more fun with procfs (netfilter)

[Alexander Viro]

% cat /proc/net/ip_conntrack |od -c
0000000   t   c   p                           6       1   2   1   0   7
[snip]
0005137
% od -c </proc/net/ip_conntrack
0000000
% cat /proc/net/ip_tables_names | od -c -w8
0000000   n   a   t  \n   f   i   l   t
0000010   e   r  \n
0000013
% od -c -w8 </proc/net/ip_tables_names
0000000   n   a   t  \n
0000004

Reason: netfilter procfs files try to fit entire records into the user
buffer.  Do a read shorter than record size and you've got zero.  And
read() returning 0 means you-know-what...

BTW, from strace output in cpuinfo bug report SuSE bash does reads by 128
bytes.  Which means that while read i; do echo $i; done </proc/net/ip_conntrack
will come out empty (lots of lines are longer than 160 characters).

I'll try to see if seq_file is suitable there, but in any case something
needs to be done - read() should return 0 _only_ at EOF and 128 bytes
definitely counts as reasonable buffer size.