diff --git a/build/service-scripts/boot-success.sh b/build/service-scripts/boot-success.sh
index cd7c7cb9..5252071c 100755
--- a/build/service-scripts/boot-success.sh
+++ b/build/service-scripts/boot-success.sh
@@ -25,6 +25,8 @@ ident() { sed -n "s/^$1=//p" "$RUN/boot-identity" 2>/dev/null | head -1; }
 other_slot() { case "$1" in a) echo b ;; b) echo a ;; esac; }
 slot="$(ident slot)"; media="$(ident media)"; state="$(ident state)"
 now() { date -Iseconds 2>/dev/null || date; }
+# Drop FILE's clean pages, so the next read is what the device holds; best effort.
+uncache() { dd if="$1" iflag=nocache count=0 status=none 2>/dev/null || :; }   # uncache FILE
 result() { printf '%s %s\n' "$*" "$(now)" > "$B/last-result.new" && mv -f "$B/last-result.new" "$B/last-result"; }
 
 if [ -n "$media" ]; then
@@ -79,12 +81,18 @@ commit_slot() {   # commit_slot <slot>: make BOOTX64.EFI this slot's kernel
         if cmp -s "$src" "$dst"; then
             say "BOOTX64.EFI already is slot $1"; rc=0
         else
-            # Copy, fsync, then rename: on FAT only the rename is not atomic.
-            cp "$src" "$dst.new" && sync -f "$dst.new" && mv -f "$dst.new" "$dst" && sync -f "$dst" && rc=0
+            # Copy, fsync, compare as the device holds it, then rename: on FAT only the
+            # rename is not atomic, and a copy that landed wrong never becomes the boot file.
+            cp "$src" "$dst.new" && sync -f "$dst.new" && uncache "$dst.new" && cmp -s "$src" "$dst.new" && mv -f "$dst.new" "$dst" && sync -f "$dst" && rc=0
             [ "$rc" -eq 0 ] && say "committed: BOOTX64.EFI is now slot $1"
         fi
-        printf '%s\n' "$1" > "$ESP_MNT/kryptik/committed-slot.new" && sync -f "$ESP_MNT/kryptik/committed-slot.new" && \
-            mv -f "$ESP_MNT/kryptik/committed-slot.new" "$ESP_MNT/kryptik/committed-slot"
+        # The record is part of the commit, and follows the boot file: kryptik-update
+        # applies only from the slot it names, so it must not name one that does not boot.
+        if [ "$rc" -eq 0 ]; then
+            { printf '%s\n' "$1" > "$ESP_MNT/kryptik/committed-slot.new" && sync -f "$ESP_MNT/kryptik/committed-slot.new" && \
+                mv -f "$ESP_MNT/kryptik/committed-slot.new" "$ESP_MNT/kryptik/committed-slot"; } \
+                || { say "the committed-slot record could not be written; the trial stays, and the next boot commits again"; rc=1; }
+        fi
     else
         say "no kernel for slot $1 on the ESP"
     fi
@@ -149,7 +157,7 @@ if [ -n "$trial" ]; then
                 say "$(kryptikd time committed "$B/release-$slot" 2>&1)"
             else
                 result "commit-failed $slot"
-                say "slot $slot is healthy but the commit failed; the committed slot is unchanged"
+                say "slot $slot is healthy but the commit failed; the trial stays for the next boot"
             fi
         else
             say "trial slot $slot came up UNHEALTHY:"
diff --git a/docs/design/boot-and-updates.md b/docs/design/boot-and-updates.md
index fe35d3ae..e1496bc1 100644
--- a/docs/design/boot-and-updates.md
+++ b/docs/design/boot-and-updates.md
@@ -150,19 +150,19 @@ payloads come from [the update channel](update-channel.md) or by hand.
 5. `boot-success` judges the trial slot: state persistent; eudev, seatd, the
    launch daemon, the net zone and the login getty up; kryptikd finding kernel
    support and the zones; an unambiguous ESP. Healthy: it copies the kernel
-   over `BOOTX64.EFI` (`.new`, fsync, rename), updates `committed-slot` alike,
-   clears the trial and forgets the entries, and the slot's kept manifest
-   raises the clock's floor if it is the newest. Unhealthy: it records that,
-   forgets the entries and reboots into the committed slot, `BootNext` being
-   spent. On a degraded state the trial record is out of reach, so
-   `committed-slot` says whether the boot is a trial. A trial that never comes
-   up also lands on the committed slot; the fallback records `trial.failed`
-   and forgets the entries, and the updater will not re-arm that payload
-   without `--retry`, which root gives (`su` from the administration login
-   on tty2; the refusal prints the command). Only a trial reboots; an
-   unhealthy committed slot is reported and left running, and so is a trial
-   on a degraded state whose ESP cannot be read, since nothing then says it
-   is one.
+   over `BOOTX64.EFI` (`.new`, fsync, compare, rename), updates
+   `committed-slot` alike, clears the trial and forgets the entries, and the
+   slot's kept manifest raises the clock's floor if it is the newest.
+   Unhealthy: it records that, forgets the entries and reboots into the
+   committed slot, `BootNext` being spent. On a degraded state the trial
+   record is out of reach, so `committed-slot` says whether the boot is a
+   trial. A trial that never comes up also lands on the committed slot; the
+   fallback records `trial.failed` and forgets the entries, and the updater
+   will not re-arm that payload without `--retry`, which root gives (`su` from
+   the administration login on tty2; the refusal prints the command). Only a
+   trial reboots; an unhealthy committed slot is reported and left running,
+   and so is a trial on a degraded state whose ESP cannot be read, since
+   nothing then says it is one.
 
    The net zone is in the check on purpose: a release whose net zone cannot
    come up could never fetch the release that fixes it. Whoever can crash
diff --git a/tools/tests/boot-success.sh b/tools/tests/boot-success.sh
index 898b6174..4bee30d6 100755
--- a/tools/tests/boot-success.sh
+++ b/tools/tests/boot-success.sh
@@ -67,6 +67,17 @@ EOF
 cat > "$T/bin/sync" <<'EOF'
 #!/bin/sh
 echo "sync $*" >> "$KTEST/syncs"
+case "$*" in
+    *committed-slot.new) [ ! -e "$KTEST/sync_fails" ] ;;
+    *BOOTX64.EFI.new) [ ! -e "$KTEST/sync_fails_boot" ] ;;
+esac
+EOF
+# cmp: the real one, but a copy of a kernel to BOOTX64.EFI.new can be made to read back wrong.
+REAL_CMP="$(command -v cmp)"
+cat > "$T/bin/cmp" <<EOF
+#!/bin/sh
+case "\$*" in *BOOTX64.EFI.new) [ -e "\$KTEST/copy_bad" ] && exit 1 ;; esac
+exec "$REAL_CMP" "\$@"
 EOF
 chmod +x "$T"/bin/*
 
@@ -129,6 +140,15 @@ check "committed-slot records b" "$(cat "$KTEST/esp/kryptik/committed-slot")" "b
 check "the new committed-slot record is fsynced under its temporary name" "$(grep -c -x "sync -f $KTEST/run/esp/kryptik/committed-slot.new" "$KTEST/syncs" 2>/dev/null)" "1"
 check "the trial record is gone" "$([[ -e "$KTEST/boot/trial" ]] && echo present || echo gone)" "gone"
 check "the trial's firmware entries and BootNext are forgotten after the commit, the committed slot gets its own, and its release goes to the clock's floor" "$CALLS" "mount -t vfat -o rw,nosuid,nodev,noexec /dev/vda1 $KTEST/run/esp umount $KTEST/run/esp efiboot forget efiboot ensure b kryptikd time committed $KTEST/boot/release-b "
+run_case recfail b "" persistent 'b\narmed=1\n' $ALL; : > "$KTEST/sync_fails"; go
+check "a committed-slot record that cannot be written fails the commit: the trial stays, for the next boot to commit again" \
+    "$RESULT|$(cat "$KTEST/esp/kryptik/committed-slot")|$([[ -e "$KTEST/boot/trial" ]] && echo kept || echo gone)|$(grep -c 'efiboot forget' "$KTEST/calls" 2>/dev/null)" "commit-failed b|a|kept|0"
+run_case bootfail b "" persistent 'b\narmed=1\n' $ALL; : > "$KTEST/sync_fails_boot"; go
+check "a boot file that cannot be replaced fails the commit, and the record goes on naming the slot BOOTX64.EFI boots" \
+    "$RESULT|$(cat "$KTEST/esp/EFI/BOOT/BOOTX64.EFI")|$(cat "$KTEST/esp/kryptik/committed-slot")|$([[ -e "$KTEST/boot/trial" ]] && echo kept || echo gone)" "commit-failed b|kernel-a|a|kept"
+run_case copybad b "" persistent 'b\narmed=1\n' $ALL; : > "$KTEST/copy_bad"; go
+check "a boot file whose copy reads back wrong is not renamed into place: the commit fails, the trial stays" \
+    "$RESULT|$(cat "$KTEST/esp/EFI/BOOT/BOOTX64.EFI")|$(cat "$KTEST/esp/kryptik/committed-slot")|$([[ -e "$KTEST/boot/trial" ]] && echo kept || echo gone)" "commit-failed b|kernel-a|a|kept"
 run_case commit0 b "" persistent 'b\narmed=0\n' $ALL; go
 check "a trial that booted before its armed=1 line was written is still a trial: committed" "$RESULT" "commit b"
 
diff --git a/tools/update/kryptik-recover b/tools/update/kryptik-recover
index 205a855b..83843912 100755
--- a/tools/update/kryptik-recover
+++ b/tools/update/kryptik-recover
@@ -59,6 +59,8 @@ slot_dev() { case "$1" in a) echo "$SA" ;; b) echo "$SB" ;; *) die "slot must be
 has_fs() { [ "$(dd if="$1" bs=1 skip=1080 count=2 status=none | od -An -tx1 | tr -d ' \n')" = "53ef" ]; }
 # Written, fsynced, renamed, as the kernels are: a power cut leaves the old file whole, not empty.
 put() { printf '%s\n' "$2" > "$1.new" && sync -f "$1.new" && mv -f "$1.new" "$1"; }   # put FILE LINE
+# Drop FILE's clean pages, so the next read is what the device holds; best effort.
+uncache() { dd if="$1" iflag=nocache count=0 status=none 2>/dev/null || :; }   # uncache FILE
 
 if [ "$STATUS" = 1 ]; then
     mount -t vfat -o ro "$ESP" /run/kryptik-recover || die "cannot mount the ESP"
@@ -91,6 +93,8 @@ commit_slot() {   # commit_slot SLOT (ESP mounted rw at /run/kryptik-recover)
     has_fs "$(slot_dev "$s")" || die "slot $s holds no filesystem; use --restore-slot $s"
     cp "$k" /run/kryptik-recover/EFI/BOOT/BOOTX64.EFI.new
     sync -f /run/kryptik-recover/EFI/BOOT/BOOTX64.EFI.new
+    uncache /run/kryptik-recover/EFI/BOOT/BOOTX64.EFI.new
+    cmp -s "$k" /run/kryptik-recover/EFI/BOOT/BOOTX64.EFI.new || die "slot $s's kernel did not copy whole; BOOTX64.EFI is unchanged"
     mv -f /run/kryptik-recover/EFI/BOOT/BOOTX64.EFI.new /run/kryptik-recover/EFI/BOOT/BOOTX64.EFI
     put /run/kryptik-recover/kryptik/committed-slot "$s" || die "cannot record slot $s as committed"
     sync
@@ -148,6 +152,9 @@ if [ -n "$RESTORE" ]; then
     mount -t vfat -o rw "$ESP" /run/kryptik-recover || die "cannot mount the target ESP"
     cp "$kdir/kryptik-$RESTORE.efi" "/run/kryptik-recover/EFI/kryptik/kryptik-$RESTORE.efi.new"
     sync -f "/run/kryptik-recover/EFI/kryptik/kryptik-$RESTORE.efi.new"
+    uncache "/run/kryptik-recover/EFI/kryptik/kryptik-$RESTORE.efi.new"
+    cmp -s "$kdir/kryptik-$RESTORE.efi" "/run/kryptik-recover/EFI/kryptik/kryptik-$RESTORE.efi.new" \
+        || die "the medium's kernel for slot $RESTORE did not copy whole; nothing was committed"
     mv -f "/run/kryptik-recover/EFI/kryptik/kryptik-$RESTORE.efi.new" "/run/kryptik-recover/EFI/kryptik/kryptik-$RESTORE.efi"
     put "/run/kryptik-recover/kryptik/version-$RESTORE" "$ver" || die "cannot record slot ${RESTORE}'s version"
     commit_slot "$RESTORE"
diff --git a/tools/update/kryptik-update b/tools/update/kryptik-update
index 8c9390d7..2508c005 100755
--- a/tools/update/kryptik-update
+++ b/tools/update/kryptik-update
@@ -76,6 +76,9 @@ esp_dev() { own_part kryptik-esp; }
 slot_dev() { own_part "kryptik-$1"; }
 hdr() { awk -F': ' -v k="$2" '$1==k {print $2; exit}' "$1"; }
 
+# Drop FILE's clean pages, so the next read is what the device holds; best effort.
+uncache() { dd if="$1" iflag=nocache count=0 status=none 2>/dev/null || :; }   # uncache FILE
+
 mount_esp() {
     dev="$(esp_dev)" || exit 1
     mkdir -p "$ESP_MNT"
@@ -363,17 +366,20 @@ cmd_apply() {
     mount_esp
     mkdir -p "$ESP_MNT/EFI/kryptik" "$ESP_MNT/kryptik"
     cp "$(payload_path "kryptik-$target.efi")" "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new" || die "staging the kernel on the ESP failed"
-    sync -f "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new"
-    # Check the staged kernel before it gets the name rollback and recovery use.
+    sync -f "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new" || die "the kernel staged on the ESP could not be flushed"
+    # Check the staged kernel, as the device holds it, before it gets the name
+    # rollback and recovery use.
+    uncache "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new"
     want_k="$H_KA"; [ "$target" = b ] && want_k="$H_KB"
     if [ "$(sha256sum "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new" | cut -c1-64)" != "$want_k" ]; then
         rm -f "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new"
         die "the kernel staged on the ESP does not hash to the manifest's kryptik-$target.efi"
     fi
     mv -f "$ESP_MNT/EFI/kryptik/kryptik-$target.efi.new" "$ESP_MNT/EFI/kryptik/kryptik-$target.efi"
-    printf '%s\n' "$VERSION" > "$ESP_MNT/kryptik/version-$target.new"
-    sync -f "$ESP_MNT/kryptik/version-$target.new"
-    mv -f "$ESP_MNT/kryptik/version-$target.new" "$ESP_MNT/kryptik/version-$target"
+    { printf '%s\n' "$VERSION" > "$ESP_MNT/kryptik/version-$target.new" \
+        && sync -f "$ESP_MNT/kryptik/version-$target.new" \
+        && mv -f "$ESP_MNT/kryptik/version-$target.new" "$ESP_MNT/kryptik/version-$target"; } \
+        || die "slot $target's version could not be recorded on the ESP"
     sync
     umount_esp
     say "kernel for slot $target installed on the ESP (version $VERSION)"