Question

I’m using Linux (Ubuntu 11.10).
Well, when I call the system call open, for example in a C program:

size_t filedesc = open("testfile.txt",O_CREAT | O_WRONLY,0640);

How can I access the partition, I mean is there a way to return the partition used?

The system call open is the defined above:

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)

If I want, I can put a printk("%s",filename) and see the path. But how I can access the partition?

An example: I have two archives example.txt in two different partitions (for example /dev/sda1 and /dev/sda2).

Then I call the system call open: Lets suppose I called the example.txt in the partition /dev/sda2.
Is there a way to acess the partition (for example, printk(KERN_ALERT "%s",partition)) using the open system call?

Answer 1

There’s nothing as simple as you might hope.

Within the do_sys_open() function, immediately before return fd;, the struct file *f points to a legitimate, opened, struct file.

The struct file contains a struct path f_path.

The struct path contains a struct vfsmount *mnt. struct vfsmount represents every mounted filesystem on the system.

The struct vfsmount contains a struct super_block *mnt_sb.

The struct super_block contains a struct block_device *s_bdev.

The struct block_device contains a struct hd_struct *bd_part.

The struct hd_struct contains a struct device __dev and an int partno. Together, these two define which partition your file is located on.

Update

I had originally stopped looking when I found the device and partition number references, since I assumed that was all that was required to put together the human-friendly string. But when looking again with fresh eyes, I see there is more:

The struct hd_struct contains a struct partition_meta_info *info.

The struct partition_meta_info contains a field:

    u8 volname[PARTITION_META_INFO_VOLNAMELTH];

This field is name of the device you’re after.