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To:     linux-security-module@vger.kernel.org
From:   Fan Wu <wufan@linux.microsoft.com>
Subject: Question about inode security blob
Message-ID: <156f6860-73a4-f754-b460-d64de40ff626@linux.microsoft.com>
Date:   Mon, 11 Jan 2021 16:56:09 -0800
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List-ID: <linux-security-module.vger.kernel.org>

Hi,

I'm trying to learn the security blob infrastructure for my future LSM 
development.

Unlike other blobs, I found inode security blob has a special pattern. I 
couldn’t find useful information on the web so I think this mail list is 
the most appropriate place to ask this question.

The BPF and SELinux will check whether the inode->i_security is NULL 
before use
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/bpf_lsm.h#n35
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/security/selinux/include/objsec.h#n164

But for smack, it doesn't do such a check
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/security/smack/smack.h#n347
Is this because smack_set_mnt_opts() already does the NULL check at
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/security/smack/smack_lsm.c#n784 
?

Also, I wonder in which situation will the inode->i_security be NULL?

Thanks, and I hope I could make my contributions to LSM soon.

Best,
Fan