On Thu, Jun 01, 2023 at 04:58:55PM +0200, Christoph Hellwig wrote:

All callers of generic_perform_write need to updated ki_pos, move it into
common code.

[another late reply, sorry]

That part is somewhat fishy - there's a case where you return a positive value
and advance ->ki_pos by more than that amount.  I really wonder if all callers
of ->write_iter() are OK with that.  Consider e.g. this:

ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count)
{
        struct fd f = fdget_pos(fd);
        ssize_t ret = -EBADF;

        if (f.file) {
                loff_t pos, *ppos = file_ppos(f.file);
                if (ppos) {
                        pos = *ppos;   
                        ppos = &pos;
                }
                ret = vfs_write(f.file, buf, count, ppos);
                if (ret >= 0 && ppos)
                        f.file->f_pos = pos;
                fdput_pos(f);
        }

        return ret;
}

ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
{
        ssize_t ret;

        if (!(file->f_mode & FMODE_WRITE))
                return -EBADF;
        if (!(file->f_mode & FMODE_CAN_WRITE))
                return -EINVAL;
        if (unlikely(!access_ok(buf, count)))
                return -EFAULT;

        ret = rw_verify_area(WRITE, file, pos, count);
        if (ret)
                return ret;
        if (count > MAX_RW_COUNT)
                count =  MAX_RW_COUNT;
        file_start_write(file);
        if (file->f_op->write)
                ret = file->f_op->write(file, buf, count, pos);
        else if (file->f_op->write_iter)
                ret = new_sync_write(file, buf, count, pos);
        else   
                ret = -EINVAL;
        if (ret > 0) {
                fsnotify_modify(file);
                add_wchar(current, ret);
        }
        inc_syscw(current);
        file_end_write(file);
        return ret;
}

static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos)
{
        struct kiocb kiocb;
        struct iov_iter iter;
        ssize_t ret; 

        init_sync_kiocb(&kiocb, filp);
        kiocb.ki_pos = (ppos ? *ppos : 0);
        iov_iter_ubuf(&iter, ITER_SOURCE, (void __user *)buf, len);

        ret = call_write_iter(filp, &kiocb, &iter);
        BUG_ON(ret == -EIOCBQUEUED);
        if (ret > 0 && ppos)
                *ppos = kiocb.ki_pos;
        return ret;
} 

Suppose ->write_iter() ends up doing returning a positive value smaller than
the increment of kiocb.ki_pos.  What do we get?  ret is positive, so
kiocb.ki_pos gets copied into *ppos, which is ksys_write's pos and there
we copy it into file->f_pos.

Is it really OK to have write() return 4096 and advance the file position
by 16K?  AFAICS, userland wouldn't get any indication of something
odd going on - just a short write to a regular file, with followup write
of remaining 12K getting quietly written in the range 16K..28K.

I don't remember what POSIX says about that, but it would qualify as
nasty surprise for any userland program - sure, one can check fsync()
results before closing the sucker and see if everything looks fine,
but the way it's usually discussed could easily lead to assumption that
(synchronous) O_DIRECT writes would not be affected by anything of that
sort.