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execve.2: Linux now imposes a floor on the ARG_MAX limit 

Starting with Linux 2.6.23, the ARG_MAX limit became settable via
(1/4 of) RLIMIT_STACK.  This broke ABI compatibility if RLIMIT_STACK
was set such that ARG_MAX was < 32 pages.  Document the fact that
since 2.6.25 Linux imposes a floor on ARG_MAX, so that the old limit
of 32 pages is guaranteed.

For some background on the changes to ARG_MAX in kernels 2.6.23 and
2.6.25, see:
    http://sourceware.org/bugzilla/show_bug.cgi?id=5786
    http://bugzilla.kernel.org/show_bug.cgi?id=10095
    http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,
    checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.

Also some reordering/rewording of the discussion of ARG_MAX.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2008-10-05 05:57:52 +02:00
parent 00e8730f78
commit f202015b45
1 changed files with 24 additions and 12 deletions
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36
man2/execve.2
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@ -34,7 +34,7 @@
.\" 2007-09-14 Ollie Wild <aaw@google.com>, mtk
.\" Add text describing limits on command-line arguments + environment
.\"
.TH EXECVE 2 2008-08-07 "Linux" "Linux Programmer's Manual"
.TH EXECVE 2 2008-10-04 "Linux" "Linux Programmer's Manual"
.SH NAME
execve \- execute program
.SH SYNOPSIS
@ -310,21 +310,33 @@ resource limit (see
that is in force at the time of the
.BR execve ()
call.
.\" FIXME .
.\" This means that as things stand sysconf(_SC_ARG_MAX) no longer returns
.\" accurate info; see http://sourceware.org/bugzilla/show_bug.cgi?id=5786
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
.BR MAX_ARG_STRLEN ),
and the maximum number of strings is 0x7FFFFFFF.
(This change allows programs to have a much larger
(Architectures with no memory management unit are excepted:
they maintain the limit that was in effect before kernel 2.6.23.)
This change allows programs to have a much larger
argument and/or environment list.
Imposing the 1/4-limit
.\" For some background on the changes to ARG_MAX in kernels 2.6.23 and
.\" 2.6.25, see:
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=5786
.\" http://bugzilla.kernel.org/show_bug.cgi?id=10095
.\" http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,
.\" checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.
For these architectures, the total size is limited to 1/4 of the allowed
stack size.
(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 limit that was in effect before kernel 2.6.23.
.\" Ollie: That doesn't include the lists of pointers, though,
.\" so the actual usage is a bit higher (1 pointer per argument).
Since Linux 2.6.25,
the kernel places a floor of 32 pages on this size limit,
so that, even when
.BR RLIMIT_STACK
is set very low,
applications are guaranteed to have at least as much argument and
environment space as was provided by Linux 2.6.23 and earlier.
(This guarantee was not provided in Linux 2.6.23 and 2.6.24.)
Additionally, the limit per string is 32 pages (the kernel constant
.BR MAX_ARG_STRLEN ),
and the maximum number of strings is 0x7FFFFFFF.
.SH "RETURN VALUE"
On success,
.BR execve ()