gard: Fix data corruption when clearing single records

Attempting to clear a specific gard record leads to corruption of the target
record rather than the expected removal:

    $ ./opal-gard -f romulus.pnor list
    No GARD entries to display
    $ ./opal-gard -f romulus.pnor create /sys0/node0/proc1
    $ ./opal-gard -f romulus.pnor list
     ID       | Error    | Type       | Path
    ---------------------------------------------------------
     00000001 | 00000000 | Manual     | /Sys0/Node0/Proc1
    =========================================================
    $ ./opal-gard -f romulus.pnor clear 00000001
    Clearing gard record 0x00000001...done
    $ ./opal-gard -f romulus.pnor list
     ID       | Error    | Type       | Path
    ---------------------------------------------------------
     00000001 | 00000000 | Unknown    | /Sys0/Node0/Proc1
    =========================================================

The GUARD partition needs to be compacted when clearing records as the
end of the list is a sentinel represented by the erased-flash state. The
compaction strategy is to read the trailing records and write them to
the offset of the record to be removed, followed by writing the sentinel
record at the offset of what was previously the last valid record.

The corruption occurs due to incorrect calculation of the offset at which the
trailing records will be written.

Cc: Skiboot Stable <skiboot-stable@lists.ozlabs.org>
Fixes: 5616c42 ("libflash/blocklevel: Make read/write be ECC agnostic for callers")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

external/gard/gard.c

@@ -570,7 +570,7 @@ static int do_clear_i(struct gard_ctx *ctx, int pos, struct gard_record *gard, v
 			return rc;
 		}
 
-		rc = blocklevel_smart_write(ctx->bl, buf_pos - sizeof(gard), buf, buf_len);
+		rc = blocklevel_smart_write(ctx->bl, buf_pos - sizeof(*gard), buf, buf_len);
 		free(buf);
 		if (rc) {
 			fprintf(stderr, "Couldn't write to flash at 0x%08x for len 0x%08x\n",

external/gard/test/results/10-clear-single.out

@@ -0,0 +1,7 @@
+Clearing the entire gard partition...done
+ ID       | Error    | Type       | Path
+---------------------------------------------------------
+ 00000001 | 00000000 | Manual     | /Sys0/Node0/Proc1
+=========================================================
+Clearing gard record 0x00000001...done
+No GARD entries to display

external/gard/test/tests/10-clear-single

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+set -e
+
+DATA=$(mktemp)
+
+cleanup() {
+	rm -f $DATA
+}
+
+trap cleanup EXIT
+
+dd if=/dev/zero of=$DATA bs=$((0x1000)) count=5 2>/dev/null
+
+run_binary "./opal-gard" "-p -e -f $DATA clear all"
+run_binary "./opal-gard" "-p -e -f $DATA create /sys0/node0/proc1"
+run_binary "./opal-gard" "-p -e -f $DATA list"
+run_binary "./opal-gard" "-p -e -f $DATA clear 00000001"
+run_binary "./opal-gard" "-p -e -f $DATA list"
+
+diff_with_result
+
+pass_test