linux-kernel - Re: [PATCH v16 3/9] libfs: Introduce case-insensitive string comparison helper

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Date: Mon, 27 May 2024 16:54:53 -0400
From: Gabriel Krisman Bertazi <krisman@...e.de>
To: Eugen Hristev <eugen.hristev@...labora.com>
Cc: Eric Biggers <ebiggers@...nel.org>,  tytso@....edu,
  adilger.kernel@...ger.ca,  linux-ext4@...r.kernel.org,
  jaegeuk@...nel.org,  chao@...nel.org,
  linux-f2fs-devel@...ts.sourceforge.net,  linux-fsdevel@...r.kernel.org,
  linux-kernel@...r.kernel.org,  kernel@...labora.com,
  viro@...iv.linux.org.uk,  brauner@...nel.org,  jack@...e.cz,  Gabriel
 Krisman Bertazi <krisman@...labora.com>
Subject: Re: [PATCH v16 3/9] libfs: Introduce case-insensitive string
 comparison helper

Eugen Hristev <eugen.hristev@...labora.com> writes:

> On 5/23/24 02:05, Gabriel Krisman Bertazi wrote:
>> Eugen Hristev <eugen.hristev@...labora.com> writes:
>> 
>>> On 5/13/24 00:27, Gabriel Krisman Bertazi wrote:
>>>> Eric Biggers <ebiggers@...nel.org> writes:
>>>>
>>>>> On Fri, Apr 05, 2024 at 03:13:26PM +0300, Eugen Hristev wrote:
>>>>
>>>>>> +		if (WARN_ON_ONCE(!fscrypt_has_encryption_key(parent)))
>>>>>> +			return -EINVAL;
>>>>>> +
>>>>>> +		decrypted_name.name = kmalloc(de_name_len, GFP_KERNEL);
>>>>>> +		if (!decrypted_name.name)
>>>>>> +			return -ENOMEM;
>>>>>> +		res = fscrypt_fname_disk_to_usr(parent, 0, 0, &encrypted_name,
>>>>>> +						&decrypted_name);
>>>>>> +		if (res < 0)
>>>>>> +			goto out;
>>>>>
>>>>> If fscrypt_fname_disk_to_usr() returns an error and !sb_has_strict_encoding(sb),
>>>>> then this function returns 0 (indicating no match) instead of the error code
>>>>> (indicating an error).  Is that the correct behavior?  I would think that
>>>>> strict_encoding should only have an effect on the actual name
>>>>> comparison.
>>>>
>>>> No. we *want* this return code to be propagated back to f2fs.  In ext4 it
>>>> wouldn't matter since the error is not visible outside of ext4_match,
>>>> but f2fs does the right thing and stops the lookup.
>>>
>>> In the previous version which I sent, you told me that the error should be
>>> propagated only in strict_mode, and if !strict_mode, it should just return no match.
>>> Originally I did not understand that this should be done only for utf8_strncasecmp
>>> errors, and not for all the errors. I will change it here to fix that.
>> 
>> Yes, it depends on which error we are talking about. For ENOMEM and
>> whatever error fscrypt_fname_disk_to_usr returns, we surely want to send
>> that back, such that f2fs can handle it (i.e abort the lookup).  Unicode
>> casefolding errors don't need to stop the lookup.
>> 
>> 
>>>> Thinking about it, there is a second problem with this series.
>>>> Currently, if we are on strict_mode, f2fs_match_ci_name does not
>>>> propagate unicode errors back to f2fs. So, once a utf8 invalid sequence
>>>> is found during lookup, it will be considered not-a-match but the lookup
>>>> will continue.  This allows some lookups to succeed even in a corrupted
>>>> directory.  With this patch, we will abort the lookup on the first
>>>> error, breaking existing semantics.  Note that these are different from
>>>> memory allocation failure and fscrypt_fname_disk_to_usr. For those, it
>>>> makes sense to abort.
>>>
>>> So , in the case of f2fs , we must not propagate utf8 errors ? It should just
>>> return no match even in strict mode ?
>>> If this helper is common for both f2fs and ext4, we have to do the same for ext4 ?
>>> Or we are no longer able to commonize the code altogether ?
>> 
>> We can have a common handler.  It doesn't matter for Ext4 because it
>> ignores all errors. Perhaps ext4 can be improved too in a different
>> patchset.
>> 
>>>> My suggestion would be to keep the current behavior.  Make
>>>> generic_ci_match only propagate non-unicode related errors back to the
>>>> filesystem.  This means that we need to move the error messages in patch
>>>> 6 and 7 into this function, so they only trigger when utf8_strncasecmp*
>>>> itself fails.
>>>>
>>>
>>> So basically unicode errors stop here, and print the error message here in that case.
>>> Am I understanding it correctly ?
>> 
>> Yes, that is it.  print the error message - only in strict mode - and
>> return not-a-match.
>> 
>> Is there any problem with this approach that I'm missing?
>
> As the printing is moved here, in the common code, we cannot use either of
> f2fs_warn nor EXT4_ERROR_INODE . Any suggestion ? Would have to be something
> meaningful for the user and ratelimited I guess.
>

Ah, that is not great, since EXT4_ERROR_INODE does more things like
annotating the error in the sb and sending a FAN_FS_ERROR to any
watchers. But still, this is a rare error and I'm not really sure we
care, nor that it should gate the rest of the series.

I'd say just use pr_err and be done with it.


-- 
Gabriel Krisman Bertazi