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SubjectOops in rpc_clnt_debugfs_register() from debugfs change
Date
I've bisected an oops that occurs in rpc_clnt_debugfs_register() trying to
dereference a pointer with -EACCES in it. This is the causing commit, though
I suspect the bug is in sunrpc expecting to see NULL rather than an error.

ff9fb72bc07705c00795ca48631f7fffe24d2c6b is the first bad commit
commit ff9fb72bc07705c00795ca48631f7fffe24d2c6b
Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: Wed Jan 23 11:28:14 2019 +0100

debugfs: return error values, not NULL

When an error happens, debugfs should return an error pointer value, not
NULL. This will prevent the totally theoretical error where a debugfs
call fails due to lack of memory, returning NULL, and that dentry value
is then passed to another debugfs call, which would end up succeeding,
creating a file at the root of the debugfs tree, but would then be
impossible to remove (because you can not remove the directory NULL).

So, to make everyone happy, always return errors, this makes the users
of debugfs much simpler (they do not have to ever check the return
value), and everyone can rest easy.
...

The attached oops occurs during boot from the gssproxy process in
rpc_clnt_debugfs_register(). The code at this point is:

0xffffffff8195cbdd <+450>: mov 0x50(%rax),%rcx <--- oopsing
0xffffffff8195cbe1 <+454>: mov $0xffffffff821cc8ba,%rdx
0xffffffff8195cbe8 <+461>: mov $0x18,%esi
0xffffffff8195cbed <+466>: lea -0x30(%rbp),%rdi
0xffffffff8195cbf1 <+470>: callq 0xffffffff819db773 <snprintf>

RAX is -EACCES.

Looking in the source:

len = snprintf(name, sizeof(name), "../../rpc_xprt/%s",
xprt->debugfs->d_name.name);

I think xprt->debugfs is the value in RAX.

(gdb) p &((struct dentry *)0)->d_name.name
$5 = (const unsigned char **) 0x50 <irq_stack_union+80>

which matches the offset on the oopsing MOV instruction.

This is with linus/master (aa0c38cf39de73bf7360a3da8f1707601261e518).

David
---
BUG: unable to handle kernel NULL pointer dereference at 0000000000000043
#PF error: [normal kernel read fault]
...
RIP: 0010:rpc_clnt_debugfs_register+0x1c2/0x27e
RSP: 0018:ffff8880be407b58 EFLAGS: 00010286
RAX: fffffffffffffff3 RBX: ffff8880d56c0e00 RCX: ab0ed8627cda32e2
RDX: 000000000000000f RSI: ffffffff82462ac0 RDI: ffff8880ce487018
RBP: ffff8880be407b88 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff826ccfac R11: 0000000000000000 R12: 0000000000000000
R13: ffffffff8241b540 R14: ffff8880ce6a1000 R15: ffff8880d56c0e00
FS: 00007f195717bc80(0000) GS:ffff8880c6d80000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000043 CR3: 00000000be40a004 CR4: 00000000001606e0
Call Trace:
rpc_client_register+0x43/0x271
rpc_new_client+0x215/0x291
rpc_create_xprt+0xa5/0x167
? rpc_xprt_debugfs_register+0x8b/0xce
? page_mapping+0x5e/0x84
rpc_create+0x143/0x15f
? __mutex_lock+0x8f/0x7b0
? set_gssp_clnt+0x13b/0x194
? mntput_no_expire+0xc0/0x39e
gssp_rpc_create+0x76/0xdb
set_gssp_clnt+0x147/0x194
? _kstrtoull+0x3b/0x8a
write_gssp+0x90/0xcc
proc_reg_write+0x3b/0x59
? proc_reg_poll+0x52/0x52
__vfs_write+0x31/0x15b
? rcu_read_lock_sched_held+0x5d/0x63
? rcu_sync_lockdep_assert+0x28/0x4e
? __sb_start_write+0xb6/0x151
? vfs_write+0xca/0x182
vfs_write+0xdb/0x182
ksys_write+0x60/0xb1
do_syscall_64+0x7d/0x1a0
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7f1957711d25
Code: 00 00 75 05 48 83 c4 58 c3 e8 f7 49 ff ff 0f 1f 80 00 00 00 00 f3 0f 1e fa 8b 05 26 f7 00 00 85 c0 75 12 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 53 c3 66 90 41 54 49 89 d4 55 48 89 f5 53 89
RSP: 002b:00007ffcc0d5f288 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000009 RCX: 00007f1957711d25
RDX: 0000000000000001 RSI: 00007ffcc0d5f296 RDI: 0000000000000009
RBP: 0000000000000000 R08: 0000000000000031 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000055be5a699950
R13: 00007ffcc0d5f5c0 R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
CR2: 0000000000000043

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