netdev - RE: /proc/net/dev regression

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Date:	Mon, 12 Jan 2015 11:47:13 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Al Viro' <viro@...IV.linux.org.uk>,
	"Carlos R. Mafra" <crmafra@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Hauke Mehrtens <hauke@...ke-m.de>,
	"John W. Linville" <linville@...driver.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: /proc/net/dev regression

From: Al Viro
> > I think the problem with wmnet is not that it was expecting the fields
> > to be aligned because it never had problems before (when definitely more
> > than 10 megabytes were received, wmnet is crappy but not _that_ crappy).
> >
> > I think the problem really was here,
> >
> > 	totalbytes_in = strtoul(&buffer[7], NULL, 10);
> >
> > After the patch the device name is 8 characters long and &buffer[7]
> > overlaps with the name instead of reading the bytes. Before the
> > patch is was fine because the call to strtoul() seems correct in the
> > sense that it would read everything until the NULL. So more than 10
> > megabytes was still ok.
> >
> > So I guess I was wrong when suggesting that the problem was the
> > alignment.
> 
> Several lines below there's this:
>                         totalpackets_out = strtoul(&buffer[74], NULL, 10);
>                         if (totalpackets_out != lastpackets_out) {
>                                 totalbytes_out = strtoul(&buffer[66], NULL, 10);
>                                 diffpackets_out += totalpackets_out - lastpackets_out;
>                                 diffbytes_out += totalbytes_out - lastbytes_out;
>                                 lastpackets_out = totalpackets_out;
>                                 lastbytes_out = totalbytes_out;
>                                 tx = True;
>                         }
> 
> So I'm afraid it *is* that crappy.  This function really should use scanf();
> note that updateStats_ipchains() in the same file does just that (well,
> fgets()+sscanf() for fsck knows what reason).  And I'd be careful with all
> those %d, actually - it's not _that_ hard to get more than 4Gb sent.
> scanf formats really ought to match the kernel-side (seq_)printf one...

IMHO it is safer to use strchr(p, ' '); to skip the interface name and then
use repeated calls to strtoull() to read the numbers.
Correctly/safely using scanf() is really too hard.

	David

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