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Re: process substitution data corruption on FreeBSD 5.3


From: Chet Ramey
Subject: Re: process substitution data corruption on FreeBSD 5.3
Date: Wed, 9 Mar 2005 21:45:17 -0500

> Machine Type: i386-portbld-freebsd5.3
> 
> Bash Version: 3.0
> Patch Level: 16
> Release Status: release
> 
> Description:
>       Process substitution doesn't work on FreeBSD 5.3 due to corruption of 
> the data. I've tested on other versions of FreeBSD (4.x) and Linux and there 
> are no problems. On all of the 5.3 boxes I have tried though with Bash 3.x 
> and Bash 2.x it does not work properly.
>       I don't know if this is a bash bug or a FreeBSD bug. I submitted a bug 
> report to FreeBSD some time ago but haven't seen any activity on it.
>       http://www.freebsd.org/cgi/query-pr.cgi?pr=77493

I think this is a FreeBSD bug.  I did some testing today using
5.4-PRERELEASE. 

The bash `read' builtin, unless it's reading from a pipe, uses
buffered reads.  When reading from stdin, upon reading a newline, it
must seek the input file pointer back to the offset of the newline to
avoid potentially stealing input from other programs. 

This seeking backward does not work when applied to named pipes on 
FreeBSD 5.[34], though there is no indication that it fails.

I don't know how FreeBSD implements named pipes, but they're not pipes:
lseek doesn't return failure when applied to them.

So the following happens: bash reads 128 bytes from the named pipe
connected to the process running `cat /etc/passwd'.  It reads 74 bytes
or so (on my system), finds a newline, returns the newline, and
attempts to seek backwards 54 bytes.  lseek appears to work and sets
the file offset to 74.  The next read, however, gets another 128 bytes
of data beginning at offset 129.  You end up missing large chunks of
the file. 

It worked on FreeBSD 4.x because that version had /dev/fd, and so bash
didn't use named pipes.  FreeBSD 5 doesn't support /dev/fd by default
for any file descriptors other than 0, 1, and 2. 

Chet

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
( ``Discere est Dolere'' -- chet )
                                                Live...Laugh...Love
Chet Ramey, ITS, CWRU    address@hidden    http://tiswww.tis.cwru.edu/~chet/




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