From 73868988c90d2701587ab2a48b5858ab935afb17 Mon Sep 17 00:00:00 2001 From: "Jose E. Marchesi" Date: Sat, 11 May 2024 23:22:13 +0200 Subject: bpf: disable strict aliasing in test_global_func9.c The BPF selftest test_global_func9.c performs type punning and breaks srict-aliasing rules. In particular, given: int global_func9(struct __sk_buff *skb) { int result = 0; [...] { const struct C c = {.x = skb->len, .y = skb->family }; result |= foo((const struct S *)&c); } } When building with strict-aliasing enabled (the default) the initialization of `c' gets optimized away in its entirely: [... no initialization of `c' ...] r1 = r10 r1 += -40 call foo w0 |= w6 Since GCC knows that `foo' accesses s->x, we get a "maybe uninitialized" warning. On the other hand, when strict-aliasing is disabled GCC only optimizes away the store to `.y': r1 = *(u32 *) (r6+0) *(u32 *) (r10+-40) = r1 ; This is .x = skb->len in `c' r1 = r10 r1 += -40 call foo w0 |= w6 In this case the warning is not emitted, because s-> is initialized. This patch disables strict aliasing in this test when building with GCC. clang seems to not optimize this particular code even when strict aliasing is enabled. Tested in bpf-next master. Signed-off-by: Jose E. Marchesi Cc: david.faust@oracle.com Cc: cupertino.miranda@oracle.com Cc: Yonghong Song Cc: Eduard Zingerman Link: https://lore.kernel.org/r/20240511212213.23418-1-jose.marchesi@oracle.com Signed-off-by: Alexei Starovoitov --- tools/testing/selftests/bpf/Makefile | 1 + 1 file changed, 1 insertion(+) (limited to 'tools/testing') diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile index ed381b0197fe..e0b3887b3d2d 100644 --- a/tools/testing/selftests/bpf/Makefile +++ b/tools/testing/selftests/bpf/Makefile @@ -53,6 +53,7 @@ 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 +progs/test_global_func9.c-CFLAGS := -fno-strict-aliasing ifneq ($(LLVM),) # Silence some warnings when compiled with clang -- cgit v1.2.3