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I'm trying to get information about file permissions. I am using the sys_access system call. Here is my code snippet:

mov eax, 33 mov ebx, fileName mov ecx, 1 int 80h cmp eax, 0 jl .error

If eax is -1 there is an error, and I am not getting one, but I need to check all the permissions of the file (owner, group, others). How do I do that?

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  • access(2) doesn't return the file permissions. It tests whether the current process has permissions specified in the second argument. Commented May 7, 2015 at 0:08
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    To get the permissions, use stat(2). It returns a structure with information about the file, including the permission mode. Commented May 7, 2015 at 0:08
  • Thank you, it solved my problem. Now i'm trying to change those permissions using chmod(), but when I'm setting the mod using tags that I've found here: [link] (tutorialspoint.com/unix_system_calls/chmod.htm), they kinda do not ework ad i want. When i set ecx to 00400 | 00200 it sets the owner to read and write and also (don not know why) guest is set that it can write. Though when I use 00400 | 00200 | 00040 its sets both read and write. Can you help me maybe with this also? Commented May 7, 2015 at 20:34
  • Try asking a new question for that. Commented May 7, 2015 at 20:37
  • Well, basically all would be the same (almost - i.e. the title), so I thought that would not be a good choice. Commented May 7, 2015 at 20:44

1 Answer 1

Reset to default
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You can use the kernel function sys_newstat (No. 106 - look at this table) to get the file permissions. The structure stat is a never ending horror, but the following example works at least on my Debian Wheezy 64 bit (NASM, 32-bit and 64-bit modes):

SECTION .data filename db '/root' ; Just an example, can be replaced with any name filename_len equ $ - filename ; Length of filename db 0 ; Terminator for `Int 80h / EAX = 106` perm_out db 'Permissions: ' perm db 'drwxrwxrwx' perm_len equ $ - perm ; Index of last character in `perm` lf db 10 perm_out_len equ $ - perm_out ; Length of `Permissions: ...\n` SECTION .bss stat resb 256 ; Way too much, but size is variable depending on OS SECTION .text global _start _start: mov eax,4 ; sys-out mov edx,filename_len ; length of string to print mov ecx,filename ; Pointer to string mov ebx,1 ; StdOut int 0x80 ; Call kernel mov eax,4 ; sys-out mov edx,1 ; Length of string to print mov ecx, lf ; Pointer to string mov ebx,1 ; StdOut int 0x80 ; Call kernel mov eax, 106 ; sys_newstat mov ebx, filename ; Pointer to ASCIIZ file-name mov ecx, stat ; Pointer to structure stat int 80h test eax, eax jz .noerr mov eax,1 ; sys_exit mov ebx,1 ; Exit code, 1=not normal int 0x80 ; Call kernel .noerr: movzx eax, word [stat + 8] ; st_mode (/usr/include/asm/stat.h) mov ebx, perm_len ; rwx bits mov ecx, 9 .L1: sub ebx, 1 shr eax, 1 jc .J1 mov byte [perm + ebx], '-' .J1: loop .L1 ; directory bit sub ebx, 1 shr eax, 6 jc .J2 mov byte [perm + ebx], '-' .J2: mov eax,4 ; sys-out mov edx,perm_out_len ; Length of string to print mov ecx,perm_out ; Pointer to string mov ebx,1 ; StdOut int 0x80 ; Call kernel mov eax,1 ; sys_exit mov ebx,0 ; Exit code, 0=normal int 0x80 ; Call kernel
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2 Comments

Thank you for your answer, I've managed to get stat() working properly. Now I'm stuck with problem that i mentioned in the comment above. Now i want to change permissions of that file (please look up in the comments).
@TreeZ: Ask a new question, tell us assembler, linker and the Linux version and show a MCVE - and maybe I will work on it tomorrow ;-)

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