Re: [PATCH] hfsplus: limit sb_maxbytes to partition size

[Hyunchul Lee <hyc.lee@gmail.com>]

On Fri, Mar 06, 2026 at 01:23:16AM +0000, Viacheslav Dubeyko wrote:
> On Fri, 2026-03-06 at 09:57 +0900, Hyunchul Lee wrote:
> > On Thu, Mar 05, 2026 at 11:21:19PM +0000, Viacheslav Dubeyko wrote:
> > > On Thu, 2026-03-05 at 10:52 +0900, Hyunchul Lee wrote:
> > > > > > 
> > > > > > Sorry it's generic/285, not generic/268.
> > > > > > in generic/285, there is a test that creates a hole exceeding the block
> > > > > > size and appends small data to the file. hfsplus fails because it fills
> > > > > > the block device and returns ENOSPC. However if it returns EFBIG
> > > > > > instead, the test is skipped.
> > > > > > 
> > > > > > For writes like xfs_io -c "pwrite 8t 512", should fops->write_iter
> > > > > > returns ENOSPC, or would it be better to return EFBIG?
> > > > > > > 
> > > > > 
> > > > > Current hfsplus_file_extend() implementation doesn't support holes. I assume you
> > > > > mean this code [1]:
> > > > > 
> > > > >         len = hip->clump_blocks;
> > > > >         start = hfsplus_block_allocate(sb, sbi->total_blocks, goal, &len);
> > > > >         if (start >= sbi->total_blocks) {
> > > > >                 start = hfsplus_block_allocate(sb, goal, 0, &len);
> > > > >                 if (start >= goal) {
> > > > >                         res = -ENOSPC;
> > > > >                         goto out;
> > > > >                 }
> > > > >         }
> > > > > 
> > > > > Am I correct?
> > > > > 
> > > > Yes,
> > > > 
> > > > hfsplus_write_begin()
> > > >   cont_write_begin()
> > > >     cont_expand_zero()
> > > > 
> > > > 1) xfs_io -c "pwrite 8t 512"
> > > > 2) hfsplus_begin_write() is called with offset 2^43 and length 512
> > > > 3) cont_expand_zero() allocates and zeroes out one block repeatedly
> > > > for the range
> > > > 0 to 2^43 - 1. To achieve this, hfsplus_write_begin() is called repeatedly.
> > > > 4) hfsplus_write_begin() allocates one block through hfsplus_get_block() =>
> > > > hfsplus_file_extend()
> > > 
> > > I think we can consider these directions:
> > > 
> > > (1) Currently, HFS+ code doesn't support holes. So, it means that
> > > hfsplus_write_begin() can check pos variable and i_size_read(inode). If pos is
> > > bigger than i_size_read(inode), then hfsplus_file_extend() will reject such
> > > request. So, we can return error code (probably, -EFBIG) for this case without
> > > calling hfsplus_file_extend(). But, from another point of view, maybe,
> > > hfsplus_file_extend() could be one place for this check. Does it make sense?
> > > 
> > > (2) I think that hfsplus_file_extend() could treat hole or absence of free
> > > blocks like -ENOSPC. Probably, we can change the error code from -ENOSPC to -
> > > EFBIG in hfsplus_write_begin(). What do you think?
> > > 
> > Even if holes are not supported, shouldn't the following writes be
> > supported?
> > 
> > xfs_io -f -c "pwrite 4k 512" <file-path>
> > 
> > If so, since we need to support cases where pos > i_size_read(inode),
> 
> The pos > i_size_read(inode) means that you create the hole. Because,

That's correct. However I believe that not supporting writes like the
one mentioned above is a significant limitation. Filesystems that don't
support sparse files, such as exFAT, allocate blocks and fill them with
zeros.

> oppositely, when HFS+ logic tries to allocate new block, then it expects to have
> pos == i_size_read(inode). And we need to take into account this code [1]:
> 
> 	if (iblock >= hip->fs_blocks) {
> 		if (!create)
> 			return 0;
> 		if (iblock > hip->fs_blocks) <-- This is the rejection of hole
> 			return -EIO;
> 		if (ablock >= hip->alloc_blocks) {
> 			res = hfsplus_file_extend(inode, false);
> 			if (res)
> 				return res;
> 		}
> 	}
> 
> The generic_write_end() changes the inode size: i_size_write(inode, pos +
> copied).

I think that it's not problem.

hfsplus_write_begin()
  cont_write_begin()
    cont_expand_zero()

cont_expand_zero() calls hfsplus_get_block() to allocate blocks between
i_size_read(inode) and pos, if pos > i_size_read(inode).

> 
> > wouldn't the condition "pos - i_size_read(inode) > free space" be better?
> > Also instead of checking every time in hfsplus_write_begin() or
> > hfsplus_file_extend(), how about implementing the check in the
> > file_operations->write_iter callback function, and returing EFBIG?
> 
> Which callback do you mean here? I am not sure that it's good idea.
> 

Here is a simple code snippet.

 static const struct file_operations hfsplus_file_operations = {
...
-       .write_iter     = generic_file_write_iter,
+       .write_iter     = hfsplus_file_write_iter,
...

+ssize_t hfsplus_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
+{
...
+       // check iocb->ki_pos is beyond i_size
+
+       ret = generic_file_write_iter(iocb, iter);

> Thanks,
> Slava.
> 
> > 
> > > > 
> > > > 
> 
> [1] https://elixir.bootlin.com/linux/v6.19/source/fs/hfsplus/extents.c#L239

-- 
Thanks,
Hyunchul