Re: [RFC/PATCH] RLIMIT_ARG_MAX

On Fri, Feb 29, 2008 at 7:39 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:


On Fri, 29 Feb 2008, Michael Kerrisk wrote:
>
> My reading of POSIX.1 (and POSIX doesn't seem very explicit on this point), is
> that the limits on argv+environ and on stack are decoupled, since POSIX
> specifies RLIMIT_STACK and sysconf(_SC_ARG_MAX) and doesn't specify any
> relationship between the two.

I agree. And clearly there _are_ relationships and always have been, but
equally clearly they simply haven't been a big issue in practice, and
nobody really cares.

Do we know that for sure?

Usually, _SC_ARG_MAX is just so much smaller than RLIMIT_STACK that it
makes no possible difference. Which I would actually argue we should just
continue with: just keep _SC_ARG_MAX a smallish, irrelevant constant.

We still have to have the compile-time ARG_MAX constant (as in *real*
constant - a #define) anyway, for traditional programs, and you might as
well make sysconf(_SC_ARG_MAX) always just match ARG_MAX.

It's not like there is likely a single user of _SC_ARG_MAX that cares.

In my initial reply, I pointed out one example where users *may* care:
NPTL uses RLIMIT_STACK to determine the size of per-thread stacks. It
is conceivable that users might want to set RLIMIT_STACK < 512k, and
that would have the effect of lowering the amount of space for
argv+eviron below what the kernel has historically guaranteed. That's
an ABI change, though it's unclear whether it would impact anyone in
practice.
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