entry0 meaning in radare2

I'm new to binary analysis. I am trying to analyse a simple program I compiled from my C code via gcc.

I followed these steps:
1. aaa
2. afl

and I got this output:

0x00000608    3 23           sym._init
0x00000630    1 8            sym.imp.puts
0x00000638    1 8            sym.imp._IO_getc
0x00000640    1 8            sym.imp.__printf_chk
0x00000648    1 8            sym.imp.__cxa_finalize
0x00000650    4 77           sym.main
0x000006a0    1 43           entry0
0x000006d0    4 50   -> 44   sym.deregister_tm_clones
0x00000710    4 66   -> 57   sym.register_tm_clones
0x00000760    5 50           sym.__do_global_dtors_aux
0x000007a0    4 48   -> 42   sym.frame_dummy
0x000007d0    1 24           sym.smth
0x000007f0    4 101          sym.__libc_csu_init
0x00000860    1 2            sym.__libc_csu_fini
0x00000864    1 9            sym._fini

I can get main is the main starting point of the program but I'm worried about what entry0 is. Apparently from what I saw is not a symbol. I tried to run ag @ entry0 and ag @ main and the graphs I saw were very different. By looking at the disassembled code I see this for entry0:

I'm supposing this might be a kind of ELF template function to load the binary and run it from main. What is entry0 really?

Sorry for keeping it so long. Thanks in advance.

You should post RE questions on https://reverseengineering.stackexchange.com/.

entry0 is an alias for the _start symbol, which corresponds to the _start function.

• The memory address of _start is the program entry point, where control is passed from the loader to the program.
• The _start function originates from a relocatable ELF object file called crt1.o that is linked into binaries that require the C runtime environment.

$ objdump -dj .text /usr/lib/x86_64-linux-gnu/crt1.o 

/usr/lib/x86_64-linux-gnu/crt1.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_start>:
   0: 31 ed                   xor    %ebp,%ebp
   2: 49 89 d1                mov    %rdx,%r9
   5: 5e                      pop    %rsi
   6: 48 89 e2                mov    %rsp,%rdx
   9: 48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
   d: 50                      push   %rax
   e: 54                      push   %rsp
   f: 49 c7 c0 00 00 00 00    mov    $0x0,%r8
  16: 48 c7 c1 00 00 00 00    mov    $0x0,%rcx
  1d: 48 c7 c7 00 00 00 00    mov    $0x0,%rdi
  24: e8 00 00 00 00          callq  29 <_start+0x29>
  29: f4                      hlt

With /bin/cat as an example:

$ readelf -h /bin/cat
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x402602                      <-----
  Start of program headers:          64 (bytes into file)
  Start of section headers:          46112 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         9
  Size of section headers:           64 (bytes)
  Number of section headers:         28
  Section header string table index: 27

The memory address of the entry point is 0x402602.

  402602:       31 ed                   xor    %ebp,%ebp
  402604:       49 89 d1                mov    %rdx,%r9
  402607:       5e                      pop    %rsi
  402608:       48 89 e2                mov    %rsp,%rdx
  40260b:       48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
  40260f:       50                      push   %rax
  402610:       54                      push   %rsp
  402611:       49 c7 c0 60 89 40 00    mov    $0x408960,%r8
  402618:       48 c7 c1 f0 88 40 00    mov    $0x4088f0,%rcx
  40261f:       48 c7 c7 40 1a 40 00    mov    $0x401a40,%rdi
  402626:       e8 d5 f1 ff ff          callq  401800 <__libc_start_main@plt>
  40262b:       f4                      hlt 

Recommended reading:

Linux x86 Program Start Up or - How the heck do we get to main()?

What is the use of _start() in C?

Generic System V ABI