Re: [Qemu-devel] sheepdog build warning

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Dec 17, 2018 at 11:03:11PM -0500, Michael S. Tsirkin wrote:
> mingw32 build on fedora fails with this warning:
> 
> /scm/qemu/block/sheepdog.c: In function 'find_vdi_name':
> /scm/qemu/block/sheepdog.c:1239:5: error: 'strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
>      strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
>      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
> make: *** [/scm/qemu/rules.mak:69: block/sheepdog.o] Error 1
> make: *** Waiting for unfinished jobs....
> 
> 
> Reading the code one sees it's working as intended:
> 
> 
> static int find_vdi_name(BDRVSheepdogState *s, const char *filename,
>                          uint32_t snapid, const char *tag, uint32_t *vid,
>                          bool lock, Error **errp)
> {
>     int ret, fd;
>     SheepdogVdiReq hdr;
>     SheepdogVdiRsp *rsp = (SheepdogVdiRsp *)&hdr;
>     unsigned int wlen, rlen = 0;
>     char buf[SD_MAX_VDI_LEN + SD_MAX_VDI_TAG_LEN];
> 
>     fd = connect_to_sdog(s, errp);
>     if (fd < 0) {
>         return fd;
>     }
> 
>     /* This pair of strncpy calls ensures that the buffer is zero-filled,
>      * which is desirable since we'll soon be sending those bytes, and
>      * don't want the send_req to read uninitialized data.
>      */
>     strncpy(buf, filename, SD_MAX_VDI_LEN);
>     strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
> 
> 
> .....
> 
> }
> 
> 
> 
> so this seems to be the case of GCC developers deciding that
> strncpy is simply a bad API and a correct use of it should be
> warned against.
> 
> I propose either
> 
> 1. simply adding
> 
> #pragma GCC diagnostic ignored "-Wstringop-truncation"
> 
> in osdep.
> 
> 2. adding an inline wrapper with said pragma in there.
> 
> 3. -Wno-stringop-truncation is the makefile
> 
> Thoughts?
> 
> -- 


So here's approach 2. However I note that a newer gcc 8.2.1
does not give this warning. Maybe detect at configure time
and suppress (option 3)?



diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 3bf48bcdec..64d8258529 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -134,6 +134,22 @@ extern int daemon(int, int);
 #define assert(x)  g_assert(x)
 #endif
 
+/*
+ * GCC 8.0 declared war on strncpy. Admittedly it's a tricky interface
+ * with unintuitive semantics, but we use it widely.
+ */
+#if defined(__GNUC__) && __GNUC__ >= 8
+static inline void qemu_strncpy(char *to, const char *from, int n)
+{
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-truncation"
+    strncpy(to, from, n);
+#pragma GCC diagnostic pop
+}
+
+#define strncpy qemu_strncpy
+#endif
+
 /*
  * According to waitpid man page:
  * WCOREDUMP