Re: [PATCH 5.4 086/255] once: add DO_ONCE_SLOW() for sleepable contexts

[Oleksandr Tymoshenko]

Hello,

This commit causes the following panic in kernel built with clang
(GCC build is not affected): 

[    8.320308] BUG: unable to handle page fault for address: ffffffff97216c6a                                        [26/4066]
[    8.330029] #PF: supervisor write access in kernel mode                                                                    
[    8.337263] #PF: error_code(0x0003) - permissions violation 
[    8.344816] PGD 12e816067 P4D 12e816067 PUD 12e817063 PMD 800000012e2001e1                                                 
[    8.354337] Oops: 0003 [#1] SMP PTI                
[    8.359178] CPU: 2 PID: 437 Comm: curl Not tainted 5.4.220 #15                                                             
[    8.367241] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015                                   
[    8.378529] RIP: 0010:__do_once_slow_done+0xf/0xa0   
[    8.384962] Code: 1b 84 db 74 0c 48 c7 c7 80 ce 8d 97 e8 fa e9 4a 00 84 db 0f 94 c0 5b 5d c3 66 90 55 48 89 e5 41 57 41 56 
53 49 89 d7 49 89 f6 <c6> 07 01 48 c7 c7 80 ce 8d 97 e8 d2 e9 4a 00 48 8b 3d 9b de c9 00                                      
[    8.409066] RSP: 0018:ffffb764c02d3c90 EFLAGS: 00010246
[    8.415697] RAX: 4f51d3d06bc94000 RBX: d474b86ddf7162eb RCX: 000000007229b1d6                                              
[    8.424805] RDX: 0000000000000000 RSI: ffffffff9791b4a0 RDI: ffffffff97216c6a                                              
[    8.434108] RBP: ffffb764c02d3ca8 R08: 0e81c130f1159fc1 R09: 1d19d60ce0b52c77                                              
[    8.443408] R10: 8ea59218e6892b1f R11: d5260237a3c1e35c R12: ffff9c3dadd42600                                              
[    8.452468] R13: ffffffff97910f80 R14: ffffffff9791b4a0 R15: 0000000000000000                                            
[    8.461416] FS:  00007eff855b40c0(0000) GS:ffff9c3db7a80000(0000) knlGS:0000000000000000                                   
[    8.471632] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033                                                              
[    8.478763] CR2: ffffffff97216c6a CR3: 000000022ded0000 CR4: 00000000000006a0                                              
[    8.487789] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000                                              
[    8.496684] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400                                              
[    8.505443] Call Trace:                                                                                                    
[    8.508568]  __inet_hash_connect+0x523/0x530                                                                               
[    8.513839]  ? inet_hash_connect+0x50/0x50                                                                                 
[    8.518818]  ? secure_ipv4_port_ephemeral+0x69/0xe0
[    8.525003]  tcp_v4_connect+0x2c5/0x410
[    8.529858]  __inet_stream_connect+0xd7/0x360
[    8.535329]  ? _raw_spin_unlock+0xe/0x10
... skipped ...


The root cause is the difference in __section macro semantics between 5.4 and
later LTS releases. On 5.4 it stringifies the argument so the ___done
symbol is created in a bogus section ".data.once", with double quotes:

% readelf -S vmlinux | grep data.once
  [ 5] ".data.once"      PROGBITS         ffffffff82216c6a  01416c6a