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

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Message-ID: <87ttipqwfn.fsf@mailhost.krisman.be>
Date: Wed, 22 May 2024 19:05:48 -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 5/13/24 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?

>>>> +	/*
>>>> +	 * Attempt a case-sensitive match first. It is cheaper and
>>>> +	 * should cover most lookups, including all the sane
>>>> +	 * applications that expect a case-sensitive filesystem.
>>>> +	 */
>>>> +	if (folded_name->name) {
>>>> +		if (dirent.len == folded_name->len &&
>>>> +		    !memcmp(folded_name->name, dirent.name, dirent.len))
>>>> +			goto out;
>>>> +		res = utf8_strncasecmp_folded(um, folded_name, &dirent);
>>>
>>> Shouldn't the memcmp be done with the original user-specified name, not the
>>> casefolded name?  I would think that the user-specified name is the one that's
>>> more likely to match the on-disk name, because of case preservation.  In most
>>> cases users will specify the same case on both file creation and later access.
>> 
>> Yes.
>> 
> so the utf8_strncasecmp_folded call here must use name->name instead of folded_name ?

No, utf8_strncasecmp_folded requires a casefolded name.  Eric's point is
that the *memcmp* should always compare against name->name since it's more
likely to match the name on disk than the folded version because the user
is probably doing a case-exact lookup.

This also means the memcmp can be moved outside the "if (folded_name->name)",
simplifying the patch!

-- 
Gabriel Krisman Bertazi