linux-kernel - Re: [PATCH 1/1] nfsd: incorrect check for debugfs returns

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Date:	Wed, 25 Mar 2015 12:41:29 -0400
From:	Chengyu Song <csong84@...ech.edu>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Taesoo Kim <taesoo@...ech.edu>, changwoo@...ech.edu,
	sanidhya@...ech.edu, Byoungyoung Lee <blee@...ech.edu>
Subject: Re: [PATCH 1/1] nfsd: incorrect check for debugfs returns

There may be a simpler solution, declare NFSD_FAULT_INJECTION has dependency
on DEBUG_FS, or automatically select DEBUG_FS. I don't think current debugfs
implementation will return any error ptr once it's configured.

I choose to check the return instead, because I was worried the debugfs interface
may change in the future.

Does this sounds like a solution? If so, I can submit a patch for Kconfig.

Best,
Chengyu

> On Mar 25, 2015, at 11:17 AM, J. Bruce Fields <bfields@...ldses.org> wrote:
> 
> On Mon, Mar 23, 2015 at 10:58:05PM -0400, Chengyu Song wrote:
>> debugfs_create_dir and debugfs_create_file may return -ENODEV when debugfs
>> is not configured, so the return value should be checked against ERROR_VALUE
>> as well, otherwise the later dereference of the dentry pointer would crash
>> the kernel.
> 
> Thanks for spotting this.  But it looks like this will cause nfsd
> startup to fail when debugfs isn't configured.  I'd rather we didn't, it
> just isn't that important.
> 
> So I'd rather just make nfsd_fault_inject_init() a void return--just do
> a dprintk as a warning in the "fail" case, and otherwise let normal
> startup continue (and check that doesn't lead to other unsafe
> dereferences of debug_dir).  Could you try that?
> 
> --b.
> 
> 
>> 
>> Signed-off-by: Chengyu Song <csong84@...ech.edu>
>> ---
>> fs/nfsd/fault_inject.c | 12 ++++++++----
>> 1 file changed, 8 insertions(+), 4 deletions(-)
>> 
>> diff --git a/fs/nfsd/fault_inject.c b/fs/nfsd/fault_inject.c
>> index c16bf5a..621d065 100644
>> --- a/fs/nfsd/fault_inject.c
>> +++ b/fs/nfsd/fault_inject.c
>> @@ -132,19 +132,23 @@ int nfsd_fault_inject_init(void)
>> 	unsigned int i;
>> 	struct nfsd_fault_inject_op *op;
>> 	umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
>> +	struct dentry *dent;
>> 
>> -	debug_dir = debugfs_create_dir("nfsd", NULL);
>> -	if (!debug_dir)
>> +	dent = debugfs_create_dir("nfsd", NULL);
>> +	if (IS_ERR_OR_NULL(dent))
>> 		goto fail;
>> +	debug_dir = dent;
>> 
>> 	for (i = 0; i < NUM_INJECT_OPS; i++) {
>> 		op = &inject_ops[i];
>> -		if (!debugfs_create_file(op->file, mode, debug_dir, op, &fops_nfsd))
>> +		dent = debugfs_create_file(op->file, mode, debug_dir, op, &fops_nfsd);
>> +		if (IS_ERR_OR_NULL(dent))
>> 			goto fail;
>> +
>> 	}
>> 	return 0;
>> 
>> fail:
>> 	nfsd_fault_inject_cleanup();
>> -	return -ENOMEM;
>> +	return dent ? PTR_ERR(dent) : -ENOMEM;
>> }
>> -- 
>> 2.1.0

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