netdev - Re: [PATCH net v4 01/12] net: core: limit nested device depth

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Date:   Tue, 01 Oct 2019 15:57:05 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Taehee Yoo <ap420073@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, linux-wireless@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        j.vosburgh@...il.com, vfalico@...il.com,
        Andy Gospodarek <andy@...yhouse.net>,
        Jiří Pírko <jiri@...nulli.us>,
        sd@...asysnail.net, Roopa Prabhu <roopa@...ulusnetworks.com>,
        saeedm@...lanox.com, manishc@...vell.com, rahulv@...vell.com,
        kys@...rosoft.com, haiyangz@...rosoft.com,
        Stephen Hemminger <stephen@...workplumber.org>,
        sashal@...nel.org, hare@...e.de, varun@...lsio.com,
        ubraun@...ux.ibm.com, kgraul@...ux.ibm.com,
        Jay Vosburgh <jay.vosburgh@...onical.com>,
        Cody Schuffelen <schuffelen@...gle.com>, bjorn@...k.no
Subject: Re: [PATCH net v4 01/12] net: core: limit nested device depth

Hi,

(jumping out now, forgive me for being so brief)

> If I understand correctly, you said about the alignment of
> "lower_level" and "upper_level".
> I thought this place is a fine position for variables as regards the
> alignment and I didn't try to put each variable in different places.
> 
> If I misunderstood your mention, please let me know.

Not sure what you mean, alignment doesn't matter for them (they're u8).

I was thinking of the packing for the overall struct, we have:

        unsigned int            max_mtu;
        unsigned short          type;
        unsigned short          hard_header_len;
        unsigned char           min_header_len;

+	unsigned char		upper_level, lower_level;

        unsigned short          needed_headroom;
        unsigned short          needed_tailroom;


Previously, there was a one byte hole at that spot due to a single
"unsigned char" (after something aligned at least 4 bytes) followed by
"unsigned short" - now you push that out a bit.

If you place the variables a bit lower, below "name_assign_type", you
probably fill a hole instead.

Check out the 'pahole' tool.

johannes