linux-ext4 - Re: [PATCH v16 3/9] libfs: Introduce case-insensitive string comparison helper

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Message-ID: <9afebadd-765f-42f3-a80b-366dd749bf48@collabora.com>
Date: Wed, 22 May 2024 17:02:53 +0300
From: Eugen Hristev <eugen.hristev@...labora.com>
To: Gabriel Krisman Bertazi <krisman@...e.de>,
 Eric Biggers <ebiggers@...nel.org>
Cc: 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

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.

> 
> 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 ?
> 
> Also, once patch 6 and 7 are added, if fscrypt fails with -EINVAL for
> any reason unrelated to unicode (like in the WARN_ON above), we will
> incorrectly print the error message saying there is a bad UTF8 string.
> 
> 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 ?
>>> +	/*
>>> +	 * 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 ?

Thanks for the review
Eugen