bpf: Build type-punning BPF selftests with -fno-strict-aliasing

A few BPF selftests perform type punning and they may break strict
aliasing rules, which are exploited by both GCC and clang by default
while optimizing.  This can lead to broken compiled programs.

This patch disables strict aliasing for these particular tests, by
mean of the -fno-strict-aliasing command line option.  This will make
sure these tests are optimized properly even if some strict aliasing
rule gets violated.

After this patch, GCC is able to build all the selftests without
warning about potential strict aliasing issue.

bpf@vger discussion on strict aliasing and BPF selftests:
https://lore.kernel.org/bpf/bae1205a-b6e5-4e46-8e20-520d7c327f7a@linux.dev/T/#t

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/bae1205a-b6e5-4e46-8e20-520d7c327f7a@linux.dev
Link: https://lore.kernel.org/bpf/20240130110343.11217-1-jose.marchesi@oracle.com

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 1a3654bcb5dd10c264536539bc73ef7b39b280c3..79253a376fc895b35826c74eb0ef518f43668019 100644 (file)
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -41,6 +41,19 @@ CFLAGS += -g $(OPT_FLAGS) -rdynamic                                  \
 LDFLAGS += $(SAN_LDFLAGS)
 LDLIBS += $(LIBELF_LIBS) -lz -lrt -lpthread
 
+# The following tests perform type punning and they may break strict
+# aliasing rules, which are exploited by both GCC and clang by default
+# while optimizing.  This can lead to broken programs.
+progs/bind4_prog.c-CFLAGS := -fno-strict-aliasing
+progs/bind6_prog.c-CFLAGS := -fno-strict-aliasing
+progs/dynptr_fail.c-CFLAGS := -fno-strict-aliasing
+progs/linked_list_fail.c-CFLAGS := -fno-strict-aliasing
+progs/map_kptr_fail.c-CFLAGS := -fno-strict-aliasing
+progs/syscall.c-CFLAGS := -fno-strict-aliasing
+progs/test_pkt_md_access.c-CFLAGS := -fno-strict-aliasing
+progs/test_sk_lookup.c-CFLAGS := -fno-strict-aliasing
+progs/timer_crash.c-CFLAGS := -fno-strict-aliasing
+
 ifneq ($(LLVM),)
 # Silence some warnings when compiled with clang
 CFLAGS += -Wno-unused-command-line-argument