repart: trim NUL bytes from verity sig split artifact

The verity signature partition content is a bare JSON object. Repart
pads it with zeros to fill the GPT partition. But when splitting out
the content as an individual file, the padding remains, so it's not
a valid text file.

jq started rejecting files with NUL bytes to fix a security issue:
https://github.com/jqlang/jq/commit/6374ae0bcdfe33a18eb0ae6db28493b1f34a0a5b

Trim the output when writing these files out.

src/repart/repart.c

@@ -7754,9 +7754,30 @@ static int context_split(Context *context) {
                 if (lseek(fd, p->offset, SEEK_SET) < 0)
                         return log_error_errno(errno, "Failed to seek to partition offset: %m");
 
-                r = copy_bytes(fd, fdt, p->new_size, COPY_REFLINK|COPY_HOLES|COPY_TRUNCATE);
-                if (r < 0)
-                        return log_error_errno(r, "Failed to copy to split partition %s: %m", p->split_path);
+                /* Verity signature partitions contain a JSON object NUL-padded out to the partition
+                 * size. The on-disk partition must keep the padding, but the split-out file is a
+                 * standalone artifact, so trim the trailing NUL bytes there to avoid tripping jq. */
+                if (partition_designator_is_verity_sig(p->type.designator)) {
+                        _cleanup_free_ char *buf = malloc(p->new_size);
+                        if (!buf)
+                                return log_oom();
+
+                        r = loop_read_exact(fd, buf, p->new_size, /* do_poll= */ false);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to read verity signature partition: %m");
+
+                        size_t len = strnlen(buf, p->new_size);
+                        if (len == 0)
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Verity signature partition is empty");
+
+                        r = loop_write(fdt, buf, len);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to write to split partition %s: %m", p->split_path);
+                } else {
+                        r = copy_bytes(fd, fdt, p->new_size, COPY_REFLINK|COPY_HOLES|COPY_TRUNCATE);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to copy to split partition %s: %m", p->split_path);
+                }
         }
 
         return 0;

test/units/TEST-58-REPART.sh

@@ -941,6 +941,7 @@ EOF
                             --dry-run=no \
                             --empty=create \
                             --size=auto \
+                            --split=yes \
                             --json=pretty \
                             --private-key="$defs/verity.key" \
                             --certificate="$defs/verity.crt" \
@@ -953,6 +954,13 @@ EOF
     assert_eq "$drh" "$hrh"
     assert_eq "$hrh" "$srh"
 
+    # The split-out verity signature file should be a valid JSON document (i.e. trailing NUL padding
+    # from the on-disk partition must be trimmed when writing the split file).
+    sig_split=$(jq -r ".[] | select(.type == \"root-${architecture}-verity-sig\") | .split_path" <<<"$output")
+    assert_neq "$sig_split" ""
+    assert_neq "$sig_split" "null"
+    jq . "$sig_split" >/dev/null
+
     # Check that offline signing works and the resulting image is valid
 
     output=$(systemd-repart --offline="$OFFLINE" \