Re: [PATCH 5/6] lib: Fix function documentation for strncpy_from_user

From: Kees Cook
Date: Thu Feb 21 2019 - 01:02:55 EST

On Wed, Feb 20, 2019 at 9:25 PM Tobin C. Harding <me@xxxxxxxx> wrote:
> On Wed, Feb 20, 2019 at 05:05:10PM -0800, Kees Cook wrote:
> > So, generally speaking, I'd love to split all strncpy* uses into
> > strscpy_zero() (when expecting to do str->str copies), and some new
> > function, named like mempadstr() or str2mem() that copies a str to a
> > __nonstring char array, with trailing padding, if there is space. Then
> > there is no more mixing the two cases and botching things.

I should use "converts" instead of "copies" above, just to drive the
point home. :)

> Oh cool, treewide changes, I'm down with that. So to v2 I'll add
> str2mem() and then attack the tree as suggested. What could possibly go
> wrong :)?

Some clear documentation needs to be written for str2mem() to really
help people understand what a "non string" character array is
(especially given that it LOOKS like it has NUL termination -- when in
fact it's just "padding").

The tree-wide changes will likely take a while (and don't need to be
part of this series unless you want to find a couple good examples)
since we have to do them case-by-case: it's not always obvious when
it's actually a non-string, so getting help from maintainers here will
be needed. (And maybe some kind of flow chart added to
Documentation/process/deprecated.rst for how to stop using strncpy()
and strlcpy().)

What I can't quite figure out yet is how to find a way for sfr to flag
newly added users of strcpy, strncpy, and strlcpy. We might need to
bring back __deprecated, but hide it behind a W=linux-next flag or
something crazy. Stephen, in your builds you're already injecting
-Wimplicit-fallthrough: do you do W=1 or anything like that? If not, I
think we need some W= setting for your linux-next builds that generate
the maintainer-nag warnings...


P.S. Here's C string API Rant (I just had to get this out, please feel
free to ignore):

strcpy returns dest ... which is already known, so it's a meaningless
return value.

strncpy returns dest (still meaningless)

strlcpy returns strlen(src) ... the length we WOULD have copied. Why
would we care about that? I'm operating on dest. Were there callers
that needed to both copy part of src and learn how long it was at the
same time?

strscpy returns -E2BIG or non-NUL bytes copied: yay, a return about
what actually happened from the operation!

... snprintf returns what it WOULD have written, much like strlcpy
above. At least snprintf has an excuse: it can be used to calculate an
allocation size (called with a NULL dest and 0 size) ... but shouldn't
"how big is this format string going to be?" be a separate function? I
wonder if we can kill all kernel uses of snprintf too (after
introducing a "how big would it be?" function and switching all other
callers over to scnprintf)...

So scnprintf() does the right thing (count of non-NUL bytes copied out).

So now our safe(r?) string API versions use different letters to show
they're safe: "s" in strcpy and "cn" in sprintf. /me cries forever.

Kees Cook