assembly - Get file size with stat syscall

Question

I'm trying to get file size wit stat syscall with assembly (nasm):

section .data
    encodeFile db "/home/user/file"

section .bss
    stat resb 64

struc STAT
    .st_dev: resd 1
    .st_ino: resd 1
    .st_mode: resw 1
    .st_nlink: resw 1
    .st_uid: resw 1
    .st_gid: resw 1
    .st_rdev: resd 1
    .st_size: resd 1
    .st_atime: resd 1
    .st_mtime: resd 1
    .st_ctime: resd 1
    .st_blksize: resd 1
    .st_blocks: resd 1
endstruc

_start:
    mov rax, 4
    mov rdi, encodeFile
    mov rsi, stat
    syscall

    mov eax, dword [stat + STAT.st_size]

There is 0 in rax after syscall executing, it's good but after mov eax, dword [stat + STAT.st_size] there is 0 too

Answer 1

It seems that you program for 64-bit Linux. It is a bit difficult to get the right structure from sys/stat.h. I created at last a C program for that:

#include <stdio.h>
#include <sys/stat.h>

int main ( void )
{
    struct stat file_stat;

    printf ("__WORDSIZE: %d
",__WORDSIZE);
    printf ("__USE_MISC: %d
",__USE_MISC);
    printf ("__USE_XOPEN2K8: %d
",__USE_XOPEN2K8);

    printf ("file_stat len: %ld
", sizeof file_stat);

    long p =  (long)(&file_stat);

    printf ("file_stat.st_dev          pos: %3ld   len: %2ld
", (long)(&file_stat.st_dev) - p,           sizeof file_stat.st_dev);
    printf ("file_stat.st_ino          pos: %3ld   len: %2ld
", (long)(&file_stat.st_ino) - p,           sizeof file_stat.st_ino);
    printf ("file_stat.st_nlink        pos: %3ld   len: %2ld
", (long)(&file_stat.st_nlink) - p,         sizeof file_stat.st_nlink);
    printf ("file_stat.st_mode         pos: %3ld   len: %2ld
", (long)(&file_stat.st_mode) - p,          sizeof file_stat.st_mode);
    printf ("file_stat.st_uid          pos: %3ld   len: %2ld
", (long)(&file_stat.st_uid) - p,           sizeof file_stat.st_uid);
    printf ("file_stat.st_gid          pos: %3ld   len: %2ld
", (long)(&file_stat.st_gid) - p,           sizeof file_stat.st_gid);
    printf ("file_stat.__pad0          pos: %3ld   len: %2ld
", (long)(&file_stat.__pad0) - p,           sizeof file_stat.__pad0);
    printf ("file_stat.st_rdev         pos: %3ld   len: %2ld
", (long)(&file_stat.st_rdev) - p,          sizeof file_stat.st_rdev);
    printf ("file_stat.st_size         pos: %3ld   len: %2ld
", (long)(&file_stat.st_size) - p,          sizeof file_stat.st_size);
    printf ("file_stat.st_blksize      pos: %3ld   len: %2ld
", (long)(&file_stat.st_blksize) - p,       sizeof file_stat.st_blksize);
    printf ("file_stat.st_blocks       pos: %3ld   len: %2ld
", (long)(&file_stat.st_blocks) - p,        sizeof file_stat.st_blocks);
    printf ("file_stat.st_atim.tv_sec  pos: %3ld   len: %2ld
", (long)(&file_stat.st_atim.tv_sec) - p,   sizeof file_stat.st_atim.tv_sec);
    printf ("file_stat.st_atim.tv_nsec pos: %3ld   len: %2ld
", (long)(&file_stat.st_atim.tv_nsec) - p,  sizeof file_stat.st_atim.tv_nsec);
    printf ("file_stat.st_mtim.tv_sec  pos: %3ld   len: %2ld
", (long)(&file_stat.st_mtim.tv_sec) - p,   sizeof file_stat.st_mtim.tv_sec);
    printf ("file_stat.st_mtim.tv_nsec pos: %3ld   len: %2ld
", (long)(&file_stat.st_mtim.tv_nsec) - p,  sizeof file_stat.st_mtim.tv_nsec);
    printf ("file_stat.st_ctim.tv_sec  pos: %3ld   len: %2ld
", (long)(&file_stat.st_ctim.tv_sec) - p,   sizeof file_stat.st_ctim.tv_sec);
    printf ("file_stat.st_ctim.tv_nsec pos: %3ld   len: %2ld
", (long)(&file_stat.st_ctim.tv_nsec) - p,  sizeof file_stat.st_ctim.tv_nsec);
    printf ("file_stat.__unused        pos: %3ld   len: %2ld
", (long)(&file_stat.__unused) - p,         sizeof file_stat.__unused);

    return 0;
}

Its output:

argv[0]: ./example_stat
__WORDSIZE: 64
__USE_MISC: 1
__USE_XOPEN2K8: 1
file_stat len: 144
file_stat.st_dev          pos:   0   len:  8
file_stat.st_ino          pos:   8   len:  8
file_stat.st_nlink        pos:  16   len:  8
file_stat.st_mode         pos:  24   len:  4
file_stat.st_uid          pos:  28   len:  4
file_stat.st_gid          pos:  32   len:  4
file_stat.__pad0          pos:  36   len:  4
file_stat.st_rdev         pos:  40   len:  8
file_stat.st_size         pos:  48   len:  8
file_stat.st_blksize      pos:  56   len:  8
file_stat.st_blocks       pos:  64   len:  8
file_stat.st_atim.tv_sec  pos:  72   len:  8
file_stat.st_atim.tv_nsec pos:  80   len:  8
file_stat.st_mtim.tv_sec  pos:  88   len:  8
file_stat.st_mtim.tv_nsec pos:  96   len:  8
file_stat.st_ctim.tv_sec  pos: 104   len:  8
file_stat.st_ctim.tv_nsec pos: 112   len:  8
file_stat.__unused        pos: 120   len: 24

This leads to the following NASM structure:

section .bss
    stat resb 144

struc STAT
    .st_dev         resq 1
    .st_ino         resq 1
    .st_nlink       resq 1
    .st_mode        resd 1
    .st_uid         resd 1
    .st_gid         resd 1
    .pad0           resb 4
    .st_rdev        resq 1
    .st_size        resq 1
    .st_blksize     resq 1
    .st_blocks      resq 1
    .st_atime       resq 1
    .st_atime_nsec  resq 1
    .st_mtime       resq 1
    .st_mtime_nsec  resq 1
    .st_ctime       resq 1
    .st_ctime_nsec  resq 1
endstruc

I tested it with GCC as linker and it worked.