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Bug#636266: marked as done (libc6: fread(buf, 1, 1) from urandom consumes 4096 bytes of entropy)

Your message dated Tue, 02 Aug 2011 18:54:41 +0200 (CEST)
with message-id <60786.10050.16516-29544-1472601700-1312304081@seznam.cz>
and subject line Re: ibc6: fread(buf, 1, 1) from urandom consumes 4096 bytes of entropy
has caused the Debian Bug report #636266,
regarding libc6: fread(buf, 1, 1) from urandom consumes 4096 bytes of entropy
to be marked as done.

This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

(NB: If you are a system administrator and have no idea what this
message is talking about, this may indicate a serious mail system
misconfiguration somewhere. Please contact owner@bugs.debian.org
immediately.)


-- 
636266: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=636266
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems
--- Begin Message --- 
Package: libc6
Version: 2.13-10
Severity: normal

fopen()/fread() is used by some software packages (I noticed it via ECL) to
read data from /dev/urandom.

But fread() reads more bytes than specified (rounded up to 4096!), thereby
decreasing the available entropy by a large amount.


To test, compile this program:
        #include <stdio.h>
        void main()
        {
                FILE *fh;

                fh = fopen("/dev/urandom", "r");
                fread(&fh, 1, 1, fh);
        }

and run via strace:

        ...
        brk(0x1e64000)                          = 0x1e64000
        open("/dev/urandom", O_RDONLY)          = 3
        fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 9), ...}) = 0
        ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff68dbb60) = -1 EINVAL
(Invalid argument)
        mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x7fac45b34000
        read(3, "\242\261\204\215\221\313\253#\336\355\336u\37\3Gl\374
\332\301\271\363Z\327\7@.ziU\324\305"..., 4096) = 4096


This is a bug IMO; with urandom it discards a lot of entropy, and with random
it might cause hangs.



-- System Information:
Debian Release: wheezy/sid
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'unstable'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.0.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=de_AT.UTF-8, LC_CTYPE=de_AT.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages libc6 depends on:
ii  libc-bin                      2.13-10    Embedded GNU C Library: Binaries
ii  libgcc1                       1:4.6.1-4  GCC support library

libc6 recommends no packages.

Versions of packages libc6 suggests:
ii  debconf [debconf-2.0]         1.5.40     Debian configuration management sy
pn  glibc-doc                     <none>     (no description available)
ii  locales                       2.13-10    Embedded GNU C Library: National L

-- debconf information:
* glibc/upgrade: true
  glibc/disable-screensaver:
  glibc/restart-failed:
* glibc/restart-services: ssh postfix openbsd-inetd cups cron atd apache2

--- End Message ---
--- Begin Message --- 
> fopen()/fread() is used by some software packages (I noticed it via ECL) to
> read data from /dev/urandom.

So fix/report bug against these packages.

Iff the application does not want to use buffered io, 
just ask for it - add "setbuf(fh, NULL)".

Your sample program should be

        #include <stdio.h>
        void main()
        {
                FILE *fh;

                fh = fopen("/dev/urandom", "r");
                setbuf(fh, NULL);
                fread(&fh, 1, 1, fh);
        }

Petr

--- End Message ---
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