[PATCH v3] udp: Ignore bogus -Wstringop-overread for write() from gcc 12.1

[PATCH v3] udp: Ignore bogus -Wstringop-overread for write() from gcc 12.1

[Stefano Brivio]

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With current OpenSUSE Tumbleweed on aarch64 (gcc-12-1.3.aarch64) and
on x86_64 (gcc-12-1.4.x86_64), but curiously not on armv7hl
(gcc-12-1.3.armv7hl), gcc warns about using the _pointer_ to the
802.3 header to write the whole frame to the tap descriptor:
  reading between 62 and 4294967357 bytes from a region of size 14

which is bogus:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103483

Probably declaring udp_sock_fill_data_v{4,6}() as noinline would
"fix" this, but that's on the data path, so I'd rather not. Use
a gcc pragma instead.

Signed-off-by: Stefano Brivio <sbrivio(a)redhat.com>
---
v2: LLVM 13.0.1 doesn't know about -Wstringop-overread, and clang-tidy
    warns about the unknown diagnostic pragma: make it conditional

v3: gcc < 11 doesn't know about it either, but it doesn't implement
    __has_warning

 udp.c  |  4 ++++
 util.h | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/udp.c b/udp.c
index f425d14..98e3eaa 100644
--- a/udp.c
+++ b/udp.c
@@ -714,8 +714,10 @@ static void udp_sock_fill_data_v4(const struct ctx *c, int n,
 	b->uh.len = htons(udp4_l2_mh_sock[n].msg_len + sizeof(b->uh));
 
 	if (c->mode == MODE_PASTA) {
+		PRAGMA_STRINGOP_OVERREAD_IGNORE
 		if (write(c->fd_tap, &b->eh, sizeof(b->eh) + ip_len) < 0)
 			debug("tap write: %s", strerror(errno));
+		PRAGMA_STRINGOP_OVERREAD_IGNORE_POP
 		pcap((char *)&b->eh, sizeof(b->eh) + ip_len);
 
 		return;
@@ -813,8 +815,10 @@ static void udp_sock_fill_data_v6(const struct ctx *c, int n,
 	b->ip6h.hop_limit = 255;
 
 	if (c->mode == MODE_PASTA) {
+		PRAGMA_STRINGOP_OVERREAD_IGNORE
 		if (write(c->fd_tap, &b->eh, sizeof(b->eh) + ip_len) < 0)
 			debug("tap write: %s", strerror(errno));
+		PRAGMA_STRINGOP_OVERREAD_IGNORE_POP
 		pcap((char *)&b->eh, sizeof(b->eh) + ip_len);
 
 		return;
diff --git a/util.h b/util.h
index f85a87a..6f2c702 100644
--- a/util.h
+++ b/util.h
@@ -102,6 +102,29 @@ enum {
 		      (void *)(arg));					\
 	} while (0)
 
+
+#ifdef __has_warning
+# if __has_warning("-Wstringop-overread")
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE				\
+     _Pragma("GCC diagnostic ignored \"-Wstringop-overread\"")
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP				\
+     _Pragma("GCC diagnostic pop")
+# else
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP
+# endif
+#else
+# if defined(__GNUC__) && __GNUC__ >= 11
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE				\
+     _Pragma("GCC diagnostic ignored \"-Wstringop-overread\"")
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP				\
+     _Pragma("GCC diagnostic pop")
+# else
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP
+# endif
+#endif
+
 #if __BYTE_ORDER == __BIG_ENDIAN
 #define L2_BUF_ETH_IP4_INIT						\
 	{								\
-- 
@@ -102,6 +102,29 @@ enum {
 		      (void *)(arg));					\
 	} while (0)
 
+
+#ifdef __has_warning
+# if __has_warning("-Wstringop-overread")
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE				\
+     _Pragma("GCC diagnostic ignored \"-Wstringop-overread\"")
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP				\
+     _Pragma("GCC diagnostic pop")
+# else
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP
+# endif
+#else
+# if defined(__GNUC__) && __GNUC__ >= 11
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE				\
+     _Pragma("GCC diagnostic ignored \"-Wstringop-overread\"")
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP				\
+     _Pragma("GCC diagnostic pop")
+# else
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE
+#  define PRAGMA_STRINGOP_OVERREAD_IGNORE_POP
+# endif
+#endif
+
 #if __BYTE_ORDER == __BIG_ENDIAN
 #define L2_BUF_ETH_IP4_INIT						\
 	{								\
-- 
2.35.1