Re: [PATCH v2 0/3] ceph: don't NULL terminate virtual xattr values

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2019-06-20 at 14:22 +0200, Geert Uytterhoeven wrote:
> Hi Jeff,
> 
> On Thu, Jun 20, 2019 at 1:41 PM Jeff Layton <jlayton@kernel.org> wrote:
> > On Thu, 2019-06-20 at 13:24 +0300, Andy Shevchenko wrote:
> > > On Wed, Jun 19, 2019 at 12:45:25PM -0400, Jeff Layton wrote:
> > > > v2: drop bogus EXPORT_SYMBOL of static function
> > > > 
> > > > The only real difference between this set and the one I sent originally
> > > > is the removal of a spurious EXPORT_SYMBOL in the snprintf patch.
> > > > 
> > > > I'm mostly sending this with a wider cc list in an effort to get a
> > > > review from the maintainers of the printf code. Basically ceph needs a
> > > > snprintf variant that does not NULL terminate in order to handle its
> > > > virtual xattrs.
> > > > 
> > > > Joe Perches had expressed some concerns about stack usage in vsnprintf
> > > > with this, but I'm not sure I really understand the basis of that
> > > > concern. If it is problematic, then I could use suggestions as to how
> > > > best to fix that up.
> > > 
> > > It might be problematic, since vsnprintf() can be called recursively.
> > > 
> > 
> > So the concern is that we'd have extra call/ret activity in the stack?
> > That seems like a lot of hand-wringing over very little, but ok if so.
> > 
> > > > ----------------------------8<-----------------------------
> > > > 
> > > > kcephfs has several "virtual" xattrs that return strings that are
> > > > currently populated using snprintf(), which always NULL terminates the
> > > > string.
> > > > 
> > > > This leads to the string being truncated when we use a buffer length
> > > > acquired by calling getxattr with a 0 size first. The last character
> > > > of the string ends up being clobbered by the termination.
> > > 
> > > So, then don't use snprintf() for this, simple memcpy() designed for that kind
> > > of things.
> > > 
> > 
> > memcpy from what? For many of these xattrs, we need to format integer
> > data into strings. I could roll my own routine to do this formatting,
> > but that's sort of what sprintf and its variants are for and I'd rather
> > not reimplement all of it from scratch.
> 
> snprintf() to a temporary buffer, and memcpy() to the final destination.
> These are all fairly small buffers (most are single integer values),
> so the overhead should be minimal, right?
> 

Yeah. I was trying to avoid having to deal with a second buffer, but
this is not a performance-critical codepath, so maybe that's the best
option.

> In fact the two largest strings are already formatted in a temporary
> buffer, so there is no reason ceph_vxattrcb_layout() cannot just use
> snprintf() now.
>
> Or perhaps this can use the existing seq_*() interface in some form?
> 

Hmm. I'll have to think about that.

> BTW, while at it, please get rid of the casts when calling snprintf(), and
> use the correct format specifiers instead.
> 

Sure, will do.

Thanks for the suggestions so far (and from Andy too),
-- 
Jeff Layton <jlayton@kernel.org>