[Qemu-devel] [PATCH v2 1/4] 9p: mitigates most QID path collisions

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

This first patch here is an updated version of Antonios Motakis'
original 4-patch set, merged to one patch:

https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg02283.html

* Updated to latest git master, specifically to new qht interface.

* Merged the original 4 patches to this single patch.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
---
 fsdev/9p-marshal.h |   4 +-
 hw/9pfs/9p.c       | 200 ++++++++++++++++++++++++++++++++++++++++++++++++-----
 hw/9pfs/9p.h       |  21 ++++++
 3 files changed, 204 insertions(+), 21 deletions(-)

diff --git a/fsdev/9p-marshal.h b/fsdev/9p-marshal.h
index c8823d878f..d1ad3645c4 100644
--- a/fsdev/9p-marshal.h
+++ b/fsdev/9p-marshal.h
@@ -10,8 +10,8 @@ typedef struct V9fsString
 typedef struct V9fsQID
 {
     int8_t type;
-    int32_t version;
-    int64_t path;
+    uint32_t version;
+    uint64_t path;
 } V9fsQID;
 
 typedef struct V9fsStat
diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 55821343e5..b9bbdcbaee 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -25,6 +25,7 @@
 #include "trace.h"
 #include "migration/blocker.h"
 #include "sysemu/qtest.h"
+#include "qemu/xxhash.h"
 
 int open_fd_hw;
 int total_open_fd;
@@ -571,14 +572,135 @@ static void coroutine_fn virtfs_reset(V9fsPDU *pdu)
                                 P9_STAT_MODE_NAMED_PIPE |   \
                                 P9_STAT_MODE_SOCKET)
 
-/* This is the algorithm from ufs in spfs */
-static void stat_to_qid(const struct stat *stbuf, V9fsQID *qidp)
+
+/* creative abuse of qemu_xxhash7, which is based on xxhash */
+static uint32_t qpp_hash(QppEntry e)
 {
-    size_t size;
+    return qemu_xxhash7(e.ino_prefix, e.dev, 0, 0, 0);
+}
+
+static uint32_t qpf_hash(QpfEntry e)
+{
+    return qemu_xxhash7(e.ino, e.dev, 0, 0, 0);
+}
+
+static bool qpp_cmp_func(const void *obj, const void *userp)
+{
+    const QppEntry *e1 = obj, *e2 = userp;
+    return (e1->dev == e2->dev) && (e1->ino_prefix == e2->ino_prefix);
+}
+
+static bool qpf_cmp_func(const void *obj, const void *userp)
+{
+    const QpfEntry *e1 = obj, *e2 = userp;
+    return (e1->dev == e2->dev) && (e1->ino == e2->ino);
+}
+
+static void qp_table_remove(void *p, uint32_t h, void *up)
+{
+    g_free(p);
+}
+
+static void qp_table_destroy(struct qht *ht)
+{
+    qht_iter(ht, qp_table_remove, NULL);
+    qht_destroy(ht);
+}
+
+static int qid_path_fullmap(V9fsPDU *pdu, const struct stat *stbuf,
+                            uint64_t *path)
+{
+    QpfEntry lookup = {
+        .dev = stbuf->st_dev,
+        .ino = stbuf->st_ino
+    }, *val;
+    uint32_t hash = qpf_hash(lookup);
+
+    /* most users won't need the fullmap, so init the table lazily */
+    if (!pdu->s->qpf_table.map) {
+        qht_init(&pdu->s->qpf_table, qpf_cmp_func, 1 << 16, QHT_MODE_AUTO_RESIZE);
+    }
+
+    val = qht_lookup(&pdu->s->qpf_table, &lookup, hash);
+
+    if (!val) {
+        if (pdu->s->qp_fullpath_next == 0) {
+            /* no more files can be mapped :'( */
+            return -ENFILE;
+        }
+
+        val = g_malloc0(sizeof(QppEntry));
+        if (!val) {
+            return -ENOMEM;
+        }
+        *val = lookup;
+
+        /* new unique inode and device combo */
+        val->path = pdu->s->qp_fullpath_next++;
+        pdu->s->qp_fullpath_next &= QPATH_INO_MASK;
+        qht_insert(&pdu->s->qpf_table, val, hash, NULL);
+    }
+
+    *path = val->path;
+    return 0;
+}
+
+/* stat_to_qid needs to map inode number (64 bits) and device id (32 bits)
+ * to a unique QID path (64 bits). To avoid having to map and keep track
+ * of up to 2^64 objects, we map only the 16 highest bits of the inode plus
+ * the device id to the 16 highest bits of the QID path. The 48 lowest bits
+ * of the QID path equal to the lowest bits of the inode number.
+ *
+ * This takes advantage of the fact that inode number are usually not
+ * random but allocated sequentially, so we have fewer items to keep
+ * track of.
+ */
+static int qid_path_prefixmap(V9fsPDU *pdu, const struct stat *stbuf,
+                                uint64_t *path)
+{
+    QppEntry lookup = {
+        .dev = stbuf->st_dev,
+        .ino_prefix = (uint16_t) (stbuf->st_ino >> 48)
+    }, *val;
+    uint32_t hash = qpp_hash(lookup);
+
+    val = qht_lookup(&pdu->s->qpp_table, &lookup, hash);
+
+    if (!val) {
+        if (pdu->s->qp_prefix_next == 0) {
+            /* we ran out of prefixes */
+            return -ENFILE;
+        }
+
+        val = g_malloc0(sizeof(QppEntry));
+        if (!val) {
+            return -ENOMEM;
+        }
+        *val = lookup;
+
+        /* new unique inode prefix and device combo */
+        val->qp_prefix = pdu->s->qp_prefix_next++;
+        qht_insert(&pdu->s->qpp_table, val, hash, NULL);
+    }
+
+    *path = ((uint64_t)val->qp_prefix << 48) | (stbuf->st_ino & QPATH_INO_MASK);
+    return 0;
+}
+
+static int stat_to_qid(V9fsPDU *pdu, const struct stat *stbuf, V9fsQID *qidp)
+{
+    int err;
+
+    /* map inode+device to qid path (fast path) */
+    err = qid_path_prefixmap(pdu, stbuf, &qidp->path);
+    if (err == -ENFILE) {
+        /* fast path didn't work, fal back to full map */
+        err = qid_path_fullmap(pdu, stbuf, &qidp->path);
+    }
+    if (err) {
+        return err;
+    }
 
-    memset(&qidp->path, 0, sizeof(qidp->path));
-    size = MIN(sizeof(stbuf->st_ino), sizeof(qidp->path));
-    memcpy(&qidp->path, &stbuf->st_ino, size);
     qidp->version = stbuf->st_mtime ^ (stbuf->st_size << 8);
     qidp->type = 0;
     if (S_ISDIR(stbuf->st_mode)) {
@@ -587,6 +709,8 @@ static void stat_to_qid(const struct stat *stbuf, V9fsQID *qidp)
     if (S_ISLNK(stbuf->st_mode)) {
         qidp->type |= P9_QID_TYPE_SYMLINK;
     }
+
+    return 0;
 }
 
 static int coroutine_fn fid_to_qid(V9fsPDU *pdu, V9fsFidState *fidp,
@@ -599,7 +723,10 @@ static int coroutine_fn fid_to_qid(V9fsPDU *pdu, V9fsFidState *fidp,
     if (err < 0) {
         return err;
     }
-    stat_to_qid(&stbuf, qidp);
+    err = stat_to_qid(pdu, &stbuf, qidp);
+    if (err < 0) {
+        return err;
+    }
     return 0;
 }
 
@@ -830,7 +957,10 @@ static int coroutine_fn stat_to_v9stat(V9fsPDU *pdu, V9fsPath *path,
 
     memset(v9stat, 0, sizeof(*v9stat));
 
-    stat_to_qid(stbuf, &v9stat->qid);
+    err = stat_to_qid(pdu, stbuf, &v9stat->qid);
+    if (err < 0) {
+        return err;
+    }
     v9stat->mode = stat_to_v9mode(stbuf);
     v9stat->atime = stbuf->st_atime;
     v9stat->mtime = stbuf->st_mtime;
@@ -891,7 +1021,7 @@ static int coroutine_fn stat_to_v9stat(V9fsPDU *pdu, V9fsPath *path,
 #define P9_STATS_ALL           0x00003fffULL /* Mask for All fields above */
 
 
-static void stat_to_v9stat_dotl(V9fsState *s, const struct stat *stbuf,
+static int stat_to_v9stat_dotl(V9fsPDU *pdu, const struct stat *stbuf,
                                 V9fsStatDotl *v9lstat)
 {
     memset(v9lstat, 0, sizeof(*v9lstat));
@@ -913,7 +1043,7 @@ static void stat_to_v9stat_dotl(V9fsState *s, const struct stat *stbuf,
     /* Currently we only support BASIC fields in stat */
     v9lstat->st_result_mask = P9_STATS_BASIC;
 
-    stat_to_qid(stbuf, &v9lstat->qid);
+    return stat_to_qid(pdu, stbuf, &v9lstat->qid);
 }
 
 static void print_sg(struct iovec *sg, int cnt)
@@ -1115,7 +1245,6 @@ static void coroutine_fn v9fs_getattr(void *opaque)
     uint64_t request_mask;
     V9fsStatDotl v9stat_dotl;
     V9fsPDU *pdu = opaque;
-    V9fsState *s = pdu->s;
 
     retval = pdu_unmarshal(pdu, offset, "dq", &fid, &request_mask);
     if (retval < 0) {
@@ -1136,7 +1265,10 @@ static void coroutine_fn v9fs_getattr(void *opaque)
     if (retval < 0) {
         goto out;
     }
-    stat_to_v9stat_dotl(s, &stbuf, &v9stat_dotl);
+    retval = stat_to_v9stat_dotl(pdu, &stbuf, &v9stat_dotl);
+    if (retval < 0) {
+        goto out;
+    }
 
     /*  fill st_gen if requested and supported by underlying fs */
     if (request_mask & P9_STATS_GEN) {
@@ -1381,7 +1513,10 @@ static void coroutine_fn v9fs_walk(void *opaque)
             if (err < 0) {
                 goto out;
             }
-            stat_to_qid(&stbuf, &qid);
+            err = stat_to_qid(pdu, &stbuf, &qid);
+            if (err < 0) {
+                goto out;
+            }
             v9fs_path_copy(&dpath, &path);
         }
         memcpy(&qids[name_idx], &qid, sizeof(qid));
@@ -1483,7 +1618,10 @@ static void coroutine_fn v9fs_open(void *opaque)
     if (err < 0) {
         goto out;
     }
-    stat_to_qid(&stbuf, &qid);
+    err = stat_to_qid(pdu, &stbuf, &qid);
+    if (err < 0) {
+        goto out;
+    }
     if (S_ISDIR(stbuf.st_mode)) {
         err = v9fs_co_opendir(pdu, fidp);
         if (err < 0) {
@@ -1593,7 +1731,10 @@ static void coroutine_fn v9fs_lcreate(void *opaque)
         fidp->flags |= FID_NON_RECLAIMABLE;
     }
     iounit =  get_iounit(pdu, &fidp->path);
-    stat_to_qid(&stbuf, &qid);
+    err = stat_to_qid(pdu, &stbuf, &qid);
+    if (err < 0) {
+        goto out;
+    }
     err = pdu_marshal(pdu, offset, "Qd", &qid, iounit);
     if (err < 0) {
         goto out;
@@ -2327,7 +2468,10 @@ static void coroutine_fn v9fs_create(void *opaque)
         }
     }
     iounit = get_iounit(pdu, &fidp->path);
-    stat_to_qid(&stbuf, &qid);
+    err = stat_to_qid(pdu, &stbuf, &qid);
+    if (err < 0) {
+        goto out;
+    }
     err = pdu_marshal(pdu, offset, "Qd", &qid, iounit);
     if (err < 0) {
         goto out;
@@ -2384,7 +2528,10 @@ static void coroutine_fn v9fs_symlink(void *opaque)
     if (err < 0) {
         goto out;
     }
-    stat_to_qid(&stbuf, &qid);
+    err = stat_to_qid(pdu, &stbuf, &qid);
+    if (err < 0) {
+        goto out;
+    }
     err =  pdu_marshal(pdu, offset, "Q", &qid);
     if (err < 0) {
         goto out;
@@ -3064,7 +3211,10 @@ static void coroutine_fn v9fs_mknod(void *opaque)
     if (err < 0) {
         goto out;
     }
-    stat_to_qid(&stbuf, &qid);
+    err = stat_to_qid(pdu, &stbuf, &qid);
+    if (err < 0) {
+        goto out;
+    }
     err = pdu_marshal(pdu, offset, "Q", &qid);
     if (err < 0) {
         goto out;
@@ -3222,7 +3372,10 @@ static void coroutine_fn v9fs_mkdir(void *opaque)
     if (err < 0) {
         goto out;
     }
-    stat_to_qid(&stbuf, &qid);
+    err = stat_to_qid(pdu, &stbuf, &qid);
+    if (err < 0) {
+        goto out;
+    }
     err = pdu_marshal(pdu, offset, "Q", &qid);
     if (err < 0) {
         goto out;
@@ -3633,6 +3786,11 @@ int v9fs_device_realize_common(V9fsState *s, const V9fsTransport *t,
         goto out;
     }
 
+    /* QID path hash table. 1 entry ought to be enough for anybody ;) */
+    qht_init(&s->qpp_table, qpp_cmp_func, 1, QHT_MODE_AUTO_RESIZE);
+    s->qp_prefix_next = 1; /* reserve 0 to detect overflow */
+    s->qp_fullpath_next = 1;
+
     s->ctx.fst = &fse->fst;
     fsdev_throttle_init(s->ctx.fst);
 
@@ -3646,6 +3804,8 @@ out:
         }
         g_free(s->tag);
         g_free(s->ctx.fs_root);
+        qp_table_destroy(&s->qpp_table);
+        qp_table_destroy(&s->qpf_table);
         v9fs_path_free(&path);
     }
     return rc;
@@ -3658,6 +3818,8 @@ void v9fs_device_unrealize_common(V9fsState *s, Error **errp)
     }
     fsdev_throttle_cleanup(s->ctx.fst);
     g_free(s->tag);
+    qp_table_destroy(&s->qpp_table);
+    qp_table_destroy(&s->qpf_table);
     g_free(s->ctx.fs_root);
 }
 
diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index 8883761b2c..44112ea97f 100644
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -8,6 +8,7 @@
 #include "fsdev/9p-iov-marshal.h"
 #include "qemu/thread.h"
 #include "qemu/coroutine.h"
+#include "qemu/qht.h"
 
 enum {
     P9_TLERROR = 6,
@@ -235,6 +236,22 @@ struct V9fsFidState
     V9fsFidState *rclm_lst;
 };
 
+#define QPATH_INO_MASK        (((unsigned long)1 << 48) - 1)
+
+/* QID path prefix entry, see stat_to_qid */
+typedef struct {
+    dev_t dev;
+    uint16_t ino_prefix;
+    uint16_t qp_prefix;
+} QppEntry;
+
+/* QID path full entry, as above */
+typedef struct {
+    dev_t dev;
+    ino_t ino;
+    uint64_t path;
+} QpfEntry;
+
 struct V9fsState
 {
     QLIST_HEAD(, V9fsPDU) free_list;
@@ -256,6 +273,10 @@ struct V9fsState
     Error *migration_blocker;
     V9fsConf fsconf;
     V9fsQID root_qid;
+    struct qht qpp_table;
+    struct qht qpf_table;
+    uint16_t qp_prefix_next;
+    uint64_t qp_fullpath_next;
 };
 
 /* 9p2000.L open flags */
-- 
2.11.0