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

Gabriel Krisman Bertazi
2024-05-12T21:28:04Z

Eric Biggers <ebiggers@kernel.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