[Bug 1693] New: nf_tables writers starve readers

[bugzilla-daemon at netfilter.org]

https://bugzilla.netfilter.org/show_bug.cgi?id=1693

            Bug ID: 1693
           Summary: nf_tables writers starve readers
           Product: nftables
           Version: unspecified
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: kernel
          Assignee: pablo at netfilter.org
          Reporter: phil at nwl.cc

If kernel's ruleset is modified, all processes currently fetching (parts of)
the ruleset receive EINTR. Current (v1.0.7 at least) nftables tool reacts to
this signal by dropping anything fetched so far and restarting.

Given a sufficiently large ruleset and a sufficiently small modification
interval, an nftables process trying to list the ruleset will block forever.
Here's a simple reproducer:

| # init
| iptables-nft -N foo
| (
|       echo "*filter";
|       for ((i = 0; i < 100000; i++)); do
|               echo "-A foo -m comment --comment \"rule $i\" -j ACCEPT"
|       done
|       echo "COMMIT"
| ) | iptables-nft-restore --noflush
|
| # flood
| while true; do
|       iptables-nft -A foo -j ACCEPT
|       iptables-nft -D foo -j ACCEPT
| done

This should pretty reliably starve calls to 'nft list ruleset'. Depending on
the system, one might even insert 'sleep' calls into the "flood"-loop.

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