What are the correct permissions while “sys_open”ing a file?

I am trying to read the contents of a file using x86 assembly on Linux. The question is, what we should put into edx – the “permissions” register for sys_open.

I’ve used open() of C before; but there wasn’t any “permissions” field. I am trying to read a file belongs to the same owner of the executable. The file’s permissions are set to 0400 (-r--------). I’ve tried opening the file with edx: empty (0), 0666 and 0400. All of them returns “-13” to eax which seems to be EACCES.

Sample code:

mov eax, 0x5;     //sys_open
push 0x6362612f //file name in hex, let's say "/abc"
mov ebx, esp;     //filename to ebx
xor ecx, ecx;     //O_RDONLY
mov edx, 0666o;   //permissions in octal for nasm
int 0x80;         //call it

What should edx to be to open a file, in general (or in this read-only case)? File’s current permissions, an empty, an arbitrary?

Thanks!

Answer

man 2 open:

if neither O_CREAT nor O_TMPFILE is specified, then mode is ignored.

As such it doesn’t matter what you put in edx if you are opening an existing file for reading. The cause of your problems is something else.

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Answer