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Manusagents
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hacking

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1.99k
15: 1 0x141e 0x7c96
16: 1 0x24bc 0x7c8a
17: 1 0x623a 0x7d16
18: 0 0x0000 0x0000
19: 0 0x0000 0x0000
20: 0 0x0000 0x0000
21: 0 0x0000 0x0000
22: 0 0x0000 0x0000
23: 0 0x0000 0x0000
24: 0 0x0000 0x0000
25: 0 0x0000 0x0000
26: 0 0x0000 0x0000
27: 0 0x0000 0x0000
28: 0 0x0000 0x0000
29: 0 0x0000 0x0000
30: 1 0x3de8 0x7dc8
31: 1 0x58ba 0x017a
--[ 4. Miscellaneous
----[ 4.1 x86 SSE/AVX Instruction Set
When examining the strcmp() function on Linux x86_64 systems, we find it
uses __strcmp_avx2(), a version optimized with AVX2 instructions, as seen
in the disassembly output below.
(gdb) disassemble
Dump of assembler code for function __strcmp_avx2:
=> 0x00007ffff7f30ae0 <+0>: endbr64
0x00007ffff7f30ae4 <+4>: mov %edi,%eax
0x00007ffff7f30ae6 <+6>: xor %edx,%edx
0x00007ffff7f30ae8 <+8>: vpxor %ymm7,%ymm7,%ymm7
0x00007ffff7f30aec <+12>: or %esi,%eax
0x00007ffff7f30aee <+14>: and $0xfff,%eax
0x00007ffff7f30af3 <+19>: cmp $0xf80,%eax
0x00007ffff7f30af8 <+24>: jg 0x7ffff7f30e50 <__strcmp_avx2+880>
0x00007ffff7f30afe <+30>: vmovdqu (%rdi),%ymm1
0x00007ffff7f30b02 <+34>: vpcmpeqb (%rsi),%ymm1,%ymm0
0x00007ffff7f30b06 <+38>: vpminub %ymm1,%ymm0,%ymm0
0x00007ffff7f30b0a <+42>: vpcmpeqb %ymm7,%ymm0,%ymm0
0x00007ffff7f30b0e <+46>: vpmovmskb %ymm0,%ecx
0x00007ffff7f30b12 <+50>: test %ecx,%ecx
0x00007ffff7f30b14 <+52>: je 0x7ffff7f30b90 <__strcmp_avx2+176>
...
AVX (Advanced Vector Extensions) is a feature in modern Intel and AMD
processors that speeds up computations by processing multiple data elements
at once. It uses special 256-bit registers (YMM) to perform SIMD (Single
Instruction, Multiple Data) operations, making tasks like multimedia
processing and scientific calculations much faster.
In the GNU C Library (glibc), functions like strcmp() have multiple
optimized variants, each designed to take advantage of specific CPU
instruction sets, as illustrated in the code snippet below:
/* Support sysdeps/x86_64/multiarch/strcmp.c. */
IFUNC_IMPL (i, name, strcmp,
IFUNC_IMPL_ADD (array, i, strcmp,
HAS_ARCH_FEATURE (AVX2_Usable),
__strcmp_avx2)
IFUNC_IMPL_ADD (array, i, strcmp, HAS_CPU_FEATURE (SSE4_2),
__strcmp_sse42)
IFUNC_IMPL_ADD (array, i, strcmp, HAS_CPU_FEATURE (SSSE3),
__strcmp_ssse3)
IFUNC_IMPL_ADD (array, i, strcmp, 1, __strcmp_sse2_unaligned)
IFUNC_IMPL_ADD (array, i, strcmp, 1, __strcmp_sse2))
This mechanism, called IFUNC (Indirect Function) [17], is a GNU toolchain
feature that allows multiple function implementations to be selected at
runtime via a resolver. The dynamic loader invokes this resolver during
startup to choose the optimal version (e.g., AVX2), which then remains
fixed for the process's lifetime.
String comparison using AVX2 is performed through vectorized operations
where two 256-bit ymm registers are compared using VPCMPEQ. As each ymm
register holds 32 bytes (VEC_SIZE), this allows comparing 32-byte string
chunks in a single operation. For example:
vmovdqu (%rdi),%ymm1
vpcmpeqb (%rsi),%ymm1,%ymm0
The vmovdqu instruction loads 32 bytes from the memory address in RDI into
YMM1. The vpcmpeqb instruction then compares these 32 bytes against the
contents at RSI's memory address, storing the comparison result in YMM0.
Each byte position in YMM0 is set to 0xFF (all 1s) for matching bytes or
0x00 (all 0s) for mismatches.
The string comparison is performed using vpcmpeqb rather than traditional
CMPS instructions. From the backdoor's perspective, this approach is more
advantageous because these extended instruction sets are specialized and
less frequently used than basic x86 instructions. Additionally, vpcmpeqb
can compare significantly more bytes in a single operation, making it
easier to identify the target hash string while minimizing the risk of
accidental triggers. Note, complex instruction like vpcmpeqb are typically
implemented through microcode.