Why writing to //./PhysicalDrive0 with _sopen_s() not working

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PaikuHan-3271 asked Mar 23, '21 PaikuHan-3271 commented Mar 23, '21

Why writing to //./PhysicalDrive0 with _sopen_s() not working

I'm trying to write to physical disk using Microsoft C's _sopen_s() function but it doesn't work at all. Here is my code:

Code-Listing 1:

 #include <stdio.h>
 #include <stdlib.h>
 #include <windows.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <share.h>
 #include <fcntl.h>
 #include <io.h>
    
 int main(int argc, char * argv[])
 {
    
  int drive_descriptor;
    
  if((errno = _sopen_s(&drive_descriptor,"//./PhysicalDrive0", _O_RDWR, _SH_DENYNO, _S_IREAD | _S_IWRITE)) != 0)
  {
  printf("Failed to open //./PhysicalDrive0 for writing : %s\n", strerror(errno));
  exit(EXIT_FAILURE);
  }
    
  return(EXIT_SUCCESS);
 }

When I compile and run the code I get, as administrator :

Failed to open //./PhysicalDrive0 for writing : Invalid argument

And without the administrator privilege I get :

Failed to open //./PhysicalDrive0 for writing : Permission denied

Note that PhysicalDrive0 is not the system drive and I'm not interested in using another function like CreateFile(). Only function that look similar to their POSIX counterpart (e.g. open(), write(), etc.) can be used as I want to make my code easily portable across platforms.

Also note that the code should be in C not C++. Unfortunately There is no C tag in Microsoft Q&A.

c++

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Answer 1 · 2021-03-23T08:07:50.077Z

WayneAKing-0228 answered Mar 23, '21 PaikuHan-3271 commented Mar 23, '21

What happens if you use _O_RDONLY instead of _O_RDWR ?

Wayne

· 5

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PaikuHan-3271 · Mar 23, 2021 at 08:18 AM

When I replace _O_RDWR with _O_RDONLY I only get the error message when not running as Administrator. And if I modify the code like this :

Code-Listing 2:

 if((errno = _sopen_s(&drive_descriptor,"//./PhysicalDrive0", _O_RDONLY, _SH_DENYNO, _S_IREAD | _S_IWRITE)) != 0)
 {
     printf("Failed to open //./PhysicalDrive0 for writing : %s\n", strerror(errno));
     exit(EXIT_FAILURE);
 }
 else
 {
     printf("Successfully opened //./PhysicalDrive0!");
 }

the following message appears:

Successfully opened //./PhysicalDrive0!

when run as Administrator.

0 ·

WayneAKing-0228 PaikuHan-3271 · Mar 23, 2021 at 08:30 AM

What kind of disk are you trying to access?

AFAIK, using "//./PhysicalDrive0" is accessing the Master
Boot Record. Perhaps requesting write permission to the
MBR is not allowed.

Wayne

0 ·

PaikuHan-3271 WayneAKing-0228 · Mar 23, 2021 at 08:48 AM

When I use CreateFile() to open and write to the physical drive (Note that I don't want to use this function for portability reason) like this:

Code-Listing 3:

 int main(int argc, char * argv[])
 {
     HANDLE drive;
     if((drive = CreateFile("//./PhysicalDrive0", GENERIC_ALL, FILE_SHARE_READ | FILE_SHARE_WRITE, 0, OPEN_EXISTING, 0, 0)) == INVALID_HANDLE_VALUE)
     {
         printf("Failed to open //./PhysicalDrive0 for writing\n");
     }
    
     byte mbr[512];
     if (!WriteFile(drive, mbr, 512, NULL, 0))
     {
         printf("Failed to write to //./PhysicalDrive0 for writing\n");
     }
     else
     {
         printf("Successfully wrote to //./PhysicalDrive0!\n");
     }
    
     return(EXIT_SUCCESS);
 }

it is writing to the MBR/GPT sector and I get the message :

Successfully wrote to //./PhysicalDrive0!

So it must be some other reason.

0 ·

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