netdev - Re: Issue with /proc/sys/net/ipv4/tcp

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Message-ID: <1444700679.21657.11.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 12 Oct 2015 18:44:39 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	wangyufen <wangyufen@...wei.com>, netdev@...r.kernel.org
Subject: Re: Issue with /proc/sys/net/ipv4/tcp_mem

On Mon, 2015-10-12 at 11:37 -0500, Eric W. Biederman wrote:
> wangyufen <wangyufen@...wei.com> writes:
> 
> > Hi,
> >
> > I tried on linux-4.1:
> >     linux:~# cat /proc/sys/net/ipv4/tcp_mem 
> >     8388608	12582912	16777216
> >     linux:~# echo 1234 >/proc/sys/net/ipv4/tcp_mem 
> >     -bash: echo: write error: Invalid argument
> >     linux:~# cat /proc/sys/net/ipv4/tcp_mem 
> >     1234	12582912	16777216
> >
> > the echo operation got error, but value already written to tcp_mem.
> >
> > I checked, patch f594d63199688ad568fb caused the issue.
> 
> 
> If your problem is that you can not write a single value and instead
> have to write all three values I don't know what to tell you.  I don't
> see how that could have ever worked.
> 
> Certainly the commit you pointed at did not change that behavior.

I would not be so sure.
Above commit added a regression for partial writes.
If a write() returns an error like EINVAL, we expect no change occurred.

Prior code was calling proc_doulongvec_minmax() using a temporary array,
and updated tcp_mem[0 .. 2] only of proc_doulongvec_minmax() returned 0

       ret = proc_doulongvec_minmax(&tmp, write, buffer, lenp, ppos);
       if (ret)
               return ret;

#ifdef CONFIG_MEMCG_KMEM
	// deleted for clarity
#endif

       net->ipv4.sysctl_tcp_mem[0] = vec[0];
       net->ipv4.sysctl_tcp_mem[1] = vec[1];
       net->ipv4.sysctl_tcp_mem[2] = vec[2];

       return 0;

We could argue it is a bug in proc_doulongvec_minmax().
This helper probably should allocate a temp buffer,
as we have the same issue with udp_mem[].

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