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

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Message-ID: <875xviyb3f.fsf@mailhost.krisman.be>
Date: Sun, 12 May 2024 17:27:48 -0400
From: Gabriel Krisman Bertazi <krisman@...e.de>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Eugen Hristev <eugen.hristev@...labora.com>,  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

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.

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.

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.

>> +	/*
>> +	 * 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.

-- 
Gabriel Krisman Bertazi