netdev - RE: [PATCH bpf-next 1/3] bpf: Propagate scalar ranges through register assignments.

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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 = &regs[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;
> >  }