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

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> 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?

I think you are right, in these particular functions, since the result
of the memory read cannot be discarded as the asm uses it.

>
>>
>> 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.

Ok.

I will send a V2 as soon as Cupertino's patch adding support for
-bpf_gcc-CFLAGS gets applied upstream.

Thanks.

>
>> +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