Re: [PATCH 0/5] kstrdup optimization

Andrew Morton
2015-01-13T23:48:52Z

On Mon, 12 Jan 2015 21:45:58 +0100 Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> On Mon, Jan 12, 2015 at 10:18 AM, Andrzej Hajda <a.hajda@samsung.com> wrote:
> > kstrdup if often used to duplicate strings where neither source neither
> > destination will be ever modified. In such case we can just reuse the source
> > instead of duplicating it. The problem is that we must be sure that
> > the source is non-modifiable and its life-time is long enough.
> >
> > I suspect the good candidates for such strings are strings located in kernel
> > .rodata section, they cannot be modifed because the section is read-only and
> > their life-time is equal to kernel life-time.
> >
> > This small patchset proposes alternative version of kstrdup - kstrdup_const,
> > which returns source string if it is located in .rodata otherwise it fallbacks
> > to kstrdup.
>
> It also introduces kfree_const(const void *x).
>
> As kfree_const() has the exact same signature as kfree(), the risk of
> accidentally passing pointers returned from kstrdup_const() to kfree() seems
> high, which may lead to memory corruption if the pointer doesn't point to
> allocated memory.

Yes, it's an ugly little patchset. But 100-200k of memory is hard to
argue with, and I'm not seeing a practical way of getting those savings
with a cleaner approach.

Hopefully a kfree(rodata-address) will promptly oops, but I haven't
tested that and it presumably depends on which flavour of
slab/sleb/slib/slob/slub you're using.