I noticed a problem building samba with ‘-DDISABLE_OPATH=1’. The resulting smbd process can hang if there is a FIFO file in the samba share. The build was done on ‘CentOS Linux release 7.6.1810 (Core)’. 

(gdb) bt
#0  0x00007f781febbe47 in openat64 () from /lib64/libc.so.6
#1  0x00007f78237f55a9 in vfswrap_openat (handle=0x56546a488950, dirfsp=0x56546a494ba0,
    smb_fname=0x56546a4c2810, fsp=0x56546a4c1510, flags=131072, mode=0)
    at ../../source3/modules/vfs_default.c:725
#2  0x00007f782388c60d in smb_vfs_call_openat (handle=0x56546a488950,
    dirfsp=0x56546a494ba0, smb_fname=0x56546a4c2810, fsp=0x56546a4c1510, flags=131072,
    mode=0) at ../../source3/smbd/vfs.c:1830
#3  0x00007f7823874c15 in non_widelink_open (dirfsp=0x56546a494ba0, fsp=0x56546a4c1510,
    smb_fname=0x56546a4c2810, flags=131072, mode=0, link_depth=0)
    at ../../source3/smbd/open.c:787
#4  0x00007f7823875037 in fd_openat (dirfsp=0x56546a494ba0, smb_fname=0x56546a4c2810,
    fsp=0x56546a4c1510, flags=0, mode=0) at ../../source3/smbd/open.c:918
#5  0x00007f78237fd119 in openat_pathref_fsp (dirfsp=0x56546a494ba0,
    smb_fname=0x56546a4c2810) at ../../source3/smbd/files.c:497
#6  0x00007f7823803994 in smbd_dirptr_get_entry (ctx=0x56546a48c5a0,
    dirptr=0x56546a488c90, mask=0x56546a497e50 "*", dirtype=22, dont_descend=false,
    ask_sharemode=true, get_dosmode_in=true, match_fn=
    0x7f782384e239 <smbd_dirptr_lanman2_match_fn>, mode_fn=
    0x7f782384e55e <smbd_dirptr_lanman2_mode_fn>, private_data=0x7ffe4a836a40,
    _fname=0x7ffe4a836a58, _smb_fname=0x7ffe4a836a50, _mode=0x7ffe4a836a64,
    _prev_offset=0x7ffe4a836a68) at ../../source3/smbd/dir.c:888
#7  0x00007f7823851cc7 in smbd_dirptr_lanman2_entry (ctx=0x56546a48c5a0,
    conn=0x56546a4960f0, dirptr=0x56546a488c90, flags2=49217,
    path_mask=0x56546a497e50 "*", dirtype=22, info_level=262, requires_resume_key=0,
    dont_descend=false, ask_sharemode=true, get_dosmode=true, align=8 '\b',
    do_pad=false, ppdata=0x56546a48c600, base_data=0x56546a4af140 "p",
    end_data=0x56546a4c0137 "", space_remaining=60378, _smb_fname=0x7ffe4a836bd8,
    got_exact_match=0x7ffe4a836bd7, _last_entry_off=0x56546a48c624, name_list=0x0,
    file_id=0x7ffe4a836bb0) at ../../source3/smbd/trans2.c:2550
#8  0x00007f78238e37af in smb2_query_directory_next_entry (req=0x56546a48c3f0)
    at ../../source3/smbd/smb2_query_directory.c:555
#9  0x00007f78238e3479 in smbd_smb2_query_directory_send (mem_ctx=0x56546a48ac00,
    ev=0x56546a4623a0, smb2req=0x56546a48ac00, fsp=0x56546a494ba0,
    in_file_info_class=37 '%', in_flags=0 '\000', in_file_index=0,
    in_output_buffer_length=65528, in_file_name=0x56546a497e50 "*")
    at ../../source3/smbd/smb2_query_directory.c:523
#10 0x00007f78238e2512 in smbd_smb2_request_process_query_directory (req=0x56546a48ac00)
    at ../../source3/smbd/smb2_query_directory.c:129

-------------------------

(gdb) f 1
#1  0x00007f78237f55a9 in vfswrap_openat (handle=0x56546a488950, dirfsp=0x56546a494ba0,
    smb_fname=0x56546a4c2810, fsp=0x56546a4c1510, flags=131072, mode=0)
    at ../../source3/modules/vfs_default.c:725
725             result = openat(fsp_get_pathref_fd(dirfsp),
(gdb) l
720             if (fsp->fsp_flags.is_pathref && !have_opath) {
721                     become_root();
722                     became_root = true;
723             }
724
725             result = openat(fsp_get_pathref_fd(dirfsp),
726                             smb_fname->base_name,
727                             flags,
728                             mode);
729
(gdb) p smb_fname->base_name
$1 = 0x56546a4c2950 "a_pipe"


--------------------------------

[miguel@mig28centos7-vm ~]$ stat a_pipe
  File: ‘a_pipe’
  Size: 0               Blocks: 0          IO Block: 4096   fifo
Device: fd02h/64770d    Inode: 9469886     Links: 1
Access: (0664/prw-rw-r--)  Uid: ( 1000/  miguel)   Gid: ( 1000/  miguel)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2021-08-25 15:25:14.137376316 -0400
Modify: 2021-08-25 15:25:14.137376316 -0400
Change: 2021-08-25 15:25:14.137376316 -0400
 Birth: -