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poor <&- behavior under ulimit -n

From: Eric Blake
Subject: poor <&- behavior under ulimit -n
Date: Wed, 27 Oct 2010 16:25:21 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100921 Fedora/3.1.4-1.fc13 Mnenhy/0.8.3 Thunderbird/3.1.4

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-redhat-linux-gnu'
-DCONF_VENDOR='redhat' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash'
-DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib  -D_GNU_SOURCE
-DRECYCLES_PIDS  -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions
-fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic
uname output: Linux office #1 SMP Tue Oct 19
04:06:30 UTC 2010 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-redhat-linux-gnu

Bash Version: 4.1
Patch Level: 7
Release Status: release

Bash does not behave well when under artificial fd pressure due to
ulimit -n.  It issues a spurious warning to stderr because it tries
to save necessary fds starting at 10.  Compare this with ksh93, which
saves fds starting at 3.

Many other shells (for example, dash or BSD /bin/sh) exit with
non-zero status if they can't use fd 10, rather than proceeding onwards.
At any rate, bash MUST exit with failure if it cannot save an fd, even
if you decide that it is unsafe to copy ksh93's action of saving at fd
3 rather than 10.  The current behavior results in bash silently
corrupting stdin!  Merely warning me that you lost stdin, but still
exiting with 0 status, is a disservice.

$ ksh -c 'ulimit -n 10; : <&-; exec 3<&0'; echo $?
$ ksh -c 'ulimit -n 3; : <&-; exec 3<&0'; echo $?
ksh[1]: open file limit exceeded [Invalid argument]
$ ksh -c 'ulimit -n 10; : <&-'; echo $?
$ bash -c 'ulimit -n 11; : <&-'; echo $?
$ # This next step should either silently work or die with an error
$ bash -c 'ulimit -n 10; : <&-'; echo $?
bash: redirection error: cannot duplicate fd: Invalid argument
$ # Proof that you silently lost stdin during the : <&- step.
$ bash -c 'ulimit -n 10; : <&-; exec 3<&0'; echo $?
bash: redirection error: cannot duplicate fd: Invalid argument
bash: 0: Bad file descriptor

The bare minimum fix is to error out if you cannot dup() a fd
to preserve it across the duration of a temporary redirection.
But the nicer fix would be to follow ksh's lead and start saving
fd's at 3, even if this means doing more bookkeeping of which fds
have been put into use by the user vs. used by bash (that is,
if stdin was saved by bash in fd 3, but then the script asks to
redirect fd 3, then bash would have to re-save fd 3 to somewhere

Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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