virtiofsd: Don't assume header layout

virtiofsd incorrectly assumed a fixed set of header layout in the virt
queue; assuming that the fuse and write headers were conveniently
separated from the data;  the spec doesn't allow us to take that
convenience, so fix it up to deal with it the hard way.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210428110100.27757-3-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

tools/virtiofsd/fuse_virtio.c

@@ -129,18 +129,55 @@ static void fv_panic(VuDev *dev, const char *err)
  * Copy from an iovec into a fuse_buf (memory only)
  * Caller must ensure there is space
  */
-static void copy_from_iov(struct fuse_buf *buf, size_t out_num,
+static size_t copy_from_iov(struct fuse_buf *buf, size_t out_num,
-                          const struct iovec *out_sg)
+                            const struct iovec *out_sg,
+                            size_t max)
 {
     void *dest = buf->mem;
+    size_t copied = 0;
 
-    while (out_num) {
+    while (out_num && max) {
         size_t onelen = out_sg->iov_len;
+        onelen = MIN(onelen, max);
         memcpy(dest, out_sg->iov_base, onelen);
         dest += onelen;
+        copied += onelen;
         out_sg++;
         out_num--;
+        max -= onelen;
     }
+
+    return copied;
+}
+
+/*
+ * Skip 'skip' bytes in the iov; 'sg_1stindex' is set as
+ * the index for the 1st iovec to read data from, and
+ * 'sg_1stskip' is the number of bytes to skip in that entry.
+ *
+ * Returns True if there are at least 'skip' bytes in the iovec
+ *
+ */
+static bool skip_iov(const struct iovec *sg, size_t sg_size,
+                     size_t skip,
+                     size_t *sg_1stindex, size_t *sg_1stskip)
+{
+    size_t vec;
+
+    for (vec = 0; vec < sg_size; vec++) {
+        if (sg[vec].iov_len > skip) {
+            *sg_1stskip = skip;
+            *sg_1stindex = vec;
+
+            return true;
+        }
+
+        skip -= sg[vec].iov_len;
+    }
+
+    *sg_1stindex = vec;
+    *sg_1stskip = 0;
+    return skip == 0;
 }
 
 /*
@@ -457,6 +494,7 @@ static void fv_queue_worker(gpointer data, gpointer user_data)
     bool allocated_bufv = false;
     struct fuse_bufvec bufv;
     struct fuse_bufvec *pbufv;
+    struct fuse_in_header inh;
 
     assert(se->bufsize > sizeof(struct fuse_in_header));
 
@@ -505,14 +543,15 @@ static void fv_queue_worker(gpointer data, gpointer user_data)
                  elem->index);
         assert(0); /* TODO */
     }
-    /* Copy just the first element and look at it */
+    /* Copy just the fuse_in_header and look at it */
-    copy_from_iov(&fbuf, 1, out_sg);
+    copy_from_iov(&fbuf, out_num, out_sg,
+                  sizeof(struct fuse_in_header));
+    memcpy(&inh, fbuf.mem, sizeof(struct fuse_in_header));
 
     pbufv = NULL; /* Compiler thinks an unitialised path */
-    if (out_num > 2 &&
+    if (inh.opcode == FUSE_WRITE &&
-        out_sg[0].iov_len == sizeof(struct fuse_in_header) &&
+        out_len >= (sizeof(struct fuse_in_header) +
-        ((struct fuse_in_header *)fbuf.mem)->opcode == FUSE_WRITE &&
+                    sizeof(struct fuse_write_in))) {
-        out_sg[1].iov_len == sizeof(struct fuse_write_in)) {
         /*
          * For a write we don't actually need to copy the
          * data, we can just do it straight out of guest memory
@@ -521,15 +560,15 @@ static void fv_queue_worker(gpointer data, gpointer user_data)
          */
         fuse_log(FUSE_LOG_DEBUG, "%s: Write special case\n", __func__);
 
-        /* copy the fuse_write_in header afte rthe fuse_in_header */
+        fbuf.size = copy_from_iov(&fbuf, out_num, out_sg,
-        fbuf.mem += out_sg->iov_len;
+                                  sizeof(struct fuse_in_header) +
-        copy_from_iov(&fbuf, 1, out_sg + 1);
+                                  sizeof(struct fuse_write_in));
-        fbuf.mem -= out_sg->iov_len;
+        /* That copy reread the in_header, make sure we use the original */
-        fbuf.size = out_sg[0].iov_len + out_sg[1].iov_len;
+        memcpy(fbuf.mem, &inh, sizeof(struct fuse_in_header));
 
         /* Allocate the bufv, with space for the rest of the iov */
         pbufv = malloc(sizeof(struct fuse_bufvec) +
-                       sizeof(struct fuse_buf) * (out_num - 2));
+                       sizeof(struct fuse_buf) * out_num);
         if (!pbufv) {
             fuse_log(FUSE_LOG_ERR, "%s: pbufv malloc failed\n",
                     __func__);
@@ -540,24 +579,37 @@ static void fv_queue_worker(gpointer data, gpointer user_data)
         pbufv->count = 1;
         pbufv->buf[0] = fbuf;
 
-        size_t iovindex, pbufvindex;
+        size_t iovindex, pbufvindex, iov_bytes_skip;
-        iovindex = 2; /* 2 headers, separate iovs */
         pbufvindex = 1; /* 2 headers, 1 fusebuf */
 
+        if (!skip_iov(out_sg, out_num,
+                      sizeof(struct fuse_in_header) +
+                      sizeof(struct fuse_write_in),
+                      &iovindex, &iov_bytes_skip)) {
+            fuse_log(FUSE_LOG_ERR, "%s: skip failed\n",
+                    __func__);
+            goto out;
+        }
+
         for (; iovindex < out_num; iovindex++, pbufvindex++) {
             pbufv->count++;
             pbufv->buf[pbufvindex].pos = ~0; /* Dummy */
             pbufv->buf[pbufvindex].flags = 0;
             pbufv->buf[pbufvindex].mem = out_sg[iovindex].iov_base;
             pbufv->buf[pbufvindex].size = out_sg[iovindex].iov_len;
+
+            if (iov_bytes_skip) {
+                pbufv->buf[pbufvindex].mem += iov_bytes_skip;
+                pbufv->buf[pbufvindex].size -= iov_bytes_skip;
+                iov_bytes_skip = 0;
+            }
         }
     } else {
         /* Normal (non fast write) path */
 
-        /* Copy the rest of the buffer */
+        copy_from_iov(&fbuf, out_num, out_sg, se->bufsize);
-        fbuf.mem += out_sg->iov_len;
+        /* That copy reread the in_header, make sure we use the original */
-        copy_from_iov(&fbuf, out_num - 1, out_sg + 1);
+        memcpy(fbuf.mem, &inh, sizeof(struct fuse_in_header));
-        fbuf.mem -= out_sg->iov_len;
         fbuf.size = out_len;
 
         /* TODO! Endianness of header */