> Date: Fri, 19 Feb 2021 10:57:30 +0100
> From: Otto Moerbeek <o...@drijf.net>
>
> Hi,
>
> working on PowerDNS Recursor, once in a while I'm seeing:
>
> #0 0x000009fd67ef09dc in
> libunwind::UnwindInfoSectionsCache::CacheTree_RB_INSERT_COLOR
> (this=<optimized out>,
> head=0x9fd67efc8e8 <libunwind::uwis_cache+8>, elm=0x9fca04be900)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/AddressSpace.hpp:243
> 243 RB_GENERATE(CacheTree, CacheItem, entry, CacheCmp);
> [Current thread is 1 (process 349420)]
> (gdb) bt
> #0 0x000009fd67ef09dc in
> libunwind::UnwindInfoSectionsCache::CacheTree_RB_INSERT_COLOR
> (this=<optimized out>,
> head=0x9fd67efc8e8 <libunwind::uwis_cache+8>, elm=0x9fca04be900)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/AddressSpace.hpp:243
> #1 0x000009fd67eeddef in
> libunwind::UnwindInfoSectionsCache::CacheTree_RB_INSERT
> (this=<optimized out>,
> head=<optimized out>, elm=<optimized out>)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/AddressSpace.hpp:243
> #2 libunwind::UnwindInfoSectionsCache::setUnwindInfoSectionsForPC
> (this=<optimized out>, key=10983975073074,
> uis=...) at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/AddressSpace.hpp:237
> #3 libunwind::UnwindCursor<libunwind::LocalAddressSpace,
> libunwind::Registers_x86_64>::setInfoBasedOnIPRegister (
> this=0x9fd2ca0aa68, isReturnAddress=<optimized out>)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/UnwindCursor.hpp:1891
> #4 0x000009fd67eedaa4 in
> libunwind::UnwindCursor<libunwind::LocalAddressSpace,
> libunwind::Registers_x86_64>::step (
> this=0x9fd2ca0aa68) at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/UnwindCursor.hpp:2031
> #5 0x000009fd67ef15a4 in unwind_phase1 (uc=<optimized out>,
> cursor=<optimized out>, exception_object=0x9fd37b24560)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/UnwindLevel1.c:46
> #6 _Unwind_RaiseException (exception_object=0x9fd37b24560)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libunwind/src/UnwindLevel1.c:363
> #7 0x000009fd67eeb533 in __cxa_throw (thrown_object=0x9fd37b24580,
> tinfo=0x9fa6c615a00 <typeinfo for PDNSException>, dest=<optimized out>)
> at
> /usr/src/gnu/lib/libcxxabi/../../../gnu/llvm/libcxxabi/src/cxa_exception.cpp:279
> #8 0x000009fa6c295955 in ComboAddress::ComboAddress (this=<optimized
> out>, str=..., port=<optimized out>)
> at ./iputils.hh:219
> #9 0x000009fa6c489970 in startFrameStreamServers (config=...) at
> pdns_recursor.cc:1248
> #10 checkFrameStreamExport (luaconfsLocal=...) at pdns_recursor.cc:1290
> #11 0x000009fa6c48158f in recursorThread (n=<optimized out>,
> ...
>
> This does not happen always, most of the time this exception is
> handled correctly, afaik.
>
> The code that twrows an exception is:
> try {
> ComboAddress address(server);
> ...
> }
> catch ...
>
> The ComboAddress constructor throws the exception (and is supposed to
> do that). It looks like libunwind gets confused somehow.
>
> Any clue?
The cache that pirofti@ added a while ago isn't thread-safe. Or maybe
this is a use-after free caused by dlcose(4). We should probably
disable/remove it while he is working on a better solution.
Unfortunately I don't think adding locking here is a good idea so this
may need a more fundamental rethink.
Upstream did add an optimization in this area a few months ago:
https://github.com/llvm/llvm-project/commit/881aba7071c6e4cc2417e875ca5027ec7c0a92a3
The version of libunwind we're using is older than that, so it may be
worth picking that up and see if that improves the original problem.
