> #include <exception>
>
> int main(void)
> {
> try { throw std::exception(); }
> catch (std::exception& e) {} // same result without &
> return 0;
> }
compiled under devel/powerpc64-gcc (gcc 5.3 based) on a FreeBSD
projects/clang380-import (-r295902, for example) that was also built
(buildworld/buildkernel) with the same devel/powerpc64-gcc demonstrates:
_Unwind_RaiseException never returns because it is stuck in a unbounded loop.
[clang380-import vs. 11.0-CURRENT is not likely to be a big distinction here.
PowerPC64 might be or might not be relative to g++ 5.3 .]
The cause has been tracked down to incomplete (and so inaccurate) .eh_frame
information as shown below.
I use a mix of . . .
# /usr/local/bin/objdump -d --prefix-addresses libcxxrt.so.1.full
output and the matching . . .
# dwarfdump -v -v -F libcxxrt.so.1.full | more
output to show this.
0000000000015350 <.__cxa_end_catch+0x490> addi r3,r31,88
0000000000015354 <.__cxa_end_catch+0x494> addi r10,r10,1
0000000000015358 <.__cxa_end_catch+0x498> stw r10,48(r9)
000000000001535c <.__cxa_end_catch+0x49c> bl 0000000000009ae0
<00000017.plt_call._Unwind_RaiseException@@GCC_3.0>
0000000000015360 <.__cxa_end_catch+0x4a0> ld r2,40(r1)
0000000000015364 <.__cxa_end_catch+0x4a4> addi r1,r1,128
0000000000015368 <.__cxa_end_catch+0x4a8> mr r4,r31
vs.
< 0><0x00015050:0x00015310><report_failure><fde offset 0x000010a8 length:
0x0000002c><eh aug data len 0x0>
0x00015050: <off cfa=00(r1) >
0x0001506c: <off cfa=176(r1) > <off r28=-32(cfa) > <off r29=-24(cfa) >
<off r30=-16(cfa) > <off r31=-8(cfa) > <off r65=16(cfa) >
0x000150b8: <off cfa=00(r1) > <off r28=-32(cfa) > <off r29=-24(cfa) >
<off r30=-16(cfa) > <off r31=-8(cfa) > <off r65=16(cfa) >
0x000150d0: <off cfa=00(r1) >
0x000150e0: <off cfa=176(r1) > <off r28=-32(cfa) > <off r29=-24(cfa) >
<off r30=-16(cfa) > <off r31=-8(cfa) > <off r65=16(cfa) >
Of note here is that the return "address" from the bl context is correctly
identified as 0000000000015360 by the .eh_frame information and its
interpretation.
But also of note there is that 0000000000015360 and 0000000000015364 also are
identified as having 0000000000015360 as their return address (part of the same
block of code as the bl as things are classified above). (That is what the live
code actually generates, up to relocation issues changing addresses.)
So when _Unwind_RaiseException attempts to walk up the stack from itself for
the first pass up the stack it gets to 0000000000015360.
And from there it gets to 0000000000015360 and loops.
And from there it gets to 0000000000015360 and loops.
And from there it gets to 0000000000015360 and loops.
And from there it gets to 0000000000015360 and loops.
. . .
This code/.eh_frame pattern occurs again a little later (and this is where the
original problem was observed for this low-level-detail view). . .
0000000000015380 <.__cxa_end_catch+0x4c0> addis r8,r2,-1
0000000000015384 <.__cxa_end_catch+0x4c4> addi r3,r31,88
0000000000015388 <.__cxa_end_catch+0x4c8> ld r10,28144(r8)
000000000001538c <.__cxa_end_catch+0x4cc> std r10,32(r31)
0000000000015390 <.__cxa_end_catch+0x4d0> lwz r10,48(r9)
0000000000015394 <.__cxa_end_catch+0x4d4> addi r10,r10,1
0000000000015398 <.__cxa_end_catch+0x4d8> stw r10,48(r9)
000000000001539c <.__cxa_end_catch+0x4dc> bl 0000000000009ae0
<00000017.plt_call._Unwind_RaiseException@@GCC_3.0>
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)
vs.
< 0><0x00015310:0x000153dc><throw_exception><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) >
This leads to (during _Unwind_RaiseException's loop):
. . .
And from there it gets to 00000000000153a0 and loops.
And from there it gets to 00000000000153a0 and loops.
And from there it gets to 00000000000153a0 and loops.
And from there it gets to 00000000000153a0 and loops.
. . .
0000000000015360 and 00000000000153a0 should each be starts of new code-block
ranges for identifying a different return address.
So C++ exception handling is broken for FreeBSD when buildworld is based on
devel/powerpc65-gcc: libcxxrt has examples of the error in it.
I'd expect that gcc5 (5.3) has other contexts where it does not get the correct
return address via its .eh_frame generated materials, at least for powerpc64.
[207359 might be considered replaced by 207471.]
===
Mark Millard
markmi at dsl-only.net
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