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[bpf-next] bpf: avoid UB in usages of the __imm_insn macro

Message ID 20240507113755.28118-1-jose.marchesi@oracle.com (mailing list archive)
State Superseded
Delegated to: BPF
Headers show
Series [bpf-next] bpf: avoid UB in usages of the __imm_insn macro | expand

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Commit Message

Jose E. Marchesi May 7, 2024, 11:37 a.m. UTC 
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 })

GCC detects this 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.  This
silences the warning and, most importantly, avoids potential
optimization problems due to breaking anti-aliasing rules.

Tested in master bpf-next.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Eduard Zingerman <eddyz87@gmail.com>
---
 tools/testing/selftests/bpf/Makefile | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
diff mbox series

Patch

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index f0c429cf4424..b0c696933d15 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -53,6 +53,21 @@  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
+# In the following tests the strict aliasing rules are broken by the
+# __imm_insn macro, that do type-punning from `struct bpf_insn' to
+# long and then uses the value.  This triggers an "is used
+# uninitialized" warning in GCC.  This in theory may also lead to
+# broken programs, so it is better to disable strict aliasing than
+# inhibiting the warning.
+progs/verifier_ref_tracking.c := -fno-strict-aliasing
+progs/verifier_unpriv.c-CFLAGS := -fno-strict-aliasing
+progs/verifier_cgroup_storage.c-CFLAGS := -fno-strict-aliasing
+progs/verifier_ld_ind.c-CFLAGS := -fno-strict-aliasing
+progs/verifier_map_ret_val.c-CFLAGS := -fno-strict-aliasing
+progs/cpumask_failure.c-CFLAGS := -fno-strict-aliasing
+progs/verifier_spill_fill.c-CFLAGS := -fno-strict-aliasing
+progs/verifier_subprog_precision.c-CFLAGS := -fno-strict-aliasing
+progs/verifier_uninit.c-CFLAGS := -fno-strict-aliasing
 
 ifneq ($(LLVM),)
 # Silence some warnings when compiled with clang