On Wed, 26 Sep 2001, Kiril Vidimce wrote:
> Is there any way to reconfigure the kernel at runtime to change the
> limit for arguments passed during an exec? [...]
include/linux/binfmts.h's MAX_ARG_PAGES define limits exec() arguments to
128k on systems with 4k pages, 256k on systems with 8k pages. Since this
define is used in the linux_binprm structure to define a static array,
there is no natural way (ioctl()) to change it runtime.
i've written a patch (attached) for 2.4.10 that implements runtime execve
argument size via /proc/sys/vm/exec-argsize. The patch also cleans up
do_execve() to be more compact. The latest versions of the patch are also
available at:
http://redhat.com/~mingo/execve-patches/
You can change the exec-argsize sysctl to any reasonable value - the
kernel will allocate at least 4 KB argument size, which is more than
enough to change the value back to a more reasonable value :) The
theoretical upper limit on x86 systems is 128 MB, due to kmalloc()
limitations. (but there might be other, mapping or sizing limitations in
libc or bash.)
systems with many users might want to set exec-argsize to a lower value
than the default.
i've tested the patch against vanilla 2.4.10 and it works just fine on
x86. A good testcase is:
ls -d `find /usr/src/linux/`
this runs just fine on my system if max-argsize is set to 1 MB.
I've seen no memory leaks in error cases either. Due to the extra
kmalloc(), execve is slower by 1-2 microseconds, but i think this is well
worth the flexibility - especially considering that execve() often takes
several milliseconds to finish.
note 1: This version of exec() gets sensitive to VM fragmentation above an
argument size is bigger than ~4MB - this is the size when the pages-array
gets larger than 4 KB, and kmalloc() uses higher-order page allocations.
note 2: it's safe to change the exec-argsize parameter in while exec()s
are in progress - old exec()s will use the older value, newer exec()s use
the newer value. The exec() code picks the parameter only once and does
not rely on it to be constant.
note 3: i've renamed struct binprm's ->page field to ->pages, to safely
break code that is not updated to use ->nr_pages yet. This breaks some
architectures in a safe way.)
Comments?
Ingo
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