Re: [PATCH] tests/9p: fix potential leak in v9fs_rreaddir()

[Greg Kurz]

Hi Christian !

On Sat, 29 Apr 2023 11:25:33 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> Free allocated directory entries in v9fs_rreaddir() if argument
> `entries` was passed as NULL, to avoid a memory leak. It is
> explicitly allowed by design for `entries` to be NULL. [1]
> 
> [1] https://lore.kernel.org/all/1690923.g4PEXVpXuU@silver
> 
> Reported-by: Coverity (CID 1487558)
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---

Good catch Coverity ! :-)

Reviewed-by: Greg Kurz <groug@kaod.org>

I still have a suggestion. See below.

>  tests/qtest/libqos/virtio-9p-client.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/tests/qtest/libqos/virtio-9p-client.c 
> b/tests/qtest/libqos/virtio-9p-client.c
> index e4a368e036..b8adc8d4b9 100644
> --- a/tests/qtest/libqos/virtio-9p-client.c
> +++ b/tests/qtest/libqos/virtio-9p-client.c
> @@ -594,6 +594,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t 
> *nentries,
>  {
>      uint32_t local_count;
>      struct V9fsDirent *e = NULL;
> +    /* only used to avoid a leak if entries was NULL */
> +    struct V9fsDirent *unused_entries = NULL;
>      uint16_t slen;
>      uint32_t n = 0;
>  
> @@ -612,6 +614,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t 
> *nentries,
>              e = g_new(struct V9fsDirent, 1);
>              if (entries) {
>                  *entries = e;
> +            } else {
> +                unused_entries = e;
>              }
>          } else {
>              e = e->next = g_new(struct V9fsDirent, 1);

This is always allocating and chaining a new entry even
though it isn't needed in the entries == NULL case.

> @@ -628,6 +632,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t 
> *nentries,
>          *nentries = n;
>      }
>  
> +    v9fs_free_dirents(unused_entries);

This is going to loop again on all entries to free them.

>      v9fs_req_free(req);
>  }
>  

If this function is to be called one day with an enormous
number of entries and entries == NULL case, this might
not scale well.

What about only allocating a single entry in this case ?

E.g.

@@ -593,7 +593,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t 
*nentries,
                    struct V9fsDirent **entries)
 {
     uint32_t local_count;
-    struct V9fsDirent *e = NULL;
+    g_autofree struct V9fsDirent *e = NULL;
     uint16_t slen;
     uint32_t n = 0;
 
@@ -611,10 +611,12 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t 
*nentries,
         if (!e) {
             e = g_new(struct V9fsDirent, 1);
             if (entries) {
-                *entries = e;
+                *entries = g_steal_pointer(e);
             }
         } else {
-            e = e->next = g_new(struct V9fsDirent, 1);
+            if (entries) {
+                e = e->next = g_new(struct V9fsDirent, 1);
+            }
         }
         e->next = NULL;
         /* qid[13] offset[8] type[1] name[s] */