NFSD: Handle full-length symlinks

I've given up on the idea of zero-copy handling of SYMLINK on the
server side. This is because the Linux VFS symlink API requires the
symlink pathname to be in a NUL-terminated kmalloc'd buffer. The
NUL-termination is going to be problematic (watching out for
landing on a page boundary and dealing with a 4096-byte pathname).

I don't believe that SYMLINK creation is on a performance path or is
requested frequently enough that it will cause noticeable CPU cache
pollution due to data copies.

There will be two places where a transport callout will be necessary
to fill in the rqstp: one will be in the svc_fill_symlink_pathname()
helper that is used by NFSv2 and NFSv3, and the other will be in
nfsd4_decode_create().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

4 files changed, 31 insertions, 43 deletions

diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
index 8d1c2d1a159b..9eb8086ea841 100644
--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -290,6 +290,7 @@ nfsd3_proc_symlink(struct svc_rqst *rqstp)
                 RETURN_STATUS(nfserr_nametoolong);
 
         argp->tname = svc_fill_symlink_pathname(rqstp, &argp->first,
+                                                page_address(rqstp->rq_arg.pages[0]),
                                                 argp->tlen);
         if (IS_ERR(argp->tname))
                 RETURN_STATUS(nfserrno(PTR_ERR(argp->tname)));
@@ -303,6 +304,7 @@ nfsd3_proc_symlink(struct svc_rqst *rqstp)
         fh_init(&resp->fh, NFS3_FHSIZE);
         nfserr = nfsd_symlink(rqstp, &resp->dirfh, argp->fname, argp->flen,
                                                    argp->tname, &resp->fh);
+        kfree(argp->tname);
         RETURN_STATUS(nfserr);
 }
 
diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
index a6faee562b31..0d20fd161225 100644
--- a/fs/nfsd/nfsproc.c
+++ b/fs/nfsd/nfsproc.c
@@ -454,6 +454,7 @@ nfsd_proc_symlink(struct svc_rqst *rqstp)
                 return nfserr_nametoolong;
 
         argp->tname = svc_fill_symlink_pathname(rqstp, &argp->first,
+                                                page_address(rqstp->rq_arg.pages[0]),
                                                 argp->tlen);
         if (IS_ERR(argp->tname))
                 return nfserrno(PTR_ERR(argp->tname));
@@ -466,6 +467,7 @@ nfsd_proc_symlink(struct svc_rqst *rqstp)
         nfserr = nfsd_symlink(rqstp, &argp->ffh, argp->fname, argp->flen,
                                                  argp->tname, &newfh);
 
+        kfree(argp->tname);
         fh_put(&argp->ffh);
         fh_put(&newfh);
         return nfserr;
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 43f88bd7b601..73e130a840ce 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -499,7 +499,8 @@ unsigned int	   svc_fill_write_vector(struct svc_rqst *rqstp,
                                          struct page **pages,
                                          struct kvec *first, size_t total);
 char              *svc_fill_symlink_pathname(struct svc_rqst *rqstp,
-                                             struct kvec *first, size_t total);
+                                             struct kvec *first, void *p,
+                                             size_t total);
 
 #define RPC_MAX_ADDRBUFLEN      (63U)
 
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 2194ed507991..d13e05f1a990 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -1577,65 +1577,48 @@ EXPORT_SYMBOL_GPL(svc_fill_write_vector);
  * svc_fill_symlink_pathname - Construct pathname argument for VFS symlink call
  * @rqstp: svc_rqst to operate on
  * @first: buffer containing first section of pathname
+ * @p: buffer containing remaining section of pathname
  * @total: total length of the pathname argument
  *
- * Returns pointer to a NUL-terminated string, or an ERR_PTR. The buffer is
- * released automatically when @rqstp is recycled.
+ * The VFS symlink API demands a NUL-terminated pathname in mapped memory.
+ * Returns pointer to a NUL-terminated string, or an ERR_PTR. Caller must free
+ * the returned string.
  */
 char *svc_fill_symlink_pathname(struct svc_rqst *rqstp, struct kvec *first,
-                                size_t total)
+                                void *p, size_t total)
 {
-        struct xdr_buf *arg = &rqstp->rq_arg;
-        struct page **pages;
-        char *result;
-
-        /* VFS API demands a NUL-terminated pathname. This function
-         * uses a page from @rqstp as the pathname buffer, to enable
-         * direct placement. Thus the total buffer size is PAGE_SIZE.
-         * Space in this buffer for NUL-termination requires that we
-         * cap the size of the returned symlink pathname just a
-         * little early.
-         */
-        if (total > PAGE_SIZE - 1)
-                return ERR_PTR(-ENAMETOOLONG);
+        size_t len, remaining;
+        char *result, *dst;
 
-        /* Some types of transport can present the pathname entirely
-         * in rq_arg.pages. If not, then copy the pathname into one
-         * page.
-         */
-        pages = arg->pages;
-        WARN_ON_ONCE(arg->page_base != 0);
-        if (first->iov_base == 0) {
-                result = page_address(*pages);
-                result[total] = '\0';
-        } else {
-                size_t len, remaining;
-                char *dst;
+        result = kmalloc(total + 1, GFP_KERNEL);
+        if (!result)
+                return ERR_PTR(-ESERVERFAULT);
 
-                result = page_address(*(rqstp->rq_next_page++));
-                dst = result;
-                remaining = total;
+        dst = result;
+        remaining = total;
 
-                len = min_t(size_t, total, first->iov_len);
+        len = min_t(size_t, total, first->iov_len);
+        if (len) {
                 memcpy(dst, first->iov_base, len);
                 dst += len;
                 remaining -= len;
+        }
 
-                /* No more than one page left */
-                if (remaining) {
-                        len = min_t(size_t, remaining, PAGE_SIZE);
-                        memcpy(dst, page_address(*pages), len);
-                        dst += len;
-                }
-
-                *dst = '\0';
+        if (remaining) {
+                len = min_t(size_t, remaining, PAGE_SIZE);
+                memcpy(dst, p, len);
+                dst += len;
         }
 
-        /* Sanity check: we don't allow the pathname argument to
+        *dst = '\0';
+
+        /* Sanity check: Linux doesn't allow the pathname argument to
          * contain a NUL byte.
          */
-        if (strlen(result) != total)
+        if (strlen(result) != total) {
+                kfree(result);
                 return ERR_PTR(-EINVAL);
+        }
         return result;
 }
 EXPORT_SYMBOL_GPL(svc_fill_symlink_pathname);