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[PATCH] doc: mention more about ksh cloexec behavior


From: Eric Blake
Subject: [PATCH] doc: mention more about ksh cloexec behavior
Date: Mon, 13 Jun 2011 17:37:08 -0600

* doc/autoconf.texi (File Descriptors): Clarify that only the exec
builtin suffers from cloexec issues, and consolidate example to
show both /bin/sh and ksh pitfalls at once.

Signed-off-by: Eric Blake <address@hidden>
---

How does this look?

 ChangeLog         |    7 +++++
 doc/autoconf.texi |   78 +++++++++++++++++++++++++---------------------------
 2 files changed, 45 insertions(+), 40 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 1fca7bb..c2e11d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2011-06-13  Eric Blake  <address@hidden>
+
+       doc: mention more about ksh cloexec behavior
+       * doc/autoconf.texi (File Descriptors): Clarify that only the exec
+       builtin suffers from cloexec issues, and consolidate example to
+       show both /bin/sh and ksh pitfalls at once.
+
 2011-05-05  Eric Blake  <address@hidden>

        doc: document dash bug with positional parameters
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 7ff693f..5571c2c 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -15435,53 +15435,51 @@ File Descriptors
 Don't rely on file descriptors 0, 1, and 2 remaining closed in a
 subsidiary program.  If any of these descriptors is closed, the
 operating system may open an unspecified file for the descriptor in the
-new process image.  Posix says this may be done only if the subsidiary
-program is set-user-ID or set-group-ID, but HP-UX 11.23 does
-it even for ordinary programs.
-
-Don't rely on open file descriptors being open in child processes.  In
address@hidden, file descriptors above 2 which are opened using
+new process image.  Posix 2008 says this may be done only if the
+subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does
+it even for ordinary programs, and the next version of Posix will allow
+HP-UX behavior.
+
+If you want a file descriptor above 2 to be inherited into a child
+process, then you must use redirections specific to that command or a
+containing subshell or command group, rather than relying on
address@hidden in the shell. In @command{ksh} as well as HP-UX
address@hidden, file descriptors above 2 which are opened using
 @samp{exec @var{n}>file} are closed by a subsequent @samp{exec} (such as
 that involved in the fork-and-exec which runs a program or script).
-Thus, using @command{sh}, we have:
-
address@hidden
-$ @kbd{cat ./descrips}
-#!/bin/sh -
-echo hello >&5
-$ @kbd{exec 5>t}
-$ @kbd{./descrips}
-$ @kbd{cat t}
-hello
-$
address@hidden example
-
address@hidden
-But using ksh:
-
address@hidden
-$ @kbd{exec 5>t}
-$ @kbd{./descrips}
-hello
-$ @kbd{cat t}
-$
address@hidden example
-
address@hidden
-Within the process which runs the @samp{descrips} script, file
-descriptor 5 is closed.

 Don't rely on duplicating a closed file descriptor to cause an
 error.  With Solaris @command{/bin/sh}, when the redirection fails, the
 output goes to the original file descriptor.

address@hidden
-$ @kbd{bash -c 'echo hi >&3' 3>&-; echo $?}
-bash: 3: Bad file descriptor
-1
-$ @kbd{/bin/sh -c 'echo hi >&3' 3>&-; echo $?}
-hi
-0
+Demonstrating these last two points, note how the child @command{sh} (in
+this case, Solaris 10 @command{/bin/sh}) leaks output to standard output
+when file descriptor 5 is closed, and how the parent @command{ksh}
+refuses to pass file descriptor 5 to the child via @command{exec}:
+
address@hidden
+$ @kbd{echo 'echo hello >&5\n' >k
+$ @kbd{sh -c 'exec 5>t; echo -`sh ./k`-`cat t`-'}
+--hello-
+$ @kbd{sh -c ':>t; exec 5>&-; echo -`sh ./k`-`cat t`-'}
+-hello--
+$ @kbd{ksh -c 'exec 5>t; echo -`ksh ./k`-`cat t`-'}
+./k[1]: 5: cannot open [Bad file number]
+---
+$ @kbd{ksh -c ':>t; exec 5>&-; echo -`ksh ./k`-`cat t`-'}
+./k[1]: 5: cannot open [Bad file number]
+---
+$ @kbd{ksh -c '(echo -`ksh ./k`-`cat t`-) 5>t'}
+--hello-
+$ @kbd{ksh -c '{ echo -`ksh ./k`-`cat t`-; } 5>t'}
+--hello-
+$ @kbd{ksh -c 'echo -`5>t ksh ./k`-`cat t`-'}
+--hello-
+$ @kbd{bash -c 'exec 5>t; echo -`bash ./k`-`cat t`-'}
+--hello-
+$ @kbd{bash -c ':>t; exec 5>&-; echo -`bash ./k`-`cat t`-'}
+./k: line 1: 5: Bad file number
+---
 @end example

 Fortunately, an attempt to close an already closed file descriptor will
-- 
1.7.4.4




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