I have seen cases of null vptr due to an incompletely constructed object, i.e., an object that's in the middle of being constructed. Allen Samuels SanDisk |a Western Digital brand 951 SanDisk Drive, Milpitas, CA 95035 T: +1 408 801 7030| M: +1 408 780 6416 allen.samuels@xxxxxxxxxxx > -----Original Message----- > From: ceph-devel-owner@xxxxxxxxxxxxxxx [mailto:ceph-devel- > owner@xxxxxxxxxxxxxxx] On Behalf Of Willem Jan Withagen > Sent: Thursday, December 22, 2016 9:41 AM > To: Ceph Development <ceph-devel@xxxxxxxxxxxxxxx> > Subject: An empty vptr in an raw object > > > I have a piece of code that actually seem to crash because the vptr is > > not set: > > (gdb) p *_raw > > $2 = {_vptr$raw = 0x0, data = 0x10cc000 "\003", len = 72, nref = {val = > > 1}, crc_spinlock = 0, crc_map = {__tree_ = { > > __begin_node_ = 0x10cc078, > > __pair1_ = > > > {<std::__1::__libcpp_compressed_pair_imp<std::__1::__tree_end_node<st > d::__1::__tree_node_base<void*>*>, > > > std::__1::allocator<std::__1::__tree_node<std::__1::__value_type<std::__1 > ::pair<unsigned > > long, unsigned long>, std::__1::pair<unsigned int, unsigned int> >, > > void*> >, 2>> = > > > {<std::__1::allocator<std::__1::__tree_node<std::__1::__value_type<std::_ > _1::pair<unsigned > > long, unsigned long>, std::__1::pair<unsigned int, unsigned int> >, > > void*> >> = {<No data fields>}, __first_ = { > > __left_ = 0x0}}, <No data fields>}, > > > > The function that crashes: > > char *buffer::ptr::c_str() { > > assert(_raw); > > if (buffer_track_c_str) > > buffer_c_str_accesses.inc(); > > char *p = _raw->get_data(); > > return p + _off; > > } > > > > And crash is actually on the return line. > > > > Any ideas as how the vptr can be empty? > > Now the _vptr$raw point is part of the internal code of the clang class > function table/constructor. Overwriting that means that class-function > references are problematic to say the least. (in this example get_data()). > > The major reason why this occurs is because an object is being zeroed in > C-style way: memset( &obj, 0, sizeof(obj)) > And thus overwriting the _vptr. > > Note that this does not bite the FreeBSD compilation, but also any other > attempts to build Ceph with clang. > > Now the strange thing is that this does not bite Clang compilation much > more. But the only test that fails is unittest_denc. So I guess that > most of the code is rather well behaved. > > And I'm off on a search to find the culprit. > > --WjW > > > > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html ��.n��������+%������w��{.n����z��u���ܨ}���Ơz�j:+v�����w����ޙ��&�)ߡ�a����z�ޗ���ݢj��w�f
