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Message-ID: <5f7e556c1e610_1a831208d2@john-XPS-13-9370.notmuch>
Date: Wed, 07 Oct 2020 16:55:24 -0700
From: John Fastabend <john.fastabend@...il.com>
To: John Fastabend <john.fastabend@...il.com>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
davem@...emloft.net
Cc: daniel@...earbox.net, john.fastabend@...il.com,
netdev@...r.kernel.org, bpf@...r.kernel.org, kernel-team@...com
Subject: RE: [PATCH bpf-next 1/3] bpf: Propagate scalar ranges through
register assignments.
John Fastabend wrote:
> Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@...nel.org>
> >
> > The llvm register allocator may use two different registers representing the
> > same virtual register. In such case the following pattern can be observed:
> > 1047: (bf) r9 = r6
> > 1048: (a5) if r6 < 0x1000 goto pc+1
> > 1050: ...
> > 1051: (a5) if r9 < 0x2 goto pc+66
> > 1052: ...
> > 1053: (bf) r2 = r9 /* r2 needs to have upper and lower bounds */
> >
> > In order to track this information without backtracking allocate ID
> > for scalars in a similar way as it's done for find_good_pkt_pointers().
> >
> > When the verifier encounters r9 = r6 assignment it will assign the same ID
> > to both registers. Later if either register range is narrowed via conditional
> > jump propagate the register state into the other register.
> >
> > Clear register ID in adjust_reg_min_max_vals() for any alu instruction.
>
> Do we also need to clear the register ID on reg0 for CALL ops into a
> helper?
>
> Looks like check_helper_call might mark reg0 as a scalar, but I don't
> see where it would clear the reg->id? Did I miss it. Either way maybe
> a comment here would help make it obvious how CALLs are handled?
>
> Thanks,
> John
OK sorry for the noise found it right after hitting send. Any call to
mark_reg_unknown will zero the id.
/* Mark a register as having a completely unknown (scalar) value. */
static void __mark_reg_unknown(const struct bpf_verifier_env *env,
struct bpf_reg_state *reg)
{
/*
* Clear type, id, off, and union(map_ptr, range) and
* padding between 'type' and union
*/
memset(reg, 0, offsetof(struct bpf_reg_state, var_off));
And check_helper_call() does,
/* update return register (already marked as written above) */
if (fn->ret_type == RET_INTEGER) {
/* sets type to SCALAR_VALUE */
mark_reg_unknown(env, regs, BPF_REG_0);
so looks good to me. In the check_func_call() case the if is_global
branch will mark_reg_unknown(). The other case only seems to do a
clear_caller_saved_regs though. Is that enough?
.John
>
> >
> > Newly allocated register ID is ignored for scalars in regsafe() and doesn't
> > affect state pruning. mark_reg_unknown() also clears the ID.
> >
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> > ---
> > kernel/bpf/verifier.c | 38 +++++++++++++++++++
> > .../testing/selftests/bpf/prog_tests/align.c | 16 ++++----
> > .../bpf/verifier/direct_packet_access.c | 2 +-
> > 3 files changed, 47 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 01120acab09a..09e17b483b0b 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -6432,6 +6432,8 @@ static int adjust_reg_min_max_vals(struct bpf_verifier_env *env,
> > src_reg = NULL;
> > if (dst_reg->type != SCALAR_VALUE)
> > ptr_reg = dst_reg;
> > + else
> > + dst_reg->id = 0;
> > if (BPF_SRC(insn->code) == BPF_X) {
> > src_reg = ®s[insn->src_reg];
> > if (src_reg->type != SCALAR_VALUE) {
> > @@ -6565,6 +6567,8 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> > /* case: R1 = R2
> > * copy register state to dest reg
> > */
> > + if (src_reg->type == SCALAR_VALUE)
> > + src_reg->id = ++env->id_gen;
> > *dst_reg = *src_reg;
> > dst_reg->live |= REG_LIVE_WRITTEN;
> > dst_reg->subreg_def = DEF_NOT_SUBREG;
> > @@ -7365,6 +7369,30 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
> > return true;
> > }
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