Re: [PATCH 1/2] nfsd: map the ELOOP to nfserr_symlink to avoid warning

[Li Lingfeng <lilingfeng3@huawei.com>]

在 2025/1/27 21:28, Chuck Lever 写道:
> On 1/26/25 9:33 PM, Li Lingfeng wrote:
>>
>> 在 2025/1/27 1:27, Chuck Lever 写道:
>>> On 1/26/25 4:50 AM, Li Lingfeng wrote:
>>>> We got -ELOOP from ext4, resulting in the following WARNING:
>>>>
>>>> VFS: Lookup of 'dc' in ext4 sdd would have caused loop
>>>> ------------[ cut here ]------------
>>>> nfsd: non-standard errno: -40
>>>> WARNING: CPU: 1 PID: 297024 at fs/nfsd/vfs.c:113 nfserrno+0xc8/0x128
>>>> Modules linked in:
>>>> CPU: 1 PID: 297024 Comm: nfsd Not tainted 6.6.0-gfa4c2159cd0d-dirty 
>>>> #21
>>>> Hardware name: linux,dummy-virt (DT)
>>>> pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>>> pc : nfserrno+0xc8/0x128
>>>> lr : nfserrno+0xc8/0x128
>>>> sp : ffff8000846475a0
>>>> x29: ffff8000846475a0 x28: 0000000000000130 x27: ffff0000d65a24e8
>>>> x26: ffff0000c7319134 x25: ffff0000d6de4240 x24: 0000000000000002
>>>> x23: ffffcda9eaac3080 x22: 00000000ffffffd8 x21: 0000000000000026
>>>> x20: ffffcda9ee055000 x19: 0000000000000000 x18: 0000000000000000
>>>> x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
>>>> x14: 0000000000000000 x13: 0000000000000001 x12: ffff60001b5ca39b
>>>> x11: 1fffe0001b5ca39a x10: ffff60001b5ca39a x9 : dfff800000000000
>>>> x8 : 00009fffe4a35c66 x7 : ffff0000dae51cd3 x6 : 0000000000000001
>>>> x5 : ffff0000dae51cd0 x4 : ffff60001b5ca39b x3 : dfff800000000000
>>>> x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0000ca5d8040
>>>> Call trace:
>>>>   nfserrno+0xc8/0x128
>>>>   nfsd4_encode_dirent_fattr+0x358/0x380
>>>>   nfsd4_encode_dirent+0x164/0x3a8
>>>>   nfsd_buffered_readdir+0x1a8/0x3a0
>>>>   nfsd_readdir+0x14c/0x188
>>>>   nfsd4_encode_readdir+0x1d4/0x370
>>>>   nfsd4_encode_operation+0x130/0x518
>>>>   nfsd4_proc_compound+0x394/0xec0
>>>>   nfsd_dispatch+0x264/0x418
>>>>   svc_process_common+0x584/0xc78
>>>>   svc_process+0x1e8/0x2c0
>>>>   svc_recv+0x194/0x2d0
>>>>   nfsd+0x198/0x378
>>>>   kthread+0x1d8/0x1f0
>>>>   ret_from_fork+0x10/0x20
>>>> Kernel panic - not syncing: kernel: panic_on_warn set ...
>>>>
>>>> The ELOOP error in Linux indicates that too many symbolic links were
>>>> encountered in resolving a path name. Mapping it to nfserr_symlink 
>>>> may be
>>>> fine.
>>>>
>>>> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
>>>> ---
>>>>   fs/nfsd/vfs.c | 1 +
>>>>   1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>>>> index 29cb7b812d71..0f727010b8cb 100644
>>>> --- a/fs/nfsd/vfs.c
>>>> +++ b/fs/nfsd/vfs.c
>>>> @@ -100,6 +100,7 @@ nfserrno (int errno)
>>>>           { nfserr_perm, -ENOKEY },
>>>>           { nfserr_no_grace, -ENOGRACE},
>>>>           { nfserr_io, -EBADMSG },
>>>> +        { nfserr_symlink, -ELOOP },
>>>>       };
>>>>       int    i;
>>>
>>> Adding ELOOP -> SYMLINK as a generic mapping could be a problem.
>>>
>>> RFC 8881 Section 15.2 does not list NFS4ERR_SYMLINK as a permissible
>>> status code for NFSv4 READDIR. Further, Section 15.4 lists only the
>>> following operations for NFS4ERR_SYMLINK:
>>>
>>> COMMIT, LAYOUTCOMMIT, LINK, LOCK, LOCKT, LOOKUP, LOOKUPP, OPEN, 
>>> READ, WRITE
>>>
>>>
>>> Which of lookup_positive_unlocked() or nfsd_cross_mnt() returned
>>> ELOOP, and why? What were the export options? What was in the file
>>> system that caused this? Can this scenario be reproduced on v6.13?
>>>
>> Hi,
>> I got a more detailed log with line numbers from our test team.
>>
>> VFS: Lookup of 'dc' in ext4 sdd would have caused loop
>> ------------[ cut here ]------------
>> nfsd: non-standard errno: -40
>> WARNING: CPU: 1 PID: 297024 at fs/nfsd/vfs.c:113 nfserrno fs/nfsd/ 
>> vfs.c:113 [inline]
>> WARNING: CPU: 1 PID: 297024 at fs/nfsd/vfs.c:113 nfserrno+0xc8/0x128 
>> fs/ nfsd/vfs.c:61
>> Modules linked in:
>> CPU: 1 PID: 297024 Comm: nfsd Not tainted 6.6.0-gfa4c2159cd0d-dirty #21
>> Hardware name: linux,dummy-virt (DT)
>> pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> pc : nfserrno fs/nfsd/vfs.c:113 [inline]
>> pc : nfserrno+0xc8/0x128 fs/nfsd/vfs.c:61
>> lr : nfserrno fs/nfsd/vfs.c:113 [inline]
>> lr : nfserrno+0xc8/0x128 fs/nfsd/vfs.c:61
>> sp : ffff8000846475a0
>> x29: ffff8000846475a0 x28: 0000000000000130 x27: ffff0000d65a24e8
>> x26: ffff0000c7319134 x25: ffff0000d6de4240 x24: 0000000000000002
>> x23: ffffcda9eaac3080 x22: 00000000ffffffd8 x21: 0000000000000026
>> x20: ffffcda9ee055000 x19: 0000000000000000 x18: 0000000000000000
>> x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
>> x14: 0000000000000000 x13: 0000000000000001 x12: ffff60001b5ca39b
>> x11: 1fffe0001b5ca39a x10: ffff60001b5ca39a x9 : dfff800000000000
>> x8 : 00009fffe4a35c66 x7 : ffff0000dae51cd3 x6 : 0000000000000001
>> x5 : ffff0000dae51cd0 x4 : ffff60001b5ca39b x3 : dfff800000000000
>> x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0000ca5d8040
>> Call trace:
>>   nfserrno fs/nfsd/vfs.c:113 [inline]
>>   nfserrno+0xc8/0x128 fs/nfsd/vfs.c:61
>>   nfsd4_encode_dirent_fattr+0x358/0x380 fs/nfsd/nfs4xdr.c:3536
>>   nfsd4_encode_dirent+0x164/0x3a8 fs/nfsd/nfs4xdr.c:3633
>>   nfsd_buffered_readdir+0x1a8/0x3a0 fs/nfsd/vfs.c:2067
>>   nfsd_readdir+0x14c/0x188 fs/nfsd/vfs.c:2123
>>   nfsd4_encode_readdir+0x1d4/0x370 fs/nfsd/nfs4xdr.c:4273
>>   nfsd4_encode_operation+0x130/0x518 fs/nfsd/nfs4xdr.c:5399
>>   nfsd4_proc_compound+0x394/0xec0 fs/nfsd/nfs4proc.c:2753
>>   nfsd_dispatch+0x264/0x418 fs/nfsd/nfssvc.c:1011
>>   svc_process_common+0x584/0xc78 net/sunrpc/svc.c:1396
>>   svc_process+0x1e8/0x2c0 net/sunrpc/svc.c:1542
>>   svc_recv+0x194/0x2d0 net/sunrpc/svc_xprt.c:877
>>   nfsd+0x198/0x378 fs/nfsd/nfssvc.c:955
>>   kthread+0x1d8/0x1f0 kernel/kthread.c:388
>>   ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:861
>>
>> Although I don't have a reproducer to reproduce this problem, I think
>> ELOOP should be returned by the following path:
>>
>> v6.6
>> nfsd4_encode_readdir
>>   nfsd_readdir
>>    nfsd_buffered_readdir
>>     nfsd4_encode_dirent // func
>>      nfsd4_encode_dirent_fattr
>>       nfsd4_encode_dirent_fattr
>>        lookup_positive_unlocked
>>         lookup_one_positive_unlocked
>>          lookup_one_unlocked // ELOOP
>>           lookup_slow
>>            __lookup_slow
>>             ext4_lookup // inode->i_op->lookup
>>              d_splice_alias
>>               // VFS: Lookup of 'dc' in ext4 sdd would have caused loop
>>
>> This scenario may be reproduced on v6.13 like this:
>> nfsd4_encode_readdir
>>   nfsd4_encode_dirlist4
>>    nfsd_readdir
>>     nfsd_buffered_readdir
>>      nfsd4_encode_entry4 // func
>>       nfsd4_encode_entry4_fattr
>>        lookup_positive_unlocked
>>         lookup_one_positive_unlocked
>>          lookup_one_unlocked
>>           lookup_slow
>>            __lookup_slow
>>             ext4_lookup // inode->i_op->lookup
>>              d_splice_alias
>
> So: lookup_positive_unlocked() is the VFS API returning it. Got it.
>
>
>> According to the information provided by the test team, the export 
>> option
>> is "rw,no_root_squash", and I'll try to reproduce the problem.
>>
>> By the way, could you suggest which NFS error code would be most
>> appropriate to map ELOOP to?
>
> NFS4ERR_SYMLINK is closest. But the spec says, you can't return that
> status for every operation; in particular, READDIR does not allow it.
> So I'm quite hesitant to correct the crash you found by adding this
> mapping to nfserrno.
>
> In this case, I wonder if READDIR can simply not return attributes
> when it hits an error.

Hi,

Do you mean to add an ELOOP check like the following and return nfs_ok
directly?

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index e67420729ecd..3a03eba9d4aa 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -3814,7 +3814,7 @@ nfsd4_encode_entry4_fattr(struct nfsd4_readdir 
*cd, const char *name,

         dentry = lookup_positive_unlocked(name, cd->rd_fhp->fh_dentry, 
namlen);
         if (IS_ERR(dentry))
-               return nfserrno(PTR_ERR(dentry));
+               return (PTR_ERR(dentry) == -ELOOP) ? nfs_ok : 
nfserrno(PTR_ERR(dentry));

         exp_get(exp);
         /*

I think it's a little weird to make this change just for ELOOP.

Thanks.

>
>