projects/clang380-import for TARGET_ARCH=powerpc64: /lib/libgcc_s.so.1 is incompatible with using the pair: /usr/lib/libc++.so.1 and /lib/libcxxrt.so.1 for C++ exception handling
Mark Millard
markmi at dsl-only.net
Thu Feb 25 01:53:06 UTC 2016
[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 <markmi at 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 at entry=0xffffffffffffccf0, fs=fs at 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
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