Messages in this thread | | | Date | Tue, 9 Jun 2020 20:15:32 -0700 | From | Eric Biggers <> | Subject | Re: [f2fs-dev] [PATCH] f2fs: add F2FS_IOC_SEC_TRIM_FILE ioctl |
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On Wed, Jun 10, 2020 at 11:05:46AM +0900, Daeho Jeong wrote: > > > Added a new ioctl to send discard commands or/and zero out > > > to whole data area of a regular file for security reason. > > > > With this ioctl available, what is the exact procedure to write and then later > > securely erase a file on f2fs? In particular, how can the user prevent f2fs > > from making multiple copies of file data blocks as part of garbage collection? > > > > To prevent the file data from garbage collecting, the user needs to > use pinfile ioctl and fallocate system call after creating the file. > The sequence is like below. > 1. create an empty file > 2. pinfile > 3. fallocate()
Is that persistent? So the file will never be moved afterwards?
Is there a place where this is (or should be) documented?
> > > + > > > + if (f2fs_readonly(sbi->sb)) > > > + return -EROFS; > > > > Isn't this redundant with mnt_want_write_file()? > > > > Also, shouldn't write access to the file be required, i.e. > > (filp->f_mode & FMODE_WRITE)? Then the f2fs_readonly() and > > mnt_want_write_file() checks would be unnecessary. > > > > Using FMODE_WRITE is more proper for this case, since we're going to > modify the data. But I think mnt_want_write_file() is still required > to prevent the filesystem from freezing or something else.
Right, the freezing check is actually still necessary. But getting write access to the mount is not necessary. I think you should use file_start_write() and file_end_write(), like vfs_write() does.
> > > > > + > > > + if (get_user(flags, (u32 __user *)arg)) > > > + return -EFAULT; > > > + if (!(flags & F2FS_TRIM_FILE_MASK)) > > > + return -EINVAL; > > > > Need to reject unknown flags: > > > > if (flags & ~F2FS_TRIM_FILE_MASK) > > return -EINVAL; > > I needed a different thing here. This was to check neither discard nor > zeroing out are not here. But we still need to check unknown flags, > too. > The below might be better. > if (!flags || flags & ~F2FS_TRIM_FILE_MASK) > return -EINVAL;
Sure, but please put parentheses around the second clause:
if (flags == 0 || (flags & ~F2FS_TRIM_FILE_MASK)) return -EINVAL;
- Eric
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