Program OK outside debugger, SIGILL under debugger when stepping? - c++

Question

I'm trying to debug a program on a BeagleBone Black. Outside the debugger it produces an incorrect result but no SIGILL. It also runs OK under the debugger without a breakpoint. However it produces a SIGILL with a breakpoint set when stepping. The program and library does not use SIGILL-based cpu feature probes. However, I don't know what GDB is doing.
Under the debugger I am seeing:
(gdb) b main
Breakpoint 1 at 0x26f20: file test.cxx, line 22.
(gdb) r
Starting program: /home/cryptopp/test.exe
Breakpoint 1, main (argc=0x1, argv=0xbeffea54) at test.cxx:22
22 byte key[16] = {0};
(gdb) n
23 byte iv[12] = {0};
(gdb)
25 GCM<AES>::Encryption enc;
(gdb)
26 enc.SetKeyWithIV(key, 16, iv, 12);
(gdb)
28 std::string plain(0x00, 16);
(gdb)
Program received signal SIGILL, Illegal instruction.
0x00026d5c in std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
()
(gdb) n
Single stepping until exit from function _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_,
which has no line number information.
Program terminated with signal SIGILL, Illegal instruction.
The program no longer exists.
And:
(gdb) shell echo _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_ | c++filt
std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
I tried searching for this issue, but I have not been able to locate a hit. I'm getting too much noise.
Why am I experiencing a SIGILL when GDB sets a breakpoint, and how do I work around it?
NEON is the problem I am trying to investigate. Here's the command line used for the program and library:
$ echo $CXXFLAGS
-DDEBUG -g3 -O0 -march=armv7-a -mtune=cortex-a8 -mfpu=neon -mfloat-abi=hard
$ g++ $CXXFLAGS test.cxx ./libcryptopp.a -o test.exe
And:
$ gdb --version
GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
$ uname -a
Linux beaglebone 4.1.15-ti-rt-r40 #1 SMP PREEMPT RT Thu Jan 7 23:32:08 UTC 2016 armv7l GNU/Linux
$ cat /proc/cpuinfo
processor : 0
model name : ARMv7 Processor rev 2 (v7l)
BogoMIPS : 996.14
Features : half thumb fastmult vfp edsp thumbee neon vfpv3 tls vfpd32
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x3
CPU part : 0xc08
CPU revision : 2
Hardware : Generic AM33XX (Flattened Device Tree)
Revision : 0000
Serial : 0000000000000000
And:
Breakpoint 1, main (argc=0x1, argv=0xbeffea54) at test.cxx:22
22 byte key[16] = {0};
(gdb) n
23 byte iv[12] = {0};
(gdb)
25 GCM<AES>::Encryption enc;
(gdb)
26 enc.SetKeyWithIV(key, 16, iv, 12);
(gdb)
28 std::string plain(0x00, 16);
(gdb)
Program received signal SIGILL, Illegal instruction.
0x00026d5c in std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
()
(gdb) up
#1 0x00026f82 in main (argc=0x1, argv=0xbeffea54) at test.cxx:28
28 std::string plain(0x00, 16);
(gdb) disass
Dump of assembler code for function main(int, char**):
0x00026f10 <+0>: push {r4, r7, lr}
0x00026f12 <+2>: sub.w sp, sp, #916 ; 0x394
0x00026f16 <+6>: add r7, sp, #16
0x00026f18 <+8>: adds r3, r7, #4
0x00026f1a <+10>: str r0, [r3, #0]
0x00026f1c <+12>: mov r3, r7
0x00026f1e <+14>: str r1, [r3, #0]
0x00026f20 <+16>: add.w r3, r7, #692 ; 0x2b4
0x00026f24 <+20>: movs r2, #0
0x00026f26 <+22>: str r2, [r3, #0]
0x00026f28 <+24>: adds r3, #4
0x00026f2a <+26>: movs r2, #0
0x00026f2c <+28>: str r2, [r3, #0]
0x00026f2e <+30>: adds r3, #4
0x00026f30 <+32>: movs r2, #0
0x00026f32 <+34>: str r2, [r3, #0]
0x00026f34 <+36>: adds r3, #4
0x00026f36 <+38>: movs r2, #0
0x00026f38 <+40>: str r2, [r3, #0]
0x00026f3a <+42>: adds r3, #4
0x00026f3c <+44>: add.w r3, r7, #680 ; 0x2a8
0x00026f40 <+48>: movs r2, #0
---Type <return> to continue, or q <return> to quit---
0x00026f42 <+50>: str r2, [r3, #0]
0x00026f44 <+52>: adds r3, #4
0x00026f46 <+54>: movs r2, #0
0x00026f48 <+56>: str r2, [r3, #0]
0x00026f4a <+58>: adds r3, #4
0x00026f4c <+60>: movs r2, #0
0x00026f4e <+62>: str r2, [r3, #0]
0x00026f50 <+64>: adds r3, #4
0x00026f52 <+66>: add.w r3, r7, #240 ; 0xf0
0x00026f56 <+70>: mov r0, r3
0x00026f58 <+72>: bl 0x2a804 <CryptoPP::GCM_Final<CryptoPP::Rijndael, (CryptoPP::GCM_TablesOption)0, true>::GCM_Final()>
0x00026f5c <+76>: add.w r1, r7, #240 ; 0xf0
0x00026f60 <+80>: add.w r2, r7, #692 ; 0x2b4
0x00026f64 <+84>: add.w r4, r7, #680 ; 0x2a8
0x00026f68 <+88>: movs r3, #12
0x00026f6a <+90>: str r3, [sp, #0]
0x00026f6c <+92>: mov r0, r1
0x00026f6e <+94>: mov r1, r2
0x00026f70 <+96>: movs r2, #16
0x00026f72 <+98>: mov r3, r4
0x00026f74 <+100>: bl 0x2da0c <CryptoPP::SimpleKeyingInterface::SetKeyWithIV(unsigned char const*, unsigned int, unsigned char const*, unsigned int)>
---Type <return> to continue, or q <return> to quit---
0x00026f78 <+104>: add.w r3, r7, #708 ; 0x2c4
0x00026f7c <+108>: mov r0, r3
0x00026f7e <+110>: blx 0x26d58 <_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_+852>
=> 0x00026f82 <+114>: add.w r2, r7, #676 ; 0x2a4
0x00026f86 <+118>: add.w r3, r7, #708 ; 0x2c4
0x00026f8a <+122>: mov r0, r2
0x00026f8c <+124>: movs r1, #0
0x00026f8e <+126>: movs r2, #16
...

Answer 1

Thanks to #ks1322, this is a known GDB/Kernel bug. See GDB crashes on debugging multithreaded program on ARM SMP dual core system in the GDB issue tracker.
According to the Debian BTS, this is also a known issue. See SIGILL when stepping through application on armhf in the Debian BTS.
The bug was refilled in hopes that it might actually be fixed sometime in the next year or two. See GDB Crash due to GDB/Kernel generated SIGILL
This is why I despise Debian's bug reporting systems. Stuff gets reported and then it just rots. Nothing gets fixed.