Date: Wed, 24 Feb 2016 17:07:59 -0800 From: Mark Millard <markmi@dsl-only.net> To: FreeBSD PowerPC ML <freebsd-ppc@freebsd.org>, FreeBSD Toolchain <freebsd-toolchain@freebsd.org> Cc: Roman Divacky <rdivacky@vlakno.cz>, Dimitry Andric <dim@FreeBSD.org> Subject: Re: 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 Message-ID: <B2EA9D1B-DCF0-4C98-AB83-F8C7B75F97BB@dsl-only.net> In-Reply-To: <5B9137BF-0E86-4C4B-A3FB-18F444A850AC@dsl-only.net> References: <5B9137BF-0E86-4C4B-A3FB-18F444A850AC@dsl-only.net>
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[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=3Dcontext@entry=3D0xffffffffffffccf0, =
fs=3Dfs@entry=3D0xffffffffffffc370) at =
/usr/src/gnu/lib/libgcc/../../../contrib/gcc/unwind-dw2.c:1371
> #1 0x00000000501cb95c in _Unwind_RaiseException (exc=3D0x50815058) at =
/usr/src/gnu/lib/libgcc/../../../contrib/gcc/unwind.inc:126
> #2 0x000000005016e3a0 in throw_exception (ex=3D0x50815000) 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>
>=20
> 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 =3D =
_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=09
> 91 /* Set up this_context to describe the current stack frame. =
*/
> 92 uw_init_context (&this_context);
> 93 cur_context =3D this_context;
> 94=09
> 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=09
> 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 =3D 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 =3D __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)
=46rom /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=3D00(r1) >=20
> 0x00015318: <off cfa=3D00(r1) > <off r31=3D-8(cfa) > <off =
r65=3Dr0 >=20
> 0x00015324: <off cfa=3D128(r1) > <off r31=3D-8(cfa) > <off =
r65=3D16(cfa) >=20
> 0x00015368: <off cfa=3D00(r1) > <off r31=3D-8(cfa) > <off =
r65=3D16(cfa) >=20
> 0x00015378: <off cfa=3D00(r1) >=20
> 0x00015380: <off cfa=3D128(r1) > <off r31=3D-8(cfa) > <off =
r65=3D16(cfa) >=20
> 0x000153a8: <off cfa=3D00(r1) > <off r31=3D-8(cfa) > <off =
r65=3D16(cfa) >=20
> 0x000153b8: <off cfa=3D00(r1) >=20
> 0x000153c0: <off cfa=3D128(r1) > <off r31=3D-8(cfa) > <off =
r65=3D16(cfa) >=20
> fde section offset 4312 0x000010d8 cie offset for fde: 4316 =
0x000010dc
> 0 DW_CFA_advance_loc 8 (2 * 4)
> 1 DW_CFA_register r65 =3D 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.
=3D=3D=3D
Mark Millard
markmi at dsl-only.net
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