 			require.Equal(t, tc.expectedBlob, result.blobRetention)
 			require.Equal(t, tc.expectedCol, result.colRetention)
diff --git a/changelog/kasey_fix-backfill-flag.md b/changelog/kasey_fix-backfill-flag.md
new file mode 100644
index 0000000000..4cb4e74216
--- /dev/null
+++ b/changelog/kasey_fix-backfill-flag.md
@@ -0,0 +1,2 @@
+#### Fixed
+- Fix validation logic for `--backfill-oldest-slot`, which was rejecting slots newer than 1056767.