Re: [PATCH bpf-next V2] bpf: avoid UB in usages of the __imm_insn macro

[Yonghong Song <yonghong.song@linux.dev>]

On 5/7/24 6:31 AM, Jose E. Marchesi wrote:
> [Differences with V1:
> - Typo fixed in patch: progs/verifier_ref_tracking.c
>    was missing -CFLAGS.]
>
> 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.

For all the modified verifier_* files below, the functions
are naked inline asm, so there is no optimization risk of breaking
anti-aliasing rules. Is this right?

>
> 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 --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index f0c429cf4424..c7507f420d9e 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-CFLAGS := -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

All these verifier_* files have __imm_insn, but I didn't see
__imm_insn usage for cpumask_failure.c. Did I miss anything?

All these verifier_* files are naked inline asm. So it should not
cause any issues with -fstrict-aliasing. Since there are no
issues for clang. Maybe just add -fno-strict-aliasing for gcc
only to silence the warning.

> +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