netdev - Re: [PATCH net-next] net: mdio: Add netlink interface

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Date:   Tue, 7 Mar 2023 11:41:04 -0500
From:   Sean Anderson <sean.anderson@...o.com>
To:     Andrew Lunn <andrew@...n.ch>,
        "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Vladimir Oltean <olteanv@...il.com>,
        linux-kernel@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Tobias Waldekranz <tobias@...dekranz.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next] net: mdio: Add netlink interface

On 3/7/23 08:47, Andrew Lunn wrote:
> On Mon, Mar 06, 2023 at 10:48:48PM +0000, Russell King (Oracle) wrote:
>> On Mon, Mar 06, 2023 at 03:45:16PM -0500, Sean Anderson wrote:
>> > +static int mdio_nl_eval(struct mdio_nl_xfer *xfer)
>> > +{
>> > +	struct mdio_nl_insn *insn;
>> > +	unsigned long timeout;
>> > +	u16 regs[8] = { 0 };
>> > +	int pc, ret = 0;
>> 
>> So "pc" is signed.
>> 
>> > +	int phy_id, reg, prtad, devad, val;
>> > +
>> > +	timeout = jiffies + msecs_to_jiffies(xfer->timeout_ms);
>> > +
>> > +	mutex_lock(&xfer->mdio->mdio_lock);
>> > +
>> > +	for (insn = xfer->prog, pc = 0;
>> > +	     pc < xfer->prog_len;
>> 
>> xfer->prog_len is signed, so this is a signed comparison.
>> 
>> > +		case MDIO_NL_OP_JEQ:
>> > +			if (__arg_ri(insn->arg0, regs) ==
>> > +			    __arg_ri(insn->arg1, regs))
>> > +				pc += (s16)__arg_i(insn->arg2);
>> 
>> This adds a signed 16-bit integer to pc, which can make pc negative.
>> 
>> And so the question becomes... what prevents pc becoming negative
>> and then trying to use a negative number as an index?
> 
> I don't know ebpf very well, but would it of caught this?  I know the
> aim of this is to be simple, but due to its simplicity, we are loosing
> out on all the inherent safety of eBPF. Is a eBPF interface all that
> complex? I assume you just need to add some way to identify MDIO
> busses and kfunc to perform a read on the bus?
Regarding eBPF over netlink, the last time this was discussed, Tobias said

> - Why not use BPF?
> 
>   That could absolutely be one way forward, but the GENL approach was
>   easy to build out-of-tree to prove the idea. Its not obvious how it
>   would work though as BPF programs typically run async on some event
>   (probe hit, packet received etc.) whereas this is a single execution
>   on behalf of a user. So to what would the program be attached? The
>   output path is also not straight forward, but it could be done with
>   perf events i suppose.

I'm not familiar enough with eBPF to comment further.

--Sean