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Date: Tue, 30 Aug 2016 11:56:33 -0700
From: Andy Lutomirski <>
To: Mickaël Salaün <>
Cc: "Serge E. Hallyn" <>, David Drysdale <>, 
	"" <>, Alexei Starovoitov <>, 
	James Morris <>, Sargun Dhillon <>, 
	Network Development <>, Casey Schaufler <>, 
	Linux API <>, Kees Cook <>, 
	LSM List <>, 
	"" <>, "David S . Miller" <>, 
	Daniel Mack <>, Arnd Bergmann <>, Will Drewry <>, 
	Paul Moore <>, Elena Reshetova <>, 
	Daniel Borkmann <>
Subject: Re: [RFC v2 06/10] landlock: Add LSM hooks

On Aug 25, 2016 12:34 PM, "Mickaël Salaün" <> wrote:
> Add LSM hooks which can be used by userland through Landlock (eBPF)
> programs. This programs are limited to a whitelist of functions (cf.
> next commit). The eBPF program context is depicted by the struct
> landlock_data (cf. include/uapi/linux/bpf.h):
> * hook: LSM hook ID (useful when using the same program for multiple LSM
>   hooks);
> * cookie: the 16-bit value from the seccomp filter that triggered this
>   Landlock program;
> * args[6]: array of LSM hook arguments.
> The LSM hook arguments can contain raw values as integers or
> (unleakable) pointers. The only way to use the pointers are to pass them
> to an eBPF function according to their types (e.g. the
> bpf_landlock_cmp_fs_beneath_with_struct_file function can use a struct
> file pointer).
> For now, there is three hooks for file system access control:
> * file_open;
> * file_permission;
> * mmap_file.

What's the purpose of exposing struct cred * to userspace?  It's
primarily just an optimization to save a bit of RAM, and it's a
dubious optimization at that.  What are you using it for?  Would it
make more sense to use struct task_struct * or struct pid * instead?

Also, exposing struct cred * has a really weird side-effect: it allows
(maybe even encourages) checking for pointer equality between two
struct cred * objects.  Doing so will have erratic results.

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