netdev - Re: [PATCH net] bpf: fix states equal logic for varlen access

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Message-ID: <20161129164907.GA22217@ast-mbp.thefacebook.com>
Date:   Tue, 29 Nov 2016 08:49:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josef Bacik <jbacik@...com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, daniel@...earbox.net,
        ast@...nel.org, jannh@...gle.com
Subject: Re: [PATCH net] bpf: fix states equal logic for varlen access

On Tue, Nov 29, 2016 at 09:45:33AM -0500, Josef Bacik wrote:
> On 11/28/2016 10:04 PM, Alexei Starovoitov wrote:
> >On Mon, Nov 28, 2016 at 02:44:10PM -0500, Josef Bacik wrote:
> >>If we have a branch that looks something like this
> >>
> >>int foo = map->value;
> >>if (condition) {
> >>  foo += blah;
> >>} else {
> >>  foo = bar;
> >>}
> >>map->array[foo] = baz;
> >>
> >>We will incorrectly assume that the !condition branch is equal to the condition
> >>branch as the register for foo will be UNKNOWN_VALUE in both cases.  We need to
> >>adjust this logic to only do this if we didn't do a varlen access after we
> >>processed the !condition branch, otherwise we have different ranges and need to
> >>check the other branch as well.
> >>
> >>Signed-off-by: Josef Bacik <jbacik@...com>
> >>---
> >> kernel/bpf/verifier.c | 10 ++++++++--
> >> 1 file changed, 8 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>index 89f787c..2c8a688 100644
> >>--- a/kernel/bpf/verifier.c
> >>+++ b/kernel/bpf/verifier.c
> >>@@ -2478,6 +2478,7 @@ static bool states_equal(struct bpf_verifier_env *env,
> >> {
> >> 	struct bpf_reg_state *rold, *rcur;
> >> 	int i;
> >>+	bool map_access = env->varlen_map_value_access;
> >
> >that's a bit misleading name for the variable.
> >Pls call it varlen_map_access.
> >
> >> 	for (i = 0; i < MAX_BPF_REG; i++) {
> >> 		rold = &old->regs[i];
> >>@@ -2489,12 +2490,17 @@ static bool states_equal(struct bpf_verifier_env *env,
> >> 		/* If the ranges were not the same, but everything else was and
> >> 		 * we didn't do a variable access into a map then we are a-ok.
> >> 		 */
> >>-		if (!env->varlen_map_value_access &&
> >>+		if (!map_access &&
> >> 		    rold->type == rcur->type && rold->imm == rcur->imm)
> >
> >just noticed that this one is missing comparing rold->id == rcur->id
> >
> 
> Do you want me to fix that here?  I'll fix up the rest of the stuff, and
> Daniels things as well.  Thanks,

Nevermind. Comparing 'id' is not needed in net, only in net-next.