linux-kernel - Re: execve manpage changes

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Message-ID: <46DEFD4C.4030805@gmx.net>
Date:	Wed, 05 Sep 2007 21:02:36 +0200
From:	Michael Kerrisk <mtk-manpages@....net>
To:	Ollie Wild <aaw@...gle.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: execve manpage changes

Hello Ollie,

Ollie Wild wrote:
> Michael,
> 
> Per our previous discussion, here are the updates to the execve(2)
> manpage resulting from the variable length argument support patch in
> linux-2.6.23-rc1.  The execve API is unmodified.  However, the
> following should be noted in the "Notes" section.
> 
> Prior to linux-2.6.23, the memory used to store the environment and
> argument list strings was limited to 32 (MAX_ARG_PAGES) pages.  On
> most architectures, the page size is 4kB, leading to a maximum size of
> 128kB.
> 
> In linux-2.6.23 and later, most architectures support a size limit
> derived from the current RLIMIT_STACK setting (see getrlimit(2)).  For
> these architectures, the total size is limited to 1/4 the allowed
> stack size, the limit per string is 32 pages (MAX_ARG_STRLEN), and the
> maximum number of strings is 0x7FFFFFFF.  Architectures with no memory
> management unit are excepted: they maintain the pre-2.6.23 limit.

Thanks.  For man-pages-2.66 I have added the following text to execve.2:

   Limits on size of arguments + environment
       Most Unix implementations impose some limit on the total size
       of the command-line argument (argv)  and  environment  (envp)
       strings  that may be passed to a new program.  POSIX.1 allows
       an implementation to advertise this limit using  the  ARG_MAX
       constant  (either  defined  in <limits.h> or available at run
       time using the call sysconf(_SC_ARG_MAX)).

       On Linux prior to kernel 2.6.23, the memory used to store the
       environment  and  argument  strings  was  limited to 32 pages
       (defined by the kernel constant MAX_ARG_PAGES).  On architec-
       tures  with  a  4-kB page size, this yields a maximum size of
       128 kB.

       On kernel 2.6.23 and later, most architectures support a size
       limit  derived from the soft RLIMIT_STACK resource limit (see
       getrlimit(2)).  For these architectures, the  total  size  is
       limited  to  1/4  of  the  allowed  stack size, the limit per
       string is 32 pages (the kernel constant MAX_ARG_STRLEN),  and
       the  maximum  number  of strings is 0x7FFFFFFF.  (This change
       allows programs to have a much larger argument  and/or  envi-
       ronment  list.   Imposing  the 1/4-limit ensures that the new
       program always has some stack space.)  Architectures with  no
       memory  management  unit  are  excepted:  they  maintain  the
       pre-2.6.23 limit.

Cheers,

Michael

-- 
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7

Want to help with man page maintenance?  Grab the latest tarball at
http://www.kernel.org/pub/linux/docs/manpages/
read the HOWTOHELP file and grep the source files for 'FIXME'.

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