AFAIK when mount command is run to mount a FS the corresponding call made in kernel is "compat_sys_mount" as per kernel 2.6.36. Now compat_sys_kernel expects a filesystem "type" name as argument but in compat_sys_mount there is no check for "type" argument as NULL, now as part of this mount path do_kern_mount also called which uses the "type" argument to compat_sys_mount and finds out the corresponding file_system_type structure from the file_system_type structure list and if not matching structure is found it returns with error.
842 asmlinkage long compat_sys_mount(const char __user * dev_name,
843 const char __user * dir_name,
844 const char __user * type, unsigned long flags, <====== type argument to compat_sys_mount
845 const void __user * data)
846 {
847 char *kernel_type;
848 unsigned long data_page;
849 char *kernel_dev;
850 char *dir_page;
851 int retval;
852
853 retval = copy_mount_string(type, &kernel_type);
1098 do_kern_mount(const char *fstype, int flags, const char *name, void *data)
1099 {
1100 struct file_system_type *type = get_fs_type(fstype); <======= same type argument to search in list
1101 struct vfsmount *mnt;
1102 if (!type) <======= if the type name entry not found in list return ERRROR
1103 return ERR_PTR(-ENODEV);
1104 mnt = vfs_kern_mount(type, flags, name, data);
1105 if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
1106 !mnt->mnt_sb->s_subtype)
1107 mnt = fs_set_subtype(mnt, fstype);
1108 put_filesystem(type);
1109 return mnt;
1110 }
Now my question is that if I don't provide FS type name to mount command how the FS gets mounted. Is there some other path for this code or if before calling compat_sys_mount i.e in user mode FS type is determined somehow?
Regards,
Piyush
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