问题
I'm having trouble determining the cause of a segfault when a program is compiled with -O3 with GCC 4.8/4.9/5.1. For GCC 4.9.x, I've seen it on Cygwin, Debian 8 (x64) and Fedora 21 (x64). Others have experienced it on GCC 4.8 and 5.1.
The program is fine under -O2, fine with other versions of GCC, and fine under other compilers (like MSVC, ICC and Clang).
Below is the crash under GDB, but nothing is jumping out at me. The source code from misc.cpp:26 is below, but its a simple XOR:
((word64*)buf)[i] ^= ((word64*)mask)[i];
The code in question checks for 64-bit word alignment prior to the cast. From the disassembly under -O3, I know it has something to do with the vmovdqa instruction:
(gdb) disass 0x0000000000539fc3
...
0x0000000000539fbc <+220>: vxorps 0x0(%r13,%r10,1),%ymm0,%ymm0
=> 0x0000000000539fc3 <+227>: vmovdqa %ymm0,0x0(%r13,%r10,1)
0x0000000000539fca <+234>: add $0x20,%r10
It appears GCC is using SSE vectors at -O3, and not using them at -O2. (Thanks to Alejandro for the suggestion).
I'm going to naively ask: does vmovdqa have alignment requirements greater than 64-bit word? Is so, why is GCC selecting it when the words are not 128-bit aligned?
What is causing the segfault here? How do I troubleshoot it further?
Also see Bug 66852 - vmovdqa instructions issued on 64-bit aligned array, causes segfault. It was filed in response to this issue, so its unconfirmed at the moment.
$ gdb ./cryptest.exe
GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
...
(gdb) r v
...
Testing MessageDigest algorithm SHA-3-224.
.....
Program received signal SIGSEGV, Segmentation fault.
0x0000000000539fc3 in CryptoPP::xorbuf (buf=0x98549a "efghijde",
mask=mask@entry=0x7fffffffbfeb "efghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", 'a' <repeats 106 times>..., count=count@entry=0x5e) at misc.cpp:26
26 ((word64*)buf)[i] ^= ((word64*)mask)[i];
(gdb) where
#0 0x0000000000539fc3 in CryptoPP::xorbuf (buf=0x98549a "efghijde",
mask=mask@entry=0x7fffffffbfeb "efghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", 'a' <repeats 106 times>..., count=count@entry=0x5e) at misc.cpp:26
#1 0x0000000000561eb0 in CryptoPP::SHA3::Update (this=0x985480,
input=0x7fffffffbfeb "efghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", 'a' <repeats 106 times>...,
length=0x5e) at sha3.cpp:264
#2 0x00000000005bac1a in CryptoPP::HashVerificationFilter::NextPutMultiple (
this=0x7fffffffd390,
inString=0x7fffffffbfeb "efghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", 'a' <repeats 106 times>...,
length=0x5e) at filters.cpp:786
#3 0x00000000005bd8a2 in NextPutMaybeModifiable (modifiable=<optimized out>,
length=0x5e,
inString=0x7fffffffbfeb "efghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", 'a' <repeats 106 times>...,
this=0x7fffffffd390) at filters.h:200
#4 CryptoPP::FilterWithBufferedInput::PutMaybeModifiable (
this=0x7fffffffd390,
inString=0x7fffffffbfeb "efghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", 'a' <repeats 106 times>...,
length=<optimized out>, messageEnd=0x0, blocking=<optimized out>,
...
-O3 disassembly and register values.
(gdb) disass 0x0000000000539fc3
Dump of assembler code for function CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long):
0x0000000000539ee0 <+0>: lea 0x8(%rsp),%r10
0x0000000000539ee5 <+5>: and $0xffffffffffffffe0,%rsp
0x0000000000539ee9 <+9>: mov %rdx,%rax
0x0000000000539eec <+12>: pushq -0x8(%r10)
0x0000000000539ef0 <+16>: push %rbp
0x0000000000539ef1 <+17>: shr $0x3,%rax
0x0000000000539ef5 <+21>: mov %rsp,%rbp
0x0000000000539ef8 <+24>: push %r15
0x0000000000539efa <+26>: push %r14
0x0000000000539efc <+28>: push %r13
0x0000000000539efe <+30>: push %r12
0x0000000000539f00 <+32>: push %r10
0x0000000000539f02 <+34>: push %rbx
0x0000000000539f03 <+35>: je 0x53a00a <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+298>
0x0000000000539f09 <+41>: lea 0x20(%rdi),%rcx
0x0000000000539f0d <+45>: cmp %rcx,%rsi
0x0000000000539f10 <+48>: lea 0x20(%rsi),%rcx
0x0000000000539f14 <+52>: setae %r8b
0x0000000000539f18 <+56>: cmp %rcx,%rdi
0x0000000000539f1b <+59>: setae %cl
0x0000000000539f1e <+62>: or %cl,%r8b
0x0000000000539f21 <+65>: je 0x53a300 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+1056>
0x0000000000539f27 <+71>: cmp $0x8,%rax
0x0000000000539f2b <+75>: jbe 0x53a300 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+1056>
0x0000000000539f31 <+81>: mov %rdi,%rcx
0x0000000000539f34 <+84>: and $0x1f,%ecx
0x0000000000539f37 <+87>: shr $0x3,%rcx
0x0000000000539f3b <+91>: neg %rcx
0x0000000000539f3e <+94>: and $0x3,%ecx
0x0000000000539f41 <+97>: cmp %rax,%rcx
0x0000000000539f44 <+100>: cmova %rax,%rcx
0x0000000000539f48 <+104>: xor %r8d,%r8d
0x0000000000539f4b <+107>: test %rcx,%rcx
0x0000000000539f4e <+110>: je 0x539f80 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+160>
0x0000000000539f50 <+112>: mov (%rsi),%r8
0x0000000000539f53 <+115>: xor %r8,(%rdi)
0x0000000000539f56 <+118>: cmp $0x1,%rcx
0x0000000000539f5a <+122>: je 0x53a371 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+1169>
0x0000000000539f60 <+128>: mov 0x8(%rsi),%r8
0x0000000000539f64 <+132>: xor %r8,0x8(%rdi)
0x0000000000539f68 <+136>: cmp $0x3,%rcx
0x0000000000539f6c <+140>: jne 0x53a366 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+1158>
0x0000000000539f72 <+146>: mov 0x10(%rsi),%r8
0x0000000000539f76 <+150>: xor %r8,0x10(%rdi)
0x0000000000539f7a <+154>: mov $0x3,%r8d
0x0000000000539f80 <+160>: mov %rax,%r11
0x0000000000539f83 <+163>: xor %r10d,%r10d
0x0000000000539f86 <+166>: sub %rcx,%r11
0x0000000000539f89 <+169>: shl $0x3,%rcx
0x0000000000539f8d <+173>: xor %ebx,%ebx
0x0000000000539f8f <+175>: lea -0x4(%r11),%r9
0x0000000000539f93 <+179>: lea (%rdi,%rcx,1),%r13
0x0000000000539f97 <+183>: shr $0x2,%r9
0x0000000000539f9b <+187>: add %rsi,%rcx
0x0000000000539f9e <+190>: add $0x1,%r9
0x0000000000539fa2 <+194>: lea 0x0(,%r9,4),%r12
0x0000000000539faa <+202>: add $0x1,%rbx
0x0000000000539fae <+206>: vmovdqu (%rcx,%r10,1),%xmm0
0x0000000000539fb4 <+212>: vinsertf128 $0x1,0x10(%rcx,%r10,1),%ymm0,%ymm0
0x0000000000539fbc <+220>: vxorps 0x0(%r13,%r10,1),%ymm0,%ymm0
=> 0x0000000000539fc3 <+227>: vmovdqa %ymm0,0x0(%r13,%r10,1)
0x0000000000539fca <+234>: add $0x20,%r10
0x0000000000539fce <+238>: cmp %r9,%rbx
0x0000000000539fd1 <+241>: jb 0x539faa <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+202>
0x0000000000539fd3 <+243>: lea (%r8,%r12,1),%rcx
0x0000000000539fd7 <+247>: cmp %r12,%r11
0x0000000000539fda <+250>: je 0x53a006 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+294>
0x0000000000539fdc <+252>: mov (%rsi,%rcx,8),%r8
0x0000000000539fe0 <+256>: xor %r8,(%rdi,%rcx,8)
0x0000000000539fe4 <+260>: lea 0x1(%rcx),%r8
0x0000000000539fe8 <+264>: cmp %r8,%rax
0x0000000000539feb <+267>: jbe 0x53a006 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+294>
0x0000000000539fed <+269>: add $0x2,%rcx
0x0000000000539ff1 <+273>: mov (%rsi,%r8,8),%r9
0x0000000000539ff5 <+277>: xor %r9,(%rdi,%r8,8)
0x0000000000539ff9 <+281>: cmp %rcx,%rax
0x0000000000539ffc <+284>: jbe 0x53a006 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+294>
0x0000000000539ffe <+286>: mov (%rsi,%rcx,8),%r8
0x000000000053a002 <+290>: xor %r8,(%rdi,%rcx,8)
0x000000000053a006 <+294>: shl $0x3,%rax
And:
(gdb) info r ymm0 r13 r10
ymm0 {v8_float = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
v4_double = {0x8000000000000000, 0x8000000000000000, 0x8000000000000000,
0x8000000000000000}, v32_int8 = {0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x65,
0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x66, 0x67, 0x68, 0x69, 0x6a,
0x6b, 0x6c, 0x6d, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x68,
0x69}, v16_int16 = {0x6766, 0x6968, 0x6b6a, 0x6665, 0x6867, 0x6a69,
0x6c6b, 0x6766, 0x6968, 0x6b6a, 0x6d6c, 0x6867, 0x6a69, 0x6c6b, 0x6e6d,
0x6968}, v8_int32 = {0x69686766, 0x66656b6a, 0x6a696867, 0x67666c6b,
0x6b6a6968, 0x68676d6c, 0x6c6b6a69, 0x69686e6d}, v4_int64 = {
0x66656b6a69686766, 0x67666c6b6a696867, 0x68676d6c6b6a6968,
0x69686e6d6c6b6a69}, v2_int128 = {0x67666c6b6a69686766656b6a69686766,
0x69686e6d6c6b6a6968676d6c6b6a6968}}
r13 0x9854a2 0x9854a2
r10 0x0 0x0
When compiled with -O2 and a breakpoint on the line in question, here's the disassembly. ((word64*)buf)[i] ^= ((word64*)mask)[i]; moved to line 31:
Breakpoint 1, CryptoPP::xorbuf (buf=0x985488 "",
mask=mask@entry=0x7fffffffc01d "The quick brown fox", 'a' <repeats 181 times>..., count=count@entry=0x13) at misc.cpp:31
31 ((word64*)buf)[i] ^= ((word64*)mask)[i];
(gdb) disass
Dump of assembler code for function CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long):
0x0000000000532150 <+0>: mov %rdx,%rcx
0x0000000000532153 <+3>: shr $0x3,%rcx
0x0000000000532157 <+7>: je 0x532170 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+32>
0x0000000000532159 <+9>: xor %eax,%eax
=> 0x000000000053215b <+11>: mov (%rsi,%rax,8),%r8
0x000000000053215f <+15>: xor %r8,(%rdi,%rax,8)
0x0000000000532163 <+19>: add $0x1,%rax
0x0000000000532167 <+23>: cmp %rcx,%rax
0x000000000053216a <+26>: jne 0x53215b <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+11>
0x000000000053216c <+28>: shl $0x3,%rcx
0x0000000000532170 <+32>: sub %rcx,%rdx
0x0000000000532173 <+35>: je 0x5321d0 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+128>
0x0000000000532175 <+37>: mov %rdx,%r8
0x0000000000532178 <+40>: add %rcx,%rdi
0x000000000053217b <+43>: add %rcx,%rsi
0x000000000053217e <+46>: shr $0x2,%r8
0x0000000000532182 <+50>: je 0x5321a8 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+88>
0x0000000000532184 <+52>: xor %eax,%eax
0x0000000000532186 <+54>: nopw %cs:0x0(%rax,%rax,1)
0x0000000000532190 <+64>: mov (%rsi,%rax,4),%ecx
0x0000000000532193 <+67>: xor %ecx,(%rdi,%rax,4)
0x0000000000532196 <+70>: add $0x1,%rax
0x000000000053219a <+74>: cmp %r8,%rax
0x000000000053219d <+77>: jne 0x532190 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+64>
0x000000000053219f <+79>: shl $0x2,%r8
0x00000000005321a3 <+83>: sub %r8,%rdx
0x00000000005321a6 <+86>: je 0x5321d8 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+136>
0x00000000005321a8 <+88>: lea (%rdi,%r8,1),%rcx
0x00000000005321ac <+92>: xor %eax,%eax
0x00000000005321ae <+94>: lea (%rsi,%r8,1),%rdi
0x00000000005321b2 <+98>: nopw 0x0(%rax,%rax,1)
0x00000000005321b8 <+104>: movzbl (%rdi,%rax,1),%esi
0x00000000005321bc <+108>: xor %sil,(%rcx,%rax,1)
0x00000000005321c0 <+112>: add $0x1,%rax
0x00000000005321c4 <+116>: cmp %rdx,%rax
0x00000000005321c7 <+119>: jb 0x5321b8 <CryptoPP::xorbuf(unsigned char*, unsigned char const*, unsigned long)+104>
0x00000000005321c9 <+121>: retq
0x00000000005321ca <+122>: nopw 0x0(%rax,%rax,1)
0x00000000005321d0 <+128>: retq
0x00000000005321d1 <+129>: nopl 0x0(%rax)
0x00000000005321d8 <+136>: retq
End of assembler dump.
From misc.cpp, line 26 is ((word64*)buf)[i] ^= ((word64*)mask)[i];.
void xorbuf(byte *buf, const byte *mask, size_t count)
{
size_t i;
if (IsAligned<word32>(buf) && IsAligned<word32>(mask))
{
if (!CRYPTOPP_BOOL_SLOW_WORD64 && IsAligned<word64>(buf) && IsAligned<word64>(mask))
{
for (i=0; i<count/8; i++)
((word64*)buf)[i] ^= ((word64*)mask)[i];
count -= 8*i;
if (!count)
return;
buf += 8*i;
mask += 8*i;
}
for (i=0; i<count/4; i++)
((word32*)buf)[i] ^= ((word32*)mask)[i];
count -= 4*i;
if (!count)
return;
buf += 4*i;
mask += 4*i;
}
for (i=0; i<count; i++)
buf[i] ^= mask[i];
}
回答1:
You could compile with g++ -Wall -Wextra -O3 -g ; you want to enable warnings, because some of them are possibly generated only in GCC passes enabled with -O3; you want to enable debugging info (-g) to use gdb but be aware that debugging info are not always reliable with strong optimizations.
You might have some pointer aliasing issues. Perhaps use (or remove) the restrict keyword.
Be sure to avoid undefined behavior. You might use -fsanitize= options (notably -fsanitize=address and -fsanitize=undefined....) to the g++ compiler (version 5 preferably) Use also valgrind.
BTW, you could use dump options like -fdump-tree-all (warning, they produces hundreds of files!) to understand more the internal behavior of g++; and you might even customize your GCC compiler with MELT.
Also, if looking at the produced assembler, compile with g++ -Wall -S -O3 -fverbose-asm since the -fverbose-asm asks GCC to emit some assembler comments "explaining" (not much, but a tiny bit) the compiled code.
来源：https://stackoverflow.com/questions/31373765/determine-cause-of-segfault-when-using-o3