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

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Message-id: <172093150291.15471.15426043640692195014@noble.neil.brown.name>
Date: Sun, 14 Jul 2024 14:31:42 +1000
From: "NeilBrown" <neilb@...e.de>
To: "Chuck Lever" <chuck.lever@...cle.com>
Cc: "Gaosheng Cui" <cuigaosheng1@...wei.com>, trondmy@...nel.org,
 anna@...nel.org, jlayton@...nel.org, kolga@...app.com, Dai.Ngo@...cle.com,
 tom@...pey.com, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, horms@...nel.org, linux-nfs@...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 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.

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
>