[clamav-users] Scan for dummy file with /dev/zero takes longer

[Taizo ITO]

Hello,

I'm trying to get some stats on how long a scan takes by different
size, but I encountered an unexpected behavior when scanning a file
generated in a specific way.
A scan for a dummy file filled with /dev/zero takes much longer than
with /dev/urandom. I think the processing time should be the same or
less.
I'd like to know how to avoid this problem because that may cause stuck service.
I'm using ClamAV version 0.101.2/25504.

You can reproduce the problem by doing the following.

A 10MB file with /dev/zero: Takes almost 7 times as long as /dev/urandom.
```
$ dd if=/dev/urandom of=dummy-dd-10MB-with-urandom.iso bs=10MB count=1
$ dd if=/dev/zero of=dummy-dd-10MB-with-zero.iso bs=10MB count=1
$ ls -ltr dummy-dd-10MB-with-*
-rw-r--r-- 1 user user 10000000 Jul  9 03:41 dummy-dd-10MB-with-zero.iso
-rw-r--r-- 1 user user 10000000 Jul  9 03:41 dummy-dd-10MB-with-urandom.iso

$ time (echo "SCAN dummy-dd-10MB-with-zero.iso" | nc -U
/var/run/clamd.scan/clamd.sock)
dummy-dd-10MB-with-zero.iso: OK
real 0m4.056s
user 0m0.008s
sys 0m0.004s

$ time (echo "SCAN dummy-dd-10MB-with-urandom.iso" | nc -U
/var/run/clamd.scan/clamd.sock)
dummy-dd-10MB-with-urandom.iso: OK
real 0m0.569s
user 0m0.012s
sys 0m0.000s
```


A 250MB file with /dev/zero: Takes almost 8 times as long as /dev/urandom.
```
$ dd if=/dev/zero of=dummy-dd-250MB-with-zero.iso bs=25MB count=10
$ dd if=/dev/urandom of=dummy-dd-250MB-with-urandom.iso bs=25MB count=10
$ ls -ltr dummy-dd-250MB-with-*
-rw-r--r-- 1 user user 250000000 Jul  9 03:44 dummy-dd-250MB-with-urandom.iso
-rw-r--r-- 1 user user 250000000 Jul  9 03:44 dummy-dd-250MB-with-zero.iso

$ time (echo "SCAN dummy-dd-250MB-with-zero.iso" | nc -U
/var/run/clamd.scan/clamd.sock)
dummy-dd-250MB-with-zero.iso: OK
real 1m42.949s
user 0m0.009s
sys 0m0.003s

$time (echo "SCAN dummy-dd-250MB-with-urandom.iso" | nc -U
/var/run/clamd.scan/clamd.sock)
dummy-dd-250MB-with-urandom.iso: OK
real 0m12.905s
user 0m0.004s
sys 0m0.007s
```

Thanks.

-- 
Taizo Ito