The ioctl encoding for this parameter is a long but the documentation says
it should be an int and the kernel drivers expect it to be an int.  If the
fuse driver treats this as a long it might end up scribbling over the stack
of a userspace process that only allocated enough space for an int.

This was previously discussed in [1] and a patch for fuse was proposed in
[2].  From what I can tell the patch in [2] was nacked in favor of adding
new, "fixed" ioctls and using those from userspace.  However there is still
no "fixed" version of these ioctls and the fact is that it's sometimes
infeasible to change all userspace to use the new one.

Handling the ioctls specially in the fuse driver seems like the most
pragmatic way for fuse servers to support them without causing crashes in
userspace applications that call them.

[1]: https://lore.kernel.org/linux-fsdevel/20131126200559.GH20559@hall.aurel32.net/T/
[2]: https://sourceforge.net/p/fuse/mailman/message/31771759/

Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Fixes: 59efec7b9039 ("fuse: implement ioctl support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fuse_writepages() ignores some errors taken from fuse_writepages_fill() I
believe it is a bug: if .writepages is called with WB_SYNC_ALL it should
either guarantee that all data was successfully saved or return error.

Fixes: 26d614df1da9 ("fuse: Implement writepages callback")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fuse_writepages_fill uses following construction:

if (wpa && ap->num_pages &&
    (A || B || C)) {
        action;
} else if (wpa && D) {
        if (E) {
                the same action;
        }
}

 - ap->num_pages check is always true and can be removed

 - "if" and "else if" calls the same action and can be merged.

Move checking A, B, C, D, E conditions to a helper, add comments.

Original-patch-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Previous patch changed handling of remount/reconfigure to ignore all
options, including those that are unknown to the fuse kernel fs.  This was
done for backward compatibility, but this likely only affects the old
mount(2) API.

The new fsconfig(2) based reconfiguration could possibly be improved.  This
would make the new API less of a drop in replacement for the old, OTOH this
is a good chance to get rid of some weirdnesses in the old API.

Several other behaviors might make sense:

 1) unknown options are rejected, known options are ignored

 2) unknown options are rejected, known options are rejected if the value
 is changed, allowed otherwise

 3) all options are rejected

Prior to the backward compatibility fix to ignore all options all known
options were accepted (1), even if they change the value of a mount
parameter; fuse_reconfigure() does not look at the config values set by
fuse_parse_param().

To fix that we'd need to verify that the value provided is the same as set
in the initial configuration (2).  The major drawback is that this is much
more complex than just rejecting all attempts at changing options (3);
i.e. all options signify initial configuration values and don't make sense
on reconfigure.

This patch opts for (3) with the rationale that no mount options are
reconfigurable in fuse.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

The command

  mount -o remount -o unknownoption /mnt/fuse

succeeds on kernel versions prior to v5.4 and fails on kernel version at or
after.  This is because fuse_parse_param() rejects any unrecognised options
in case of FS_CONTEXT_FOR_RECONFIGURE, just as for FS_CONTEXT_FOR_MOUNT.

This causes a regression in case the fuse filesystem is in fstab, since
remount sends all options found there to the kernel; even ones that are
meant for the initial mount and are consumed by the userspace fuse server.

Fix this by ignoring mount options, just as fuse_remount_fs() did prior to
the conversion to the new API.

Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Fixes: c30da2e981a7 ("fuse: convert to use the new mount API")
Cc: <stable@vger.kernel.org> # v5.4
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

s_op->remount_fs() is only called from legacy_reconfigure(), which is not
used after being converted to the new API.

Convert to using ->reconfigure().  This restores the previous behavior of
syncing the filesystem and rejecting MS_MANDLOCK on remount.

Fixes: c30da2e981a7 ("fuse: convert to use the new mount API")
Cc: <stable@vger.kernel.org> # v5.4
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fuse_writepages_fill() calls tree_insert() with ap->num_pages = 0 which
triggers the following warning:

 WARNING: CPU: 1 PID: 17211 at fs/fuse/file.c:1728 tree_insert+0xab/0xc0 [fuse]
 RIP: 0010:tree_insert+0xab/0xc0 [fuse]
 Call Trace:
  fuse_writepages_fill+0x5da/0x6a0 [fuse]
  write_cache_pages+0x171/0x470
  fuse_writepages+0x8a/0x100 [fuse]
  do_writepages+0x43/0xe0

Fix up the warning and clean up the code around rb-tree insertion:

 - Rename tree_insert() to fuse_insert_writeback() and make it return the
   conflicting entry in case of failure

 - Re-add tree_insert() as a wrapper around fuse_insert_writeback()

 - Rename fuse_writepage_in_flight() to fuse_writepage_add() and reverse
   the meaning of the return value to mean

    + "true" in case the writepage entry was successfully added

    + "false" in case it was in-fligt queued on an existing writepage
       entry's auxiliary list or the existing writepage entry's temporary
       page updated

   Switch from fuse_find_writeback() + tree_insert() to
   fuse_insert_writeback()

 - Move setting orig_pages to before inserting/updating the entry; this may
   result in the orig_pages value being discarded later in case of an
   in-flight request

 - In case of a new writepage entry use fuse_writepage_add()
   unconditionally, only set data->wpa if the entry was added.

Fixes: 6b2fb79963fb ("fuse: optimize writepages search")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Original-path-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

In fuse_writepage_end() the old writepages entry needs to be removed from
the rbtree before inserting the new one, otherwise tree_insert() would
fail.  This is a very rare codepath and no reproducer exists.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Pull fuse updates from Miklos Szeredi:

 - Fix a rare deadlock in virtiofs

 - Fix st_blocks in writeback cache mode

 - Fix wrong checks in splice move causing spurious warnings

 - Fix a race between a GETATTR request and a FUSE_NOTIFY_INVAL_INODE
   notification

 - Use rb-tree instead of linear search for pages currently under
   writeout by userspace

 - Fix copy_file_range() inconsistencies

* tag 'fuse-update-5.8' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse:
  fuse: copy_file_range should truncate cache
  fuse: fix copy_file_range cache issues
  fuse: optimize writepages search
  fuse: update attr_version counter on fuse_notify_inval_inode()
  fuse: don't check refcount after stealing page
  fuse: fix weird page warning
  fuse: use dump_page
  virtiofs: do not use fuse_fill_super_common() for device installation
  fuse: always allow query of st_dev
  fuse: always flush dirty data on close(2)
  fuse: invalidate inode attr in writeback cache mode
  fuse: Update stale comment in queue_interrupt()
  fuse: BUG_ON correction in fuse_dev_splice_write()
  virtiofs: Add mount option and atime behavior to the doc
  virtiofs: schedule blocking async replies in separate worker

Merge more updates from Andrew Morton:
 "More mm/ work, plenty more to come

  Subsystems affected by this patch series: slub, memcg, gup, kasan,
  pagealloc, hugetlb, vmscan, tools, mempolicy, memblock, hugetlbfs,
  thp, mmap, kconfig"

* akpm: (131 commits)
  arm64: mm: use ARCH_HAS_DEBUG_WX instead of arch defined
  x86: mm: use ARCH_HAS_DEBUG_WX instead of arch defined
  riscv: support DEBUG_WX
  mm: add DEBUG_WX support
  drivers/base/memory.c: cache memory blocks in xarray to accelerate lookup
  mm/thp: rename pmd_mknotpresent() as pmd_mkinvalid()
  powerpc/mm: drop platform defined pmd_mknotpresent()
  mm: thp: don't need to drain lru cache when splitting and mlocking THP
  hugetlbfs: get unmapped area below TASK_UNMAPPED_BASE for hugetlbfs
  sparc32: register memory occupied by kernel as memblock.memory
  include/linux/memblock.h: fix minor typo and unclear comment
  mm, mempolicy: fix up gup usage in lookup_node
  tools/vm/page_owner_sort.c: filter out unneeded line
  mm: swap: memcg: fix memcg stats for huge pages
  mm: swap: fix vmstats for huge pages
  mm: vmscan: limit the range of LRU type balancing
  mm: vmscan: reclaim writepage is IO cost
  mm: vmscan: determine anon/file pressure balance at the reclaim root
  mm: balance LRU lists based on relative thrashing
  mm: only count actual rotations as LRU reclaim cost
  ...