[In part: My references to libc++ should have been to libcxxrt. Also: devel/powerpc64-gcc produced the incomplete .eh_frame information.]
On 2016-Feb-24, at 5:07 PM, Mark Millard <mar...@dsl-only.net> wrote: > > [Deliberate top posting an history removal for significant new information.] > > I've finally traced the low level details of the powerpc64 > _Unwind_RaiseException stuck looping failure. What I've submitted into the > defect is basically that for clang 3.8.0's .eh_frame information generation > in libcxxrt's code there is an error in the form of an incompleteness. . . > > > Starting from the observed low-level evidence based on observation via gdb > and such: > > A backtrace while stopped during the unbounded looping is: > >> #0 uw_update_context (context=context@entry=0xffffffffffffccf0, >> fs=fs@entry=0xffffffffffffc370) at >> /usr/src/gnu/lib/libgcc/../../../contrib/gcc/unwind-dw2.c:1371 >> #1 0x00000000501cb95c in _Unwind_RaiseException (exc=0x50815058) at >> /usr/src/gnu/lib/libgcc/../../../contrib/gcc/unwind.inc:126 >> #2 0x000000005016e3a0 in throw_exception (ex=0x50815000) at >> /usr/src/lib/libcxxrt/../../contrib/libcxxrt/exception.cc:751 >> #3 0x0000000010000d50 in main () at exception_test.cpp:5 > > _Unwind_RaiseException never returns for source code that looks like: > >> #include <exception> >> >> int main(void) >> { >> try { throw std::exception(); } >> catch (std::exception& e) {} >> return 0; >> } > > where in /usr/lib/libc++.so.1 there is: > >> 736 static void throw_exception(__cxa_exception *ex) >> 737 { >> . . . >> 751 _Unwind_Reason_Code err = >> _Unwind_RaiseException(&ex->unwindHeader); >> . . . >> 756 } > > The unbounded loop in _Unwind_RaiseException is in the code: > >> 85 _Unwind_Reason_Code >> 86 _Unwind_RaiseException(struct _Unwind_Exception *exc) >> 87 { >> 88 struct _Unwind_Context this_context, cur_context; >> 89 _Unwind_Reason_Code code; >> 90 >> 91 /* Set up this_context to describe the current stack frame. */ >> 92 uw_init_context (&this_context); >> 93 cur_context = this_context; >> 94 >> 95 /* Phase 1: Search. Unwind the stack, calling the personality routine >> 96 with the _UA_SEARCH_PHASE flag set. Do not modify the stack yet. >> */ >> 97 while (1) >> 98 { >> 99 _Unwind_FrameState fs; >> 100 >> 101 /* Set up fs to describe the FDE for the caller of cur_context. >> The >> 102 first time through the loop, that means __cxa_throw. */ >> 103 code = uw_frame_state_for (&cur_context, &fs); >> . . . >> 125 /* Update cur_context to describe the same frame as fs. */ >> 126 uw_update_context (&cur_context, &fs); >> 127 } >> . . . >> 140 } > > The uw_update_context call is doing the following before returning: > >> 1367 /* Compute the return address now, since the return address column >> 1368 can change from frame to frame. */ >> 1369 context->ra = __builtin_extract_return_addr >> 1370 (_Unwind_GetPtr (context, fs->retaddr_column)); > > with context->ra before and after the call both being 0x5016e3a0 . In fact it > was 0x5016e3a0 for the prior uw_frame_state_for call as well and continues to > be so each loop iteration once the problem starts. > > As for the code around 0x5016e3a0: > >> 0x5016e398 <throw_exception(__cxxabiv1::__cxa_exception*)+136>: stw >> r10,48(r9) >> 0x5016e39c <throw_exception(__cxxabiv1::__cxa_exception*)+140>: bl >> 0x50162ae0 <00000017.plt_call._Unwind_RaiseException@@GCC_3.0> >> 0x5016e3a0 <throw_exception(__cxxabiv1::__cxa_exception*)+144>: ld >> r2,40(r1) >> 0x5016e3a4 <throw_exception(__cxxabiv1::__cxa_exception*)+148>: addi >> r1,r1,128 >> 0x5016e3a8 <throw_exception(__cxxabiv1::__cxa_exception*)+152>: mr >> r4,r31 >> 0x5016e3ac <throw_exception(__cxxabiv1::__cxa_exception*)+156>: ld >> r0,16(r1) > > From /usr/local/bin/objdump for FreeBSD projects/clang380-import -r295902's > /usr/lib/libc++.so.1 for the same code (to match up with the .eh_frame dwarf > information): > >> 0000000000015398 <.__cxa_end_catch+0x4d8> stw r10,48(r9) >> 000000000001539c <.__cxa_end_catch+0x4dc> bl 0000000000009ae0 >> <CXXABI_1.3+0x9ae0> >> 00000000000153a0 <.__cxa_end_catch+0x4e0> ld r2,40(r1) >> 00000000000153a4 <.__cxa_end_catch+0x4e4> addi r1,r1,128 >> 00000000000153a8 <.__cxa_end_catch+0x4e8> mr r4,r31 >> 00000000000153ac <.__cxa_end_catch+0x4ec> ld r0,16(r1) > > The code block above from 153a0 up to 153a8 is being given 153a0 as its > "return address" (context->ra) by uw_update_context via interpreting the > dwarf .eh_frame information. So once in that range there it never leaves that > range. > > The relevant dwarfdump output spanning that area is: > (Note that 153a0 up to 153a8 is part of the range that includes the bl to > _Unwind_RaiseException .) > >> < 0><0x00015310:0x000153dc><><fde offset 0x000010d8 length: >> 0x00000034><eh aug data len 0x0> >> 0x00015310: <off cfa=00(r1) > >> 0x00015318: <off cfa=00(r1) > <off r31=-8(cfa) > <off r65=r0 > >> 0x00015324: <off cfa=128(r1) > <off r31=-8(cfa) > <off r65=16(cfa) > >> 0x00015368: <off cfa=00(r1) > <off r31=-8(cfa) > <off r65=16(cfa) > >> 0x00015378: <off cfa=00(r1) > >> 0x00015380: <off cfa=128(r1) > <off r31=-8(cfa) > <off r65=16(cfa) > >> 0x000153a8: <off cfa=00(r1) > <off r31=-8(cfa) > <off r65=16(cfa) > >> 0x000153b8: <off cfa=00(r1) > >> 0x000153c0: <off cfa=128(r1) > <off r31=-8(cfa) > <off r65=16(cfa) > >> fde section offset 4312 0x000010d8 cie offset for fde: 4316 0x000010dc >> 0 DW_CFA_advance_loc 8 (2 * 4) >> 1 DW_CFA_register r65 = r0 >> 4 DW_CFA_offset r31 -8 (1 * -8) >> 6 DW_CFA_advance_loc 12 (3 * 4) >> 7 DW_CFA_def_cfa_offset 128 >> 10 DW_CFA_offset_extended_sf r65 16 (-2 * -8) >> 13 DW_CFA_advance_loc 68 (17 * 4) >> 14 DW_CFA_remember_state >> 15 DW_CFA_def_cfa_offset 0 >> 17 DW_CFA_advance_loc 16 (4 * 4) >> 18 DW_CFA_restore_extended r65 >> 20 DW_CFA_restore r31 >> 21 DW_CFA_advance_loc 8 (2 * 4) >> 22 DW_CFA_restore_state >> 23 DW_CFA_advance_loc 40 (10 * 4) >> 24 DW_CFA_remember_state >> 25 DW_CFA_def_cfa_offset 0 >> 27 DW_CFA_advance_loc 16 (4 * 4) >> 28 DW_CFA_restore_extended r65 >> 30 DW_CFA_restore r31 >> 31 DW_CFA_advance_loc 8 (2 * 4) >> 32 DW_CFA_restore_state >> 33 DW_CFA_nop >> 34 DW_CFA_nop >> 35 DW_CFA_nop >> 36 DW_CFA_nop >> 37 DW_CFA_nop >> 38 DW_CFA_nop > > Note that the stack pointer manipulation (incrementing/popping) *after* the > return from the bl. 15339c, 153a0, and 153a4 should be 3 or so distinct > contexts in the .eh_frame information but are not. The code is: > >> 000000000001539c <.__cxa_end_catch+0x4dc> bl 0000000000009ae0 >> <CXXABI_1.3+0x9ae0> >> 00000000000153a0 <.__cxa_end_catch+0x4e0> ld r2,40(r1) >> 00000000000153a4 <.__cxa_end_catch+0x4e4> addi r1,r1,128 > > So there is a missing step or more of frame-context tracking in the .eh_frame > information. I should have noted that devel/powerpc64-gcc was used for buildworld of projects/clang380-import -r295902 (and other versions) --and so for libcxxrt as well. So it is g++ 5.3 that put out the incomplete .eh_frame frame-context tracking, not clang 3.8.0. There is also the point that I typed "libc++" in places that I should have typed "libcxxrt". === Mark Millard markmi at dsl-only.net _______________________________________________ freebsd-toolchain@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-toolchain To unsubscribe, send any mail to "freebsd-toolchain-unsubscr...@freebsd.org"