xemu/hw/i386/kvm/xenstore_impl.c

/*
 * QEMU Xen emulation: The actual implementation of XenStore
 *
 * Copyright © 2023 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Authors: David Woodhouse <dwmw2@infradead.org>, Paul Durrant <paul@xen.org>
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or later.
 * See the COPYING file in the top-level directory.
 */

#include "qemu/osdep.h"
#include "qom/object.h"

#include "xen_xenstore.h"
#include "xenstore_impl.h"

#include "hw/xen/interface/io/xs_wire.h"

#define XS_MAX_WATCHES          128
#define XS_MAX_DOMAIN_NODES     1000
#define XS_MAX_NODE_SIZE        2048
#define XS_MAX_TRANSACTIONS     10
#define XS_MAX_PERMS_PER_NODE   5

#define XS_VALID_CHARS "abcdefghijklmnopqrstuvwxyz" \
                       "ABCDEFGHIJKLMNOPQRSTUVWXYZ" \
                       "0123456789-/_"

typedef struct XsNode {
    uint32_t ref;
    GByteArray *content;
    GHashTable *children;
    uint64_t gencnt;
#ifdef XS_NODE_UNIT_TEST
    gchar *name; /* debug only */
#endif
} XsNode;

struct XenstoreImplState {
    XsNode *root;
    unsigned int nr_nodes;
};

static inline XsNode *xs_node_new(void)
{
    XsNode *n = g_new0(XsNode, 1);
    n->ref = 1;

#ifdef XS_NODE_UNIT_TEST
    nr_xs_nodes++;
    xs_node_list = g_list_prepend(xs_node_list, n);
#endif
    return n;
}

static inline XsNode *xs_node_ref(XsNode *n)
{
    /* With just 10 transactions, it can never get anywhere near this. */
    g_assert(n->ref < INT_MAX);

    g_assert(n->ref);
    n->ref++;
    return n;
}

static inline void xs_node_unref(XsNode *n)
{
    if (!n) {
        return;
    }
    g_assert(n->ref);
    if (--n->ref) {
        return;
    }

    if (n->content) {
        g_byte_array_unref(n->content);
    }
    if (n->children) {
        g_hash_table_unref(n->children);
    }
#ifdef XS_NODE_UNIT_TEST
    g_free(n->name);
    nr_xs_nodes--;
    xs_node_list = g_list_remove(xs_node_list, n);
#endif
    g_free(n);
}

/* For copying from one hash table to another using g_hash_table_foreach() */
static void do_insert(gpointer key, gpointer value, gpointer user_data)
{
    g_hash_table_insert(user_data, g_strdup(key), xs_node_ref(value));
}

static XsNode *xs_node_copy(XsNode *old)
{
    XsNode *n = xs_node_new();

    n->gencnt = old->gencnt;
    if (old->children) {
        n->children = g_hash_table_new_full(g_str_hash, g_str_equal, g_free,
                                            (GDestroyNotify)xs_node_unref);
        g_hash_table_foreach(old->children, do_insert, n->children);
    }
    if (old && old->content) {
        n->content = g_byte_array_ref(old->content);
    }
    return n;
}

/* Returns true if it made a change to the hash table */
static bool xs_node_add_child(XsNode *n, const char *path_elem, XsNode *child)
{
    assert(!strchr(path_elem, '/'));

    if (!child) {
        assert(n->children);
        return g_hash_table_remove(n->children, path_elem);
    }

#ifdef XS_NODE_UNIT_TEST
    g_free(child->name);
    child->name = g_strdup(path_elem);
#endif
    if (!n->children) {
        n->children = g_hash_table_new_full(g_str_hash, g_str_equal, g_free,
                                            (GDestroyNotify)xs_node_unref);
    }

    /*
     * The documentation for g_hash_table_insert() says that it "returns a
     * boolean value to indicate whether the newly added value was already
     * in the hash table or not."
     *
     * It could perhaps be clearer that returning TRUE means it wasn't,
     */
    return g_hash_table_insert(n->children, g_strdup(path_elem), child);
}

struct walk_op {
    struct XenstoreImplState *s;
    char path[XENSTORE_ABS_PATH_MAX + 2]; /* Two NUL terminators */
    int (*op_fn)(XsNode **n, struct walk_op *op);
    void *op_opaque;
    void *op_opaque2;

    unsigned int dom_id;

    /* The number of nodes which will exist in the tree if this op succeeds. */
    unsigned int new_nr_nodes;

    /*
     * This is maintained on the way *down* the walk to indicate
     * whether nodes can be modified in place or whether COW is
     * required. It starts off being true, as we're always going to
     * replace the root node. If we walk into a shared subtree it
     * becomes false. If we start *creating* new nodes for a write,
     * it becomes true again.
     *
     * Do not use it on the way back up.
     */
    bool inplace;
    bool mutating;
    bool create_dirs;
};

static int xs_node_add_content(XsNode **n, struct walk_op *op)
{
    GByteArray *data = op->op_opaque;

    if (op->dom_id) {
        /*
         * The real XenStored includes permissions and names of child nodes
         * in the calculated datasize but life's too short. For a single
         * tenant internal XenStore, we don't have to be quite as pedantic.
         */
        if (data->len > XS_MAX_NODE_SIZE) {
            return E2BIG;
        }
    }
    /* We *are* the node to be written. Either this or a copy. */
    if (!op->inplace) {
        XsNode *old = *n;
        *n = xs_node_copy(old);
        xs_node_unref(old);
    }

    if ((*n)->content) {
        g_byte_array_unref((*n)->content);
    }
    (*n)->content = g_byte_array_ref(data);
    return 0;
}

static int xs_node_get_content(XsNode **n, struct walk_op *op)
{
    GByteArray *data = op->op_opaque;
    GByteArray *node_data;

    assert(op->inplace);
    assert(*n);

    node_data = (*n)->content;
    if (node_data) {
        g_byte_array_append(data, node_data->data, node_data->len);
    }

    return 0;
}

static int node_rm_recurse(gpointer key, gpointer value, gpointer user_data)
{
    struct walk_op *op = user_data;
    XsNode *n = value;
    bool this_inplace = op->inplace;

    if (n->ref != 1) {
        op->inplace = 0;
    }

    if (n->children) {
        g_hash_table_foreach_remove(n->children, node_rm_recurse, op);
    }
    op->new_nr_nodes--;

    /*
     * Actually deleting the child here is just an optimisation; if we
     * don't then the final unref on the topmost victim will just have
     * to cascade down again repeating all the g_hash_table_foreach()
     * calls.
     */
    return this_inplace;
}

static int xs_node_rm(XsNode **n, struct walk_op *op)
{
    bool this_inplace = op->inplace;

    /* Keep count of the nodes in the subtree which gets deleted. */
    if ((*n)->children) {
        g_hash_table_foreach_remove((*n)->children, node_rm_recurse, op);
    }
    op->new_nr_nodes--;

    if (this_inplace) {
        xs_node_unref(*n);
    }
    *n = NULL;
    return 0;
}

/*
 * Passed a full reference in *n which it may free if it needs to COW.
 *
 * When changing the tree, the op->inplace flag indicates whether this
 * node may be modified in place (i.e. it and all its parents had a
 * refcount of one). If walking down the tree we find a node whose
 * refcount is higher, we must clear op->inplace and COW from there
 * down. Unless we are creating new nodes as scaffolding for a write
 * (which works like 'mkdir -p' does). In which case those newly
 * created nodes can (and must) be modified in place again.
 */
static int xs_node_walk(XsNode **n, struct walk_op *op)
{
    char *child_name = NULL;
    size_t namelen;
    XsNode *old = *n, *child = NULL;
    bool stole_child = false;
    bool this_inplace;
    int err;

    namelen = strlen(op->path);

    /* Is there a child, or do we hit the double-NUL termination? */
    if (op->path[namelen + 1]) {
        char *slash;
        child_name = op->path + namelen + 1;
        slash = strchr(child_name, '/');
        if (slash) {
            *slash = '\0';
        }
        op->path[namelen] = '/';
    }

    /* If we walk into a subtree which is shared, we must COW */
    if (op->mutating && old->ref != 1) {
        op->inplace = false;
    }

    if (!child_name) {
        /* This is the actual node on which the operation shall be performed */
        err = op->op_fn(n, op);
        goto out;
    }

    /* op->inplace will be further modified during the recursion */
    this_inplace = op->inplace;

    if (old && old->children) {
        child = g_hash_table_lookup(old->children, child_name);
        /* This is a *weak* reference to 'child', owned by the hash table */
    }

    if (child) {
        xs_node_ref(child);
        /*
         * Now we own it too. But if we can modify inplace, that's going to
         * foil the check and force it to COW. We want to be the *only* owner
         * so that it can be modified in place, so remove it from the hash
         * table in that case. We'll add it (or its replacement) back later.
         */
        if (op->mutating && this_inplace) {
            g_hash_table_remove(old->children, child_name);
            stole_child = true;
        }
    } else if (op->create_dirs) {
        if (op->dom_id && op->new_nr_nodes >= XS_MAX_DOMAIN_NODES) {
            err = ENOSPC;
            goto out;
        }
        op->new_nr_nodes++;
        child = xs_node_new();

        /*
         * If we're creating a new child, we can clearly modify it (and its
         * children) in place from here on down.
         */
        op->inplace = true;
    } else {
        err = ENOENT;
        goto out;
    }

    /*
     * Except for the temporary child-stealing as noted, our node has not
     * changed yet. We don't yet know the overall operation will complete.
     */
    err = xs_node_walk(&child, op);
    if (err || !op->mutating) {
        if (stole_child) {
            /* Put it back as it was. */
            g_hash_table_replace(old->children, g_strdup(child_name), child);
        } else {
            xs_node_unref(child);
        }
        goto out;
    }

    /*
     * Now we know the operation has completed successfully and we're on
     * the way back up. Make the change, substituting 'child' in the
     * node at our level.
     */
    if (!this_inplace) {
        *n = xs_node_copy(old);
        xs_node_unref(old);
    }

    /*
     * The child may be NULL here, for a remove operation. Either way,
     * xs_node_add_child() will do the right thing and return a value
     * indicating whether it changed the parent's hash table or not.
     *
     * We bump the parent gencnt if it adds a child that we *didn't*
     * steal from it in the first place, or if child==NULL and was
     * thus removed (whether we stole it earlier and didn't put it
     * back, or xs_node_add_child() actually removed it now).
     */
    if ((xs_node_add_child(*n, child_name, child) && !stole_child) || !child) {
        (*n)->gencnt++;
    }

 out:
    op->path[namelen] = '\0';
    if (!namelen) {
        /*
         * On completing the recursion back up the path walk and reaching the
         * top, assign the new node count if the operation was successful.
         */
        if (!err && op->mutating) {
            op->s->nr_nodes = op->new_nr_nodes;
        }
    }
    return err;
}

static void append_directory_item(gpointer key, gpointer value,
                                  gpointer user_data)
{
    GList **items = user_data;

    *items = g_list_insert_sorted(*items, g_strdup(key), (GCompareFunc)strcmp);
}

/* Populates items with char * names which caller must free. */
static int xs_node_directory(XsNode **n, struct walk_op *op)
{
    GList **items = op->op_opaque;

    assert(op->inplace);
    assert(*n);

    if ((*n)->children) {
        g_hash_table_foreach((*n)->children, append_directory_item, items);
    }

    if (op->op_opaque2) {
        *(uint64_t *)op->op_opaque2 = (*n)->gencnt;
    }

    return 0;
}

static int validate_path(char *outpath, const char *userpath,
                         unsigned int dom_id)
{
    size_t i, pathlen = strlen(userpath);

    if (!pathlen || userpath[pathlen] == '/' || strstr(userpath, "//")) {
        return EINVAL;
    }
    for (i = 0; i < pathlen; i++) {
        if (!strchr(XS_VALID_CHARS, userpath[i])) {
            return EINVAL;
        }
    }
    if (userpath[0] == '/') {
        if (pathlen > XENSTORE_ABS_PATH_MAX) {
            return E2BIG;
        }
        memcpy(outpath, userpath, pathlen + 1);
    } else {
        if (pathlen > XENSTORE_REL_PATH_MAX) {
            return E2BIG;
        }
        snprintf(outpath, XENSTORE_ABS_PATH_MAX, "/local/domain/%u/%s", dom_id,
                 userpath);
    }
    return 0;
}


static int init_walk_op(XenstoreImplState *s, struct walk_op *op,
                        xs_transaction_t tx_id, unsigned int dom_id,
                        const char *path, XsNode ***rootp)
{
    int ret = validate_path(op->path, path, dom_id);
    if (ret) {
        return ret;
    }

    /*
     * We use *two* NUL terminators at the end of the path, as during the walk
     * we will temporarily turn each '/' into a NUL to allow us to use that
     * path element for the lookup.
     */
    op->path[strlen(op->path) + 1] = '\0';
    op->path[0] = '\0';
    op->inplace = true;
    op->mutating = false;
    op->create_dirs = false;
    op->dom_id = dom_id;
    op->s = s;

    if (tx_id == XBT_NULL) {
        *rootp = &s->root;
        op->new_nr_nodes = s->nr_nodes;
    } else {
        return ENOENT;
    }

    return 0;
}

int xs_impl_read(XenstoreImplState *s, unsigned int dom_id,
                 xs_transaction_t tx_id, const char *path, GByteArray *data)
{
    /*
     * The data GByteArray shall exist, and will be freed by caller.
     * Just g_byte_array_append() to it.
     */
    struct walk_op op;
    XsNode **n;
    int ret;

    ret = init_walk_op(s, &op, tx_id, dom_id, path, &n);
    if (ret) {
        return ret;
    }
    op.op_fn = xs_node_get_content;
    op.op_opaque = data;
    return xs_node_walk(n, &op);
}

int xs_impl_write(XenstoreImplState *s, unsigned int dom_id,
                  xs_transaction_t tx_id, const char *path, GByteArray *data)
{
    /*
     * The data GByteArray shall exist, will be freed by caller. You are
     * free to use g_byte_array_steal() and keep the data. Or just ref it.
     */
    struct walk_op op;
    XsNode **n;
    int ret;

    ret = init_walk_op(s, &op, tx_id, dom_id, path, &n);
    if (ret) {
        return ret;
    }
    op.op_fn = xs_node_add_content;
    op.op_opaque = data;
    op.mutating = true;
    op.create_dirs = true;
    return xs_node_walk(n, &op);
}

int xs_impl_directory(XenstoreImplState *s, unsigned int dom_id,
                      xs_transaction_t tx_id, const char *path,
                      uint64_t *gencnt, GList **items)
{
    /*
     * The items are (char *) to be freed by caller. Although it's consumed
     * immediately so if you want to change it to (const char *) and keep
     * them, go ahead and change the caller.
     */
    struct walk_op op;
    XsNode **n;
    int ret;

    ret = init_walk_op(s, &op, tx_id, dom_id, path, &n);
    if (ret) {
        return ret;
    }
    op.op_fn = xs_node_directory;
    op.op_opaque = items;
    op.op_opaque2 = gencnt;
    return xs_node_walk(n, &op);
}

int xs_impl_transaction_start(XenstoreImplState *s, unsigned int dom_id,
                              xs_transaction_t *tx_id)
{
    return ENOSYS;
}

int xs_impl_transaction_end(XenstoreImplState *s, unsigned int dom_id,
                            xs_transaction_t tx_id, bool commit)
{
    return ENOSYS;
}

int xs_impl_rm(XenstoreImplState *s, unsigned int dom_id,
               xs_transaction_t tx_id, const char *path)
{
    struct walk_op op;
    XsNode **n;
    int ret;

    ret = init_walk_op(s, &op, tx_id, dom_id, path, &n);
    if (ret) {
        return ret;
    }
    op.op_fn = xs_node_rm;
    op.mutating = true;
    return xs_node_walk(n, &op);
}

int xs_impl_get_perms(XenstoreImplState *s, unsigned int dom_id,
                      xs_transaction_t tx_id, const char *path, GList **perms)
{
    /*
     * The perms are (char *) in the <perm-as-string> wire format to be
     * freed by the caller.
     */
    return ENOSYS;
}

int xs_impl_set_perms(XenstoreImplState *s, unsigned int dom_id,
                      xs_transaction_t tx_id, const char *path, GList *perms)
{
    /*
     * The perms are (const char *) in the <perm-as-string> wire format.
     */
    return ENOSYS;
}

int xs_impl_watch(XenstoreImplState *s, unsigned int dom_id, const char *path,
                  const char *token, xs_impl_watch_fn fn, void *opaque)
{
    /*
     * When calling the callback @fn, note that the path should
     * precisely match the relative path that the guest provided, even
     * if it was a relative path which needed to be prefixed with
     * /local/domain/${domid}/
     */
    return ENOSYS;
}

int xs_impl_unwatch(XenstoreImplState *s, unsigned int dom_id,
                    const char *path, const char *token,
                    xs_impl_watch_fn fn, void *opaque)
{
    /*
     * When calling the callback @fn, note that the path should
     * precisely match the relative path that the guest provided, even
     * if it was a relative path which needed to be prefixed with
     * /local/domain/${domid}/
     */
    return ENOSYS;
}

int xs_impl_reset_watches(XenstoreImplState *s, unsigned int dom_id)
{
    /* Remove the watch that matches all four criteria */
    return ENOSYS;
}

XenstoreImplState *xs_impl_create(void)
{
    XenstoreImplState *s = g_new0(XenstoreImplState, 1);

    s->nr_nodes = 1;
    s->root = xs_node_new();
#ifdef XS_NODE_UNIT_TEST
    s->root->name = g_strdup("/");
#endif
    return s;
}