Re: [f2fs-dev] Using secure erase in f2fs

[Mikhail Novosyolov via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

02.12.2020 21:24, Mikhail Novosyolov пишет:
> Hello, f2fs developers!
>
> I saw commit 9af846486d781a63 "f2fs: add F2FS_IOC_SEC_TRIM_FILE ioctl" in Linux kernel
> https://github.com/torvalds/linux/commit/9af846486d781a63de025a5f502c515268e48790#
>
> I saw usage of it in Android only
> https://android.googlesource.com/platform/system/vold/+/master/secdiscard.cpp
>
> Do I understand correctly that userspace code decides himself where the region to erase starts and when to call that ioctl?
>
> I am interested in it to implement realtime secure erase - overwriting files with zeros or random data (I am not interestied in sending a discard/trim command) - when files are deleted or are removed completely.
> This code seems to be a good solution of this problem, but the problem is that kernel cannot trigger it by itself.
What if do something like this?

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 7d8578401267..1e72ac27bfdf 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1166,6 +1166,10 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
                     SECTOR_FROM_BLOCK(start),
                     SECTOR_FROM_BLOCK(len),
                     GFP_NOFS, 0, &bio);
+        __blkdev_issue_write_zeroes(bdev,
+                    SECTOR_FROM_BLOCK(start),
+                    SECTOR_FROM_BLOCK(len),
+                    GFP_NOFS, 0, &bio);
 submit:
         if (err) {
             spin_lock_irqsave(&dc->lock, flags);

after of before __blkdev_issue_discard()?

Is it safe to fill the region which is being discarded with zeroes?

Will it work and will it fit with background housekeeping and discard queue?

>
> I think it should be possible to call this from another part of f2fs when a file is being edited or unlinked, but I am not sure from where exactly.
>
> F2FS has background housekeeping, it would be nice if it could issue secure trim in the background.
>
> Also, I am not sure that it will work with symlinks correctly, for example.
>
> Please give a direction.
>


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