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Why does this program receive a SEGV?

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1

I have the following program, which (I think) allocates a bunch of virtual memory (by changing the system break with sys_brk), and then attempts to write to the newly allocated memory. However, I am receiving a SIGSEGV (on the very first memory write, and I don't understand why).

[OP@localhost sys_brk]$ cat out_of_memory.s 
 .section .text
 .globl _start

_start:
 mov $12, %rax
 mov $0x1000000, %rdi
 syscall


 # i: index of memory we are writing to (%rax)
 mov $0x403000, %rax
evil_loop_start:
 cmp $0x1000000, %rax
 jge evil_loop_end

 mov %rax, (%rax)
 add 0x8, %rax
 jmp evil_loop_start
evil_loop_end:
 hlt

linux assembly x86-64 system-calls gas

share|improve this question

edited Mar 28 at 3:46

Michael Petch

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asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  call sys_brk with the value of 0 to get the current brk in RAX. Add the amount of extra space you want to RAX and then call sys_brk again with the value as a parameter.
  
  – Michael Petch
  Mar 28 at 3:35
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  You also need to use add $0x8, %rax to add 8 to rax. You forgot the $ for an immediate value. HLT will segfault as well. You should use sys_exit syscall
  
  – Michael Petch
  Mar 28 at 3:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MichaelPetch Thanks. Why does HLT segfault?
  
  – extremeaxe5
  Mar 28 at 6:38
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  HLT is a privileged instruction and can't be run in user mode (ring 3). It can only be used in the kernel (ring 0) or real mode.
  
  – Michael Petch
  Mar 28 at 6:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MichaelPetch I was under the impression that segfault occurred when a program attempted an invalid memory access.
  
  – extremeaxe5
  Mar 28 at 6:43
  

  
  
  
  

 | 
show 4 more comments

1

I have the following program, which (I think) allocates a bunch of virtual memory (by changing the system break with sys_brk), and then attempts to write to the newly allocated memory. However, I am receiving a SIGSEGV (on the very first memory write, and I don't understand why).

[OP@localhost sys_brk]$ cat out_of_memory.s 
 .section .text
 .globl _start

_start:
 mov $12, %rax
 mov $0x1000000, %rdi
 syscall


 # i: index of memory we are writing to (%rax)
 mov $0x403000, %rax
evil_loop_start:
 cmp $0x1000000, %rax
 jge evil_loop_end

 mov %rax, (%rax)
 add 0x8, %rax
 jmp evil_loop_start
evil_loop_end:
 hlt

linux assembly x86-64 system-calls gas

share|improve this question

edited Mar 28 at 3:46

Michael Petch

31.2k66 gold badges6060 silver badges119119 bronze badges

asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  call sys_brk with the value of 0 to get the current brk in RAX. Add the amount of extra space you want to RAX and then call sys_brk again with the value as a parameter.
  
  – Michael Petch
  Mar 28 at 3:35
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  You also need to use add $0x8, %rax to add 8 to rax. You forgot the $ for an immediate value. HLT will segfault as well. You should use sys_exit syscall
  
  – Michael Petch
  Mar 28 at 3:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MichaelPetch Thanks. Why does HLT segfault?
  
  – extremeaxe5
  Mar 28 at 6:38
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  HLT is a privileged instruction and can't be run in user mode (ring 3). It can only be used in the kernel (ring 0) or real mode.
  
  – Michael Petch
  Mar 28 at 6:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MichaelPetch I was under the impression that segfault occurred when a program attempted an invalid memory access.
  
  – extremeaxe5
  Mar 28 at 6:43
  

  
  
  
  

 | 
show 4 more comments

I have the following program, which (I think) allocates a bunch of virtual memory (by changing the system break with sys_brk), and then attempts to write to the newly allocated memory. However, I am receiving a SIGSEGV (on the very first memory write, and I don't understand why).

[OP@localhost sys_brk]$ cat out_of_memory.s 
 .section .text
 .globl _start

_start:
 mov $12, %rax
 mov $0x1000000, %rdi
 syscall


 # i: index of memory we are writing to (%rax)
 mov $0x403000, %rax
evil_loop_start:
 cmp $0x1000000, %rax
 jge evil_loop_end

 mov %rax, (%rax)
 add 0x8, %rax
 jmp evil_loop_start
evil_loop_end:
 hlt

linux assembly x86-64 system-calls gas

share|improve this question

edited Mar 28 at 3:46

Michael Petch

31.2k66 gold badges6060 silver badges119119 bronze badges

asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges

[OP@localhost sys_brk]$ cat out_of_memory.s 
 .section .text
 .globl _start

_start:
 mov $12, %rax
 mov $0x1000000, %rdi
 syscall


 # i: index of memory we are writing to (%rax)
 mov $0x403000, %rax
evil_loop_start:
 cmp $0x1000000, %rax
 jge evil_loop_end

 mov %rax, (%rax)
 add 0x8, %rax
 jmp evil_loop_start
evil_loop_end:
 hlt

share|improve this question

edited Mar 28 at 3:46

Michael Petch

31.2k66 gold badges6060 silver badges119119 bronze badges

asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges

share|improve this question

edited Mar 28 at 3:46

Michael Petch

31.2k66 gold badges6060 silver badges119119 bronze badges

asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges

edited Mar 28 at 3:46

Michael Petch

31.2k66 gold badges6060 silver badges119119 bronze badges

edited Mar 28 at 3:46

Michael Petch

31.2k66 gold badges6060 silver badges119119 bronze badges

asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges

asked Mar 28 at 3:22

extremeaxe5extremeaxe5

21411 silver badge77 bronze badges