Re: [PATCH V2 1/2] string: Add stracpy and stracpy_pad mechanisms

[Joe Perches <joe@perches.com>]

On Wed, 2019-09-25 at 14:50 -0700, Andrew Morton wrote:
> On Tue, 23 Jul 2019 06:51:36 -0700 Joe Perches <joe@perches.com> wrote:
> 
> > Several uses of strlcpy and strscpy have had defects because the
> > last argument of each function is misused or typoed.
> > 
> > Add macro mechanisms to avoid this defect.
> > 
> > stracpy (copy a string to a string array) must have a string
> > array as the first argument (dest) and uses sizeof(dest) as the
> > count of bytes to copy.
> > 
> > These mechanisms verify that the dest argument is an array of
> > char or other compatible types like u8 or s8 or equivalent.
> > 
> > A BUILD_BUG is emitted when the type of dest is not compatible.
> > 
> 
> I'm still reluctant to merge this because we don't have code in -next
> which *uses* it.  You did have a patch for that against v1, I believe? 
> Please dust it off and send it along?

https://lore.kernel.org/lkml/CAHk-=wgqQKoAnhmhGE-2PBFt7oQs9LLAATKbYa573UO=DPBE0Q@mail.gmail.com/

I gave up, especially after the snark from Linus
where he wrote I don't understand this stuff.

He's just too full of himself here merely using
argument from authority.

Creating and using a function like copy_string with
both source and destination lengths specified is
is also potentially a large source of defects where
the stracpy macro atop strscpy does not have a
defect path other than the src not being a string
at all.

I think the analysis of defects in string function
in the kernel is overly difficult today given the
number of possible uses of pointer and length in
strcpy/strncpy/strlcpy/stracpy.

I think also that there is some sense in what he
wrote against the "word salad" use of str<foo>cpy,
but using stracpy as a macro when possible instead
of strscpy also makes the analysis of defects rather
simpler.

The trivial script cocci I posted works well for the
simple cases.

https://lore.kernel.org/cocci/66fcdbf607d7d0bea41edb39e5579d63b62b7d84.camel@perches.com/

The more complicated cocci script Julia posted is
still not quite correct as it required intermediate
compilation for verification of specified lengths.

https://lkml.org/lkml/2019/7/25/1406

Tell me again if you still want it and maybe the
couple conversions that mm/ would get.

via:

$ spatch --all-includes --in-place -sp-file str.cpy.cocci mm
$ git diff --stat -p mm
--
 mm/dmapool.c | 2 +-
 mm/zswap.c   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/dmapool.c b/mm/dmapool.c
index fe5d33060415..b3a4feb423f8 100644
--- a/mm/dmapool.c
+++ b/mm/dmapool.c
@@ -158,7 +158,7 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
 	if (!retval)
 		return retval;
 
-	strlcpy(retval->name, name, sizeof(retval->name));
+	stracpy(retval->name, name);
 
 	retval->dev = dev;
 
diff --git a/mm/zswap.c b/mm/zswap.c
index 08b6cefae5d8..c6cd38de185a 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -533,7 +533,7 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 	}
 	pr_debug("using %s zpool\n", zpool_get_type(pool->zpool));
 
-	strlcpy(pool->tfm_name, compressor, sizeof(pool->tfm_name));
+	stracpy(pool->tfm_name, compressor);
 	pool->tfm = alloc_percpu(struct crypto_comp *);
 	if (!pool->tfm) {
 		pr_err("percpu alloc failed\n");