Re: [PATCH] fs/ntfs3: disable page fault during ntfs_fiemap()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2023/04/12 22:13, Matthew Wilcox wrote:
>> Also, since Documentation/filesystems/fiemap.rst says that "If an error
>> is encountered while copying the extent to user memory, -EFAULT will be
>> returned.", I assume that ioctl(FS_IOC_FIEMAP) users can handle -EFAULT
>> error.
> 
> What?  No, that doesn't mean "You can return -EFAULT because random luck".
> That means "If you pass it an invalid address, you'll get -EFAULT back".
> 
> NACK.

Then, why does fiemap.rst say "If an error is encountered" rather than
"If an invalid address is passed" ?

Does the definition of -EFAULT limited to "the caller does not have permission
to access this address" ?

----------
int fiemap_fill_next_extent(struct fiemap_extent_info *fieinfo, u64 logical,
			    u64 phys, u64 len, u32 flags)
{
	struct fiemap_extent extent;
	struct fiemap_extent __user *dest = fieinfo->fi_extents_start;

	/* only count the extents */
	if (fieinfo->fi_extents_max == 0) {
		fieinfo->fi_extents_mapped++;
		return (flags & FIEMAP_EXTENT_LAST) ? 1 : 0;
	}

	if (fieinfo->fi_extents_mapped >= fieinfo->fi_extents_max)
		return 1;

	if (flags & SET_UNKNOWN_FLAGS)
		flags |= FIEMAP_EXTENT_UNKNOWN;
	if (flags & SET_NO_UNMOUNTED_IO_FLAGS)
		flags |= FIEMAP_EXTENT_ENCODED;
	if (flags & SET_NOT_ALIGNED_FLAGS)
		flags |= FIEMAP_EXTENT_NOT_ALIGNED;

	memset(&extent, 0, sizeof(extent));
	extent.fe_logical = logical;
	extent.fe_physical = phys;
	extent.fe_length = len;
	extent.fe_flags = flags;

	dest += fieinfo->fi_extents_mapped;
	if (copy_to_user(dest, &extent, sizeof(extent)))
		return -EFAULT;

	fieinfo->fi_extents_mapped++;
	if (fieinfo->fi_extents_mapped == fieinfo->fi_extents_max)
		return 1;
	return (flags & FIEMAP_EXTENT_LAST) ? 1 : 0;
}
----------

If copy_to_user() must not fail other than "the caller does not have
permission to access this address", what should we do for now?
Just remove ntfs_fiemap() and return -EOPNOTSUPP ?