[Changes from V2:
- no-strict-aliasing is only applied when building with GCC.
- cpumask_failure.c is excluded, as it doesn't use __imm_insn.]
The __imm_insn macro is defined in bpf_misc.h as:
#define __imm_insn(name, expr) [name]"i"(*(long *)&(expr))
This may lead to type-punning and strict aliasing rules violations in
it's typical usage where the address of a struct bpf_insn is passed as
expr, like in:
__imm_insn(st_mem,
BPF_ST_MEM(BPF_W, BPF_REG_1, offsetof(struct __sk_buff, mark), 42))
Where:
#define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
((struct bpf_insn) { \
.code = BPF_ST | BPF_SIZE(SIZE) | BPF_MEM, \
.dst_reg = DST, \
.src_reg = 0, \
.off = OFF, \
.imm = IMM })
In all the actual instances of this in the BPF selftests the value is
fed to a volatile asm statement as soon as it gets read from memory,
and thus it is unlikely anti-aliasing rules breakage may lead to
misguided optimizations.
However, GCC detects the potential problem (indirectly) by issuing a
warning stating that a temporary <Uxxxxxx> is used uninitialized,
where the temporary corresponds to the memory read by *(long *).
This patch adds -fno-strict-aliasing to the compilation flags of the
particular selftests that do type punning via __imm_insn, only for
GCC.
Tested in master bpf-next.
No regressions.
Signed-off-by: Jose E. Marchesi <jose.marchesi@xxxxxxxxxx>
Cc: david.faust@xxxxxxxxxx
Cc: cupertino.miranda@xxxxxxxxxx
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
tools/testing/selftests/bpf/Makefile | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 511ea84139a3..e962a0bd8a78 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -86,6 +86,19 @@ progs/btf_dump_test_case_namespacing.c-bpf_gcc-CFLAGS := -Wno-error
progs/btf_dump_test_case_packing.c-bpf_gcc-CFLAGS := -Wno-error
progs/btf_dump_test_case_padding.c-bpf_gcc-CFLAGS := -Wno-error
progs/btf_dump_test_case_syntax.c-bpf_gcc-CFLAGS := -Wno-error
+
+# The following tests do type-punning, via the __imm_insn macro, from
+# `struct bpf_insn' to long and then uses the value. This triggers an
+# "is used uninitialized" warning in GCC due to strict-aliasing
+# rules.
+progs/verifier_ref_tracking.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_unpriv.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_cgroup_storage.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_ld_ind.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_map_ret_val.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_spill_fill.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_subprog_precision.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
+progs/verifier_uninit.c-bpf_gcc-CFLAGS := -fno-strict-aliasing
endif
ifneq ($(CLANG_CPUV4),)
--
2.30.2
