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Message-ID: <CAADnVQ+3go895wfmdCDnxt8FHhD9VMhtDZrPfe6i90LEBOonPA@mail.gmail.com>
Date: Fri, 12 Jan 2024 11:10:57 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Maxim Mikityanskiy <maxtram95@...il.com>
Cc: Eduard Zingerman <eddyz87@...il.com>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
Shung-Hsi Yu <shung-hsi.yu@...e.com>, John Fastabend <john.fastabend@...il.com>,
Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Jesper Dangaard Brouer <hawk@...nel.org>, bpf <bpf@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>, Maxim Mikityanskiy <maxim@...valent.com>
Subject: Re: [PATCH bpf-next v2 10/15] bpf: Track spilled unbounded scalars
On Mon, Jan 8, 2024 at 12:53 PM Maxim Mikityanskiy <maxtram95@...il.com> wrote:
>
> From: Maxim Mikityanskiy <maxim@...valent.com>
>
> Support the pattern where an unbounded scalar is spilled to the stack,
> then boundary checks are performed on the src register, after which the
> stack frame slot is refilled into a register.
>
> Before this commit, the verifier didn't treat the src register and the
> stack slot as related if the src register was an unbounded scalar. The
> register state wasn't copied, the id wasn't preserved, and the stack
> slot was marked as STACK_MISC. Subsequent boundary checks on the src
> register wouldn't result in updating the boundaries of the spilled
> variable on the stack.
>
> After this commit, the verifier will preserve the bond between src and
> dst even if src is unbounded, which permits to do boundary checks on src
> and refill dst later, still remembering its boundaries. Such a pattern
> is sometimes generated by clang when compiling complex long functions.
>
> One test is adjusted to reflect the fact that an untracked register is
> marked as precise at an earlier stage, and one more test is adjusted to
> reflect that now unbounded scalars are tracked.
>
> Signed-off-by: Maxim Mikityanskiy <maxim@...valent.com>
> Acked-by: Eduard Zingerman <eddyz87@...il.com>
> ---
> kernel/bpf/verifier.c | 7 +------
> tools/testing/selftests/bpf/progs/verifier_spill_fill.c | 6 +++---
> tools/testing/selftests/bpf/verifier/precise.c | 6 +++---
> 3 files changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 055fa8096a08..e7fff5f5aa1d 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4389,11 +4389,6 @@ static bool __is_scalar_unbounded(struct bpf_reg_state *reg)
> reg->u32_min_value == 0 && reg->u32_max_value == U32_MAX;
> }
>
> -static bool register_is_bounded(struct bpf_reg_state *reg)
> -{
> - return reg->type == SCALAR_VALUE && !__is_scalar_unbounded(reg);
> -}
> -
> static bool __is_pointer_value(bool allow_ptr_leaks,
> const struct bpf_reg_state *reg)
> {
> @@ -4504,7 +4499,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> return err;
>
> mark_stack_slot_scratched(env, spi);
> - if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) && env->bpf_capable) {
> + if (reg && !(off % BPF_REG_SIZE) && reg->type == SCALAR_VALUE && env->bpf_capable) {
> bool reg_value_fits;
>
> reg_value_fits = get_reg_width(reg) <= BITS_PER_BYTE * size;
> diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> index b05aab925ee5..57eb70e100a3 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> @@ -452,9 +452,9 @@ l0_%=: r1 >>= 16; \
> SEC("raw_tp")
> __log_level(2)
> __success
> -__msg("fp-8=0m??mmmm")
> -__msg("fp-16=00mm??mm")
> -__msg("fp-24=00mm???m")
> +__msg("fp-8=0m??scalar()")
> +__msg("fp-16=00mm??scalar()")
> +__msg("fp-24=00mm???scalar()")
> __naked void spill_subregs_preserve_stack_zero(void)
> {
> asm volatile (
> diff --git a/tools/testing/selftests/bpf/verifier/precise.c b/tools/testing/selftests/bpf/verifier/precise.c
> index 8a2ff81d8350..0a9293a57211 100644
> --- a/tools/testing/selftests/bpf/verifier/precise.c
> +++ b/tools/testing/selftests/bpf/verifier/precise.c
> @@ -183,10 +183,10 @@
> .prog_type = BPF_PROG_TYPE_XDP,
> .flags = BPF_F_TEST_STATE_FREQ,
> .errstr = "mark_precise: frame0: last_idx 7 first_idx 7\
> - mark_precise: frame0: parent state regs=r4 stack=:\
> + mark_precise: frame0: parent state regs=r4 stack=-8:\
> mark_precise: frame0: last_idx 6 first_idx 4\
> - mark_precise: frame0: regs=r4 stack= before 6: (b7) r0 = -1\
> - mark_precise: frame0: regs=r4 stack= before 5: (79) r4 = *(u64 *)(r10 -8)\
> + mark_precise: frame0: regs=r4 stack=-8 before 6: (b7) r0 = -1\
> + mark_precise: frame0: regs=r4 stack=-8 before 5: (79) r4 = *(u64 *)(r10 -8)\
> mark_precise: frame0: regs= stack=-8 before 4: (7b) *(u64 *)(r3 -8) = r0\
> mark_precise: frame0: parent state regs=r0 stack=:\
> mark_precise: frame0: last_idx 3 first_idx 3\
Yesterday I've applied patches 1 through 11 to bpf-next.
Then Yonghong found that removal of register_is_bounded()
in this patch 10 makes __is_scalar_unbounded() unused which
breaks build.
So I dropped patches 10 and 11.
Today we found out that test_verifier is broken with patches 1 through 9.
Turned out that this hunk for verifier/precise.c in patch 10 should have been
in patch 8.
I manually took it and force pushed bpf-next again.
Please test bisectability of the series more carefully in the future.
As far as register_is_bounded() issue.
Maybe order patch 14 that uses __is_scalar_unbounded() first and
then add this patch 10 ?
Other ideas?
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