netdev - Re: [PATCH -next,v2] gss_krb5: refactor code to return correct PTR_ERR in krb5

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Message-ID: <8A935EDD-1959-474E-BB5B-92E0F8C2CF2A@oracle.com>
Date: Sun, 14 Jul 2024 16:18:37 +0000
From: Chuck Lever III <chuck.lever@...cle.com>
To: Neil Brown <neilb@...e.de>
CC: Gaosheng Cui <cuigaosheng1@...wei.com>,
        Trond Myklebust
	<trondmy@...nel.org>,
        Anna Schumaker <anna@...nel.org>, Jeff Layton
	<jlayton@...nel.org>,
        Olga Kornievskaia <kolga@...app.com>, Dai Ngo
	<dai.ngo@...cle.com>,
        Tom Talpey <tom@...pey.com>, "David S. Miller"
	<davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Simon Horman
	<horms@...nel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH -next,v2] gss_krb5: refactor code to return correct
 PTR_ERR in krb5_DK



> On Jul 14, 2024, at 12:31 AM, NeilBrown <neilb@...e.de> wrote:
> 
> On Sat, 13 Jul 2024, Chuck Lever wrote:
>> On Fri, Jul 12, 2024 at 09:39:08AM -0400, Chuck Lever wrote:
>>> On Fri, Jul 12, 2024 at 03:24:23PM +0800, Gaosheng Cui wrote:
>>>> Refactor the code in krb5_DK to return PTR_ERR when an error occurs.
>>> 
>>> My understanding of the current code is that if either
>>> crypto_alloc_sync_skcipher() or crypto_sync_skcipher_blocksize()
>>> fails, then krb5_DK() returns -EINVAL. At the only call site for
>>> krb5_DK(), that return code is unconditionally discarded. Thus I
>>> don't see that the proposed change is necessary or improves
>>> anything.
>> 
>> My understanding is wrong  ;-)
> 
> True, but I think your conclusion was correct.
> 
> krb5_DK() returns zero or -EINVAL.
> It is only used by krb5_derive_key_v2(), which returns zero or -EINVAL,
> or -ENOMEM.

These are really the only three interesting return codes.
Leaking other error codes to callers is not desirable, IMO.

But looking at the current implementation of
crypto_alloc_sync_skcipher(), it returns either
ERR_PTR(-EINVAL) or a valid pointer; it doesn't return any
other error value. Since it never returns -ENOMEM, there
still doesn't seem to be a technical reason for modifying
krb5_DK() to pass errors through.


> krb4_derive_key_v2() is only used as a ->derive_key() method.
> This is called from krb5_derive_key(), and various unit tests in
> gss_krb5_tests.c
> 
> krb5_derive_key() is only called in gss_krb5_mech.c, and each call site
> is of the form:
>  if (krb5_derive_key(...)) goto out;
> so it doesn't matter what error is returned.
> 
> The unit test calls are all followed by
> KUNIT_ASSERT_EQ(test, err, 0);
> so the only place the err is used is (presumably) in failure reports
> from the unit tests.
> 
> So the proposed change seems unnecessary from a practical perspective.
> 
> Maybe it is justified from an aesthetic perspective, but I think that
> should be clearly stated in the commit message.  e.g.
> 
>  This change has no practical effect as all non-zero error statuses
>  are treated equally, however the distinction between EINVAL and ENOMEM
>  may be relevant at some future time and it seems cleaner to maintain
>  the distinction.
> 
> NeilBrown
> 
> 
>> 
>> The return code isn't discarded. A non-zero return code from
>> krb5_DK() is carried back up the call stack. The logic in
>> krb5_derive_key_v2() does not use the kernel's usual error flow
>> form, so I missed this.
>> 
>> However, it still isn't clear to me why the error behavior here
>> needs to change. It's possible, for example, that -EINVAL is
>> perfectly adequate to indicate when sync_skcipher() can't find the
>> specified encryption algorithm (gk5e->encrypt_name).
>> 
>> Specifying the wrong encryption type: -EINVAL. That makes sense.
>> 
>> 
>>>> Signed-off-by: Gaosheng Cui <cuigaosheng1@...wei.com>
>>>> ---
>>>> v2: Update IS_ERR to PTR_ERR, thanks very much!
>>>> net/sunrpc/auth_gss/gss_krb5_keys.c | 8 ++++++--
>>>> 1 file changed, 6 insertions(+), 2 deletions(-)
>>>> 
>>>> diff --git a/net/sunrpc/auth_gss/gss_krb5_keys.c b/net/sunrpc/auth_gss/gss_krb5_keys.c
>>>> index 4eb19c3a54c7..5ac8d06ab2c0 100644
>>>> --- a/net/sunrpc/auth_gss/gss_krb5_keys.c
>>>> +++ b/net/sunrpc/auth_gss/gss_krb5_keys.c
>>>> @@ -164,10 +164,14 @@ static int krb5_DK(const struct gss_krb5_enctype *gk5e,
>>>> goto err_return;
>>>> 
>>>> cipher = crypto_alloc_sync_skcipher(gk5e->encrypt_name, 0, 0);
>>>> - if (IS_ERR(cipher))
>>>> + if (IS_ERR(cipher)) {
>>>> + ret = PTR_ERR(cipher);
>>>> goto err_return;
>>>> + }
>>>> +
>>>> blocksize = crypto_sync_skcipher_blocksize(cipher);
>>>> - if (crypto_sync_skcipher_setkey(cipher, inkey->data, inkey->len))
>>>> + ret = crypto_sync_skcipher_setkey(cipher, inkey->data, inkey->len);
>>>> + if (ret)
>>>> goto err_free_cipher;
>>>> 
>>>> ret = -ENOMEM;
>>>> -- 
>>>> 2.25.1
>>>> 
>>> 
>>> -- 
>>> Chuck Lever
>>> 
>> 
>> -- 
>> Chuck Lever
>> 
> 

--
Chuck Lever