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. > > The convention with xattrs is to not store the termination with string > > data, given that we have the length. This is how setfattr/getfattr > > operate. > > Fine. > > > This patch makes ceph's virtual xattrs not include NULL termination > > when formatting their values. In order to handle this, a new > > snprintf_noterm function is added, and ceph is changed over to use > > this to populate the xattr value buffer. > > In terms of vsnprintf(), and actually compiler point of view, it's not a string > anymore, it's a text-based data. > > Personally, I don't see an advantage of a deep intrusion into vsnprintf(). > The wrapper can be made to achieve this w/o touching the generic code. Thus, > you can quickly and cleanly fix the issue, while discussing this with wider > audience. > Sorry, if I'm being dense but I'm not sure I follow here. Are you suggesting I should just copy/paste most of vsnprintf into a new function that just leaves off the termination at the end, and leave the original alone? That seems like a bit of a waste, but if that's the consensus then ok. > > Finally, we fix ceph to > > return -ERANGE properly when the string didn't fit in the buffer. > > > > Jeff Layton (3): > > lib/vsprintf: add snprintf_noterm > > ceph: don't NULL terminate virtual xattr strings > > ceph: return -ERANGE if virtual xattr value didn't fit in buffer > > > > fs/ceph/xattr.c | 49 +++++++------- > > include/linux/kernel.h | 2 + > > lib/vsprintf.c | 144 ++++++++++++++++++++++++++++------------- > > 3 files changed, 129 insertions(+), 66 deletions(-) > > > > -- > > 2.21.0 > > -- Jeff Layton <jlayton@xxxxxxxxxx>
