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9pfs: allocate space for guest originated empty strings 

If a guest sends an empty string paramater to any 9P operation, the current
code unmarshals it into a V9fsString equal to { .size = 0, .data = NULL }.

This is unfortunate because it can cause NULL pointer dereference to happen
at various locations in the 9pfs code. And we don't want to check str->data
everywhere we pass it to strcmp() or any other function which expects a
dereferenceable pointer.

This patch enforces the allocation of genuine C empty strings instead, so
callers don't have to bother.

Out of all v9fs_iov_vunmarshal() users, only v9fs_xattrwalk() checks if
the returned string is empty. It now uses v9fs_string_size() since
name.data cannot be NULL anymore.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
[groug, rewritten title and changelog,
 fix empty string check in v9fs_xattrwalk()]
Signed-off-by: Greg Kurz <groug@kaod.org>
This commit is contained in:
Li Qiang 2016-10-17 14:13:58 +02:00 committed by Greg Kurz
parent ad728364e3
commit ba42ebb863
2 changed files with 2 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
fsdev/9p-iov-marshal.c
View File

@ -125,7 +125,7 @@ ssize_t v9fs_iov_vunmarshal(struct iovec *out_sg, int out_num, size_t offset,
str->data = g_malloc(str->size + 1);
copied = v9fs_unpack(str->data, out_sg, out_num, offset,
str->size);
if (copied > 0) {
if (copied >= 0) {
str->data[str->size] = 0;
} else {
v9fs_string_free(str);

2
hw/9pfs/9p.c
View File

@ -3174,7 +3174,7 @@ static void v9fs_xattrwalk(void *opaque)
goto out;
}
v9fs_path_copy(&xattr_fidp->path, &file_fidp->path);
if (name.data == NULL) {
if (!v9fs_string_size(&name)) {
/*
* listxattr request. Get the size first
*/