Re: [PATCH 0/3] add new strscpy() API for string copy

[Chris Metcalf]

On 05/14/2015 07:10 PM, Michael Ellerman wrote:
> On Thu, 2015-04-30 at 12:01 -0400, Chris Metcalf wrote:
> > This patch series addresses limitations in strncpy() and strlcpy();
> > both the old APIs are unpleasant, as Linus nicely summarized here
> > a couple of days ago:
> >
> >    https://lkml.org/lkml/2015/4/28/570
> >
> > and of course as other folks (Greg K-H and Linus again) said last year:
> >
> >    https://plus.google.com/+gregkroahhartman/posts/1amLbuhWbh5
> >
> > The proposed new API (strscpy(), for "s"afe string copy) has an
> > easy-to-use API for detecting buffer overflow, avoids unsafe truncation
> > by default, and isn't subject to thread-safety attacks like the current
> > strlcpy implementation.  See patch 2/3 for more on why strscpy() is a
> > good thing.
> +1 on the concept.

Thanks.

> > To make strscpy() work more efficiently I did the minimum tweaking
> > necessary to allow <asm/word-at-a-time.h> to work on all architectures,
> > though of course individual maintainers can still make their versions
> > more efficient as needed.
> >
> > It's likely not necessary for per-architecture implementations of
> > strscpy() to be written, but I stuck with the standard __HAVE_ARCH_XXX
> > model just for consistency with the rest of <linux/string.h>.
> >
> > I tested the implementation with a simple user-space harness, so I
> > believe it is correct for the corner cases I could think of.  In
> > particular I pairwise-tested all the unaligned values of source and
> > dest, and tested the restriction on src page-crossing at all
> > unaligned offsets approaching the page boundary.
> Can you please put that in tools/testing/selftests and merge it as part of the
> series? That way I can run the tests and be confident it works on powerpc.

Unfortunately, the strscpy patch series only changes the one previous
user of the API, which is a tile-architecture-only driver piece, not
particularly useful for anyone else for testing.

The testing I did pulled strscpy() and word-at-a-time out into a
separate, standalone userspace implementation, and tested it there,
rather than doing tests through the syscall API like 
tools/testing/selftests.
So I don't really see a way of committing my test framework, other
than as a real Kconfig-enabled boot-time self-test or some such;
I can certainly do that but I don't know how excited people are to
have that additional level of source-code and Kconfig bloat.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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