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Questions to bash "read" builtin functionality


From: Fiedler Roman
Subject: Questions to bash "read" builtin functionality
Date: Fri, 14 Dec 2012 11:28:03 +0000

Hello list,

One of our bash-scrips failed with very low frequency but randomly. The result 
was that exactly 1 byte was lost, so the string returned by "read -t 1" was too 
short. The culprit seems to be the built-in read function itself, the 
probability of failure was about 1:100000 in our case.

After analysis and creation of a reliable reproducer, I'm not sure if this is a 
programming error in bash or a bug in our script as the result of suboptimal 
documentation.

* Description:

When "read" builtin is used with timeout setting, both with "-t" option or by 
setting the "TMOUT" environment variable, and the timeout occurs when some 
bytes of line were already received by bash, then those bytes are silently 
dropped. The next "read" returns the remainder of the line.

A reproducer for Ubuntu precise is

base64 -d <<BASE64-EOF | tar -xj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==
BASE64-EOF
(export TMOUT=1; ./FragmentedSend.py | ./BashReadTest)

* Discussion:

The topic has two aspects:

** What is the intended read behavior in that case, is it a bug?

Man-page is silent on that. So even if current behavior is intended, in my 
opinion the possible loss of data in "read" should be mentioned.

              -t timeout
                     Cause read to time out and return failure if  a  complete
                     line  of input is not read within timeout seconds.  time-
                     out may be a decimal number  with  a  fractional  portion
                     following  the decimal point.  This option is only effec-
                     tive if read is reading input from a terminal,  pipe,  or
                     other  special  file;  it has no effect when reading from
                     regular files.  If timeout is 0, read returns success  if
                     input  is  available  on  the  specified file descriptor,
                     failure otherwise.  The exit status is greater  than  128
                     if the timeout is exceeded.

**Mild security aspect:

Since the TMOUT variable might not be filtered when invoking programs via 
"sudo" or within other restricted environments, it might be possible for an 
unprivileged user to invoke programs that use "read" in the same problematic 
way as I did. If he finds ways to influence the timing of the data reaching 
this function, he might be able to remove parts of the input or just cause 
random data corruption.

I would guess, that it would be a quite rare case, e.g. when someone 
implemented following script for login checking

cat [somefile] | sed [regex check for line sanity] | while read authConfLine; do
 ....Handle it
done

somefile content:

root allprivs Remaining line name of admin
userx someprivs FrankChangedItViaChFn userx allprivs FrankDidIt


Any comments?
Roman

DI Roman Fiedler
Engineer
Safety & Security Department
Assistive Healthcare Information Technology

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