On Sat, Jun 15, 2019 at 11:08:21AM -0400, Theodore Ts'o wrote:
> On Thu, Jun 06, 2019 at 08:51:59AM -0700, Eric Biggers wrote:
> > From: Eric Biggers <[email protected]>
> >
> > Add a function for filesystems to call to implement the
> > FS_IOC_ENABLE_VERITY ioctl. This ioctl enables fs-verity on a file.
> >
> > See the "FS_IOC_ENABLE_VERITY" section of
> > Documentation/filesystems/fsverity.rst for the documentation.
> >
> > Signed-off-by: Eric Biggers <[email protected]>
>
> > diff --git a/fs/verity/enable.c b/fs/verity/enable.c
> > new file mode 100644
> > index 000000000000..7e7ef9d3c376
> > --- /dev/null
> > +++ b/fs/verity/enable.c
> > + /* Tell the filesystem to finish enabling verity on the file */
> > + err = vops->end_enable_verity(filp, desc, desc_size, params.tree_size);
> > + if (err) {
> > + fsverity_err(inode, "%ps() failed with err %d",
> > + vops->end_enable_verity, err);
> > + fsverity_free_info(vi);
> > + } else {
> > + /* Successfully enabled verity */
> > +
> > + WARN_ON(!IS_VERITY(inode));
> > +
> > + /*
> > + * Readers can start using ->i_verity_info immediately, so it
> > + * can't be rolled back once set. So don't set it until just
> > + * after the filesystem has successfully enabled verity.
> > + */
> > + fsverity_set_info(inode, vi);
> > + }
>
> If end_enable_Verity() retuns success, and IS_VERITY is not set, I
> would think that we should report the error via fsverity_err() and
> return an error to userspace, and *not* call fsverity_set_info(). I
> don't think the stack trace printed by WARN_ON is going to very
> interesting, since the call path which gets us to enable_verity() is
> not going to be surprising.
>
I want to keep it as WARN_ON() because if it happens it's a kernel bug, and
WARNs are reported as bugs by automated tools. But I can do the following so it
returns an error code too:
@@ -229,11 +235,12 @@ static int enable_verity(struct file *filp,
fsverity_err(inode, "%ps() failed with err %d",
vops->end_enable_verity, err);
fsverity_free_info(vi);
+ } else if (WARN_ON(!IS_VERITY(inode))) {
+ err = -EINVAL;
+ fsverity_free_info(vi);
} else {
/* Successfully enabled verity */
- WARN_ON(!IS_VERITY(inode));
-
/*
* Readers can start using ->i_verity_info immediately, so it
* can't be rolled back once set. So don't set it until just
> > +
> > + if (inode->i_size <= 0) {
> > + err = -EINVAL;
> > + goto out_unlock;
> > + }
>
> How hard would it be to support fsverity for zero-length files? There
> would be no Merkle tree, but there still would be an fsverity header
> file on which we can calculate a checksum for the digital signature.
>
> - Ted
>
Empty files would have to be special-cased, e.g. defining the root hash to be
all 0's, since there are no blocks to checksum. It would be straightforward,
but it would still be a special case, e.g.:
diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index ee9dd578e59fb..e859a2b6a4310 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -112,6 +112,12 @@ static int build_merkle_tree(struct inode *inode,
unsigned int level;
int err = -ENOMEM;
+ if (inode->i_size == 0) {
+ /* Empty file is a special case; root hash is all 0's */
+ memset(root_hash, 0, params->digest_size);
+ return 0;
+ }
+
On the other hand, *not* supporting empty files is a special case from the
user's point of view. It means that fs-verity isn't supported on every possible
file. Thinking about it, that's probably worse than having a special case in
the *implementation*.
So now I'm leaning towards changing it to support empty files.
- Eric
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