linux-ext4 - Re: [PATCH v4 5/5] fs: unicode: Add utf8 module and a unicode layer

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Date:   Fri, 26 Mar 2021 02:56:01 +0530
From:   Shreeya Patel <shreeya.patel@...labora.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, jaegeuk@...nel.org,
        chao@...nel.org, krisman@...labora.com, drosen@...gle.com,
        yuchao0@...wei.com, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, kernel@...labora.com,
        andre.almeida@...labora.com
Subject: Re: [PATCH v4 5/5] fs: unicode: Add utf8 module and a unicode layer


On 26/03/21 1:56 am, Shreeya Patel wrote:
>
> On 26/03/21 1:10 am, Eric Biggers wrote:
>> On Thu, Mar 25, 2021 at 05:38:11AM +0530, Shreeya Patel wrote:
>>> Also, indirect calls using function pointers are easily exploitable by
>>> speculative execution attacks, hence use static_call() in unicode.h and
>>> unicode-core.c files inorder to prevent these attacks by making direct
>>> calls and also to improve the performance of function pointers.
>> I don't think you need to worry about avoiding indirect calls to prevent
>> speculative execution attacks.  That's what the mitigations like 
>> Retpoline are
>> for.  Instead my concern was just that indirect calls are *slow*, 
>> especially
>> when those mitigations are enabled.  Some of the casefolding 
>> operations are
>> called a lot (e.g., repeatedly during path resolution), and it would be
>> desirable to avoid adding more overhead there.
>>
>>> diff --git a/fs/unicode/Kconfig b/fs/unicode/Kconfig
>>> index 2c27b9a5cd6c..2961b0206b4d 100644
>>> --- a/fs/unicode/Kconfig
>>> +++ b/fs/unicode/Kconfig
>>> @@ -8,7 +8,16 @@ config UNICODE
>>>         Say Y here to enable UTF-8 NFD normalization and NFD+CF 
>>> casefolding
>>>         support.
>>>   +# UTF-8 encoding can be compiled as a module using UNICODE_UTF8 
>>> option.
>>> +# Having UTF-8 encoding as a module will avoid carrying large
>>> +# database table present in utf8data.h_shipped into the kernel
>>> +# by being able to load it only when it is required by the filesystem.
>>> +config UNICODE_UTF8
>>> +    tristate "UTF-8 module"
>>> +    depends on UNICODE
>>> +    default m
>>> +
>> The help for UNICODE still says that it enables UTF-8 support. But 
>> now there is
>> a separate option that people will need to remember to enable.
>>
>> Please document each of these options properly.
>>
>> Perhaps EXT4_FS and F2FS_FS just should select UNICODE_UTF8 if 
>> UNICODE, so that
>> UNICODE_UTF8 doesn't have to be a user-selectable symbol?
>
>
> It is not a user-selectable symbol. It depends on UNICODE and if 
> someone enables it,
> by default UNICODE_UTF8 will be enabled as a module.
>
>

Sorry, I think I misunderstood when you said EXT4_FS and F2FS_FS should 
select
UNICODE_UTF8 if UNICODE is enabled. I now get it that you don't want 
them to be deselectable.
I'll make this change. Thanks


>>> +DEFINE_STATIC_CALL(validate, unicode_validate_static_call);
>>> +EXPORT_STATIC_CALL(validate);
>> Global symbols can't have generic names like "validate".  Please add an
>> appropriate prefix like "unicode_".
>>
>> Also, the thing called "unicode_validate_static_call" isn't actually 
>> a static
>> call as the name suggests, but rather the default function used by 
>> the static
>> call.  It should be called something like unicode_validate_default.
>>
>> Likewise for all the others.
>
>
> Thanks for your reviews, I'll make the change suggested by you in v5.
>
>
>>
>> - Eric
>>