[PATCH v3 8/8] initramfs_test: add filename padding test case

David Disseldorp
2025-08-19T03:50:37Z

Confirm that cpio filenames with multiple trailing zeros (accounted for
in namesize) extract successfully.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
init/initramfs_test.c | 68 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 67 insertions(+), 1 deletion(-)

diff --git a/init/initramfs_test.c b/init/initramfs_test.c
index 517e5e04e5ccf..da16b012322b9 100644
--- a/init/initramfs_test.c
+++ b/init/initramfs_test.c
@@ -45,8 +45,11 @@ static size_t fill_cpio(struct initramfs_test_cpio *cs, size_t csz, char *out)
c->mtime, c->filesize, c->devmajor, c->devminor,
c->rdevmajor, c->rdevminor, c->namesize, c->csum,
c->fname) + 1;
+
pr_debug("packing (%zu): %.*s\n", thislen, (int)thislen, pos);
- off += thislen;
+ if (thislen != CPIO_HDRLEN + c->namesize)
+ pr_debug("padded to: %u\n", CPIO_HDRLEN + c->namesize);
+ off += CPIO_HDRLEN + c->namesize;
while (off & 3)
out[off++] = '\0';

@@ -383,6 +386,68 @@ static void __init initramfs_test_many(struct kunit *test)
kfree(cpio_srcbuf);
}

+/*
+ * An initramfs filename is namesize in length, including the zero-terminator.
+ * A filename can be zero-terminated prior to namesize, with the remainder used
+ * as padding. This can be useful for e.g. alignment of file data segments with
+ * a 4KB filesystem block, allowing for extent sharing (reflinks) between cpio
+ * source and destination. This hack works with both GNU cpio and initramfs, as
+ * long as PATH_MAX isn't exceeded.
+ */
+static void __init initramfs_test_fname_pad(struct kunit *test)
+{
+ char *err;
+ size_t len;
+ struct file *file;
+ char fdata[] = "this file data is aligned at 4K in the archive";
+ struct test_fname_pad {
+ char padded_fname[4096 - CPIO_HDRLEN];
+ char cpio_srcbuf[CPIO_HDRLEN + PATH_MAX + 3 + sizeof(fdata)];
+ } *tbufs = kzalloc(sizeof(struct test_fname_pad), GFP_KERNEL);
+ struct initramfs_test_cpio c[] = { {
+ .magic = "070701",
+ .ino = 1,
+ .mode = S_IFREG | 0777,
+ .uid = 0,
+ .gid = 0,
+ .nlink = 1,
+ .mtime = 1,
+ .filesize = 0,
+ .devmajor = 0,
+ .devminor = 1,
+ .rdevmajor = 0,
+ .rdevminor = 0,
+ /* align file data at 4K archive offset via padded fname */
+ .namesize = 4096 - CPIO_HDRLEN,
+ .csum = 0,
+ .fname = tbufs->padded_fname,
+ .data = fdata,
+ .filesize = sizeof(fdata),
+ } };
+
+ memcpy(tbufs->padded_fname, "padded_fname", sizeof("padded_fname"));
+ len = fill_cpio(c, ARRAY_SIZE(c), tbufs->cpio_srcbuf);
+
+ err = unpack_to_rootfs(tbufs->cpio_srcbuf, len);
+ KUNIT_EXPECT_NULL(test, err);
+
+ file = filp_open(c[0].fname, O_RDONLY, 0);
+ if (IS_ERR(file)) {
+ KUNIT_FAIL(test, "open failed");
+ goto out;
+ }
+
+ /* read back file contents into @cpio_srcbuf and confirm match */
+ len = kernel_read(file, tbufs->cpio_srcbuf, c[0].filesize, NULL);
+ KUNIT_EXPECT_EQ(test, len, c[0].filesize);
+ KUNIT_EXPECT_MEMEQ(test, tbufs->cpio_srcbuf, c[0].data, len);
+
+ fput(file);
+ KUNIT_EXPECT_EQ(test, init_unlink(c[0].fname), 0);
+out:
+ kfree(tbufs);
+}
+
/*
* The kunit_case/_suite struct cannot be marked as __initdata as this will be
* used in debugfs to retrieve results after test has run.
@@ -394,6 +459,7 @@ static struct kunit_case __refdata initramfs_test_cases[] = {
KUNIT_CASE(initramfs_test_csum),
KUNIT_CASE(initramfs_test_hardlink),
KUNIT_CASE(initramfs_test_many),
+ KUNIT_CASE(initramfs_test_fname_pad),
{},
};

--
2.43.0