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@xxxxxxxxxx> 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@xxxxxxxxxx>
