From: Eduard Zingerman <eddyz87@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>,
bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
martin.lau@kernel.org, kernel-team@meta.com
Subject: Re: [PATCH v2 bpf-next 06/10] bpf: preserve constant zero when doing partial register restore
Date: Tue, 21 Nov 2023 22:22:59 +0200 [thread overview]
Message-ID: <4a91459cfdc249dfa6fc0dcc166ed5629f00def1.camel@gmail.com> (raw)
In-Reply-To: <CAEf4BzbeNwJSDo0t=NxK4dL2m2m9O30=iAxTata1kK9p-uxxxQ@mail.gmail.com>
On Tue, 2023-11-21 at 10:14 -0800, Andrii Nakryiko wrote:
> On Tue, Nov 21, 2023 at 8:20 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
> >
> > On Mon, 2023-11-20 at 16:22 -0800, Andrii Nakryiko wrote:
> > > Similar to special handling of STACK_ZERO, when reading 1/2/4 bytes from
> > > stack from slot that has register spilled into it and that register has
> > > a constant value zero, preserve that zero and mark spilled register as
> > > precise for that. This makes spilled const zero register and STACK_ZERO
> > > cases equivalent in their behavior.
> > >
> > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > > ---
> >
> > Acked-by: Eduard Zingerman <eddyz87@gmail.com>
> >
> > [...]
> >
> > > + if (spill_cnt == size &&
> > > + tnum_is_const(reg->var_off) && reg->var_off.value == 0) {
> > > + __mark_reg_const_zero(&state->regs[dst_regno]);
> > > + /* this IS register fill, so keep insn_flags */
> > > + } else if (zero_cnt == size) {
> > > + /* similarly to mark_reg_stack_read(), preserve zeroes */
> > > + __mark_reg_const_zero(&state->regs[dst_regno]);
> > > + insn_flags = 0; /* not restoring original register state */
> >
> > nit: In case if there would be v3, could you please
> > leave a comment here, something like below:
> >
> > when check_stack_write_fixed_off() puts STACK_ZERO marks
> > for writes, not aligned on register boundary, it marks source
> > registers precise. Thus, additional precision propagation is
> > necessary in this case and insn_flags could be cleared.
> >
> > or something along these lines?
> >
>
> Hm... this seems misleading, precision propagation of original
> register on write is orthogonal to this. The real reason why we clear
> insn_flags is because there is no *register* fill here. There might
> not be any spilled register at all here, or a completely irrelevant
> spilled register. This instruction is basically `r1 = 0;`, though
> obfuscated as stack dereference. So from the backtracking side of
> things, there is no stack access here.
Hm, I mostly agree. This comment would be relevant only before patch #8
in this series.
next prev parent reply other threads:[~2023-11-21 20:23 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-21 0:22 [PATCH v2 bpf-next 00/10] Complete BPF verifier precision tracking support for register spills Andrii Nakryiko
2023-11-21 0:22 ` [PATCH v2 bpf-next 01/10] bpf: support non-r10 register spill/fill to/from stack in precision tracking Andrii Nakryiko
2023-11-21 0:22 ` [PATCH v2 bpf-next 02/10] selftests/bpf: add stack access precision test Andrii Nakryiko
2023-11-21 0:42 ` Eduard Zingerman
2023-11-21 0:22 ` [PATCH v2 bpf-next 03/10] bpf: fix check for attempt to corrupt spilled pointer Andrii Nakryiko
2023-11-21 0:22 ` [PATCH v2 bpf-next 04/10] bpf: preserve STACK_ZERO slots on partial reg spills Andrii Nakryiko
2023-11-21 0:22 ` [PATCH v2 bpf-next 05/10] selftests/bpf: validate STACK_ZERO is preserved on subreg spill Andrii Nakryiko
2023-11-21 0:55 ` Eduard Zingerman
2023-11-21 0:22 ` [PATCH v2 bpf-next 06/10] bpf: preserve constant zero when doing partial register restore Andrii Nakryiko
2023-11-21 16:20 ` Eduard Zingerman
2023-11-21 18:14 ` Andrii Nakryiko
2023-11-21 20:22 ` Eduard Zingerman [this message]
2023-11-21 0:22 ` [PATCH v2 bpf-next 07/10] selftests/bpf: validate zero preservation for sub-slot loads Andrii Nakryiko
2023-11-21 16:20 ` Eduard Zingerman
2023-11-21 18:15 ` Andrii Nakryiko
2023-11-21 0:22 ` [PATCH v2 bpf-next 08/10] bpf: track aligned STACK_ZERO cases as imprecise spilled registers Andrii Nakryiko
2023-11-21 20:38 ` Alexei Starovoitov
2023-11-21 22:01 ` Andrii Nakryiko
2023-11-21 0:22 ` [PATCH v2 bpf-next 09/10] selftests/bpf: validate precision logic in partial_stack_load_preserves_zeros Andrii Nakryiko
2023-11-21 16:20 ` Eduard Zingerman
2023-11-21 0:22 ` [PATCH v2 bpf-next 10/10] bpf: use common instruction history across all states Andrii Nakryiko
2023-11-21 16:20 ` Eduard Zingerman
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