Re: [PATCH 1/4] ns: add bpf hooks

[Matt Bobrowski <mattbobrowski@google.com>]

On Fri, Feb 27, 2026 at 11:33:56AM +0100, Christian Brauner wrote:
> On Tue, Feb 24, 2026 at 01:16:01AM +0000, Matt Bobrowski wrote:
> > On Fri, Feb 20, 2026 at 01:38:29AM +0100, Christian Brauner wrote:
> > > Add the three namespace lifecycle hooks and make them available to bpf
> > > lsm program types. This allows bpf to supervise namespace creation. I'm
> > > in the process of adding various "universal truth" bpf programs to
> > > systemd that will make use of this. This e.g., allows to lock in a
> > > program into a given set of namespaces.
> > > 
> > > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > > ---
> > >  include/linux/bpf_lsm.h | 21 +++++++++++++++++++++
> > >  kernel/bpf/bpf_lsm.c    | 25 +++++++++++++++++++++++++
> > >  kernel/nscommon.c       |  9 ++++++++-
> > >  kernel/nsproxy.c        |  7 +++++++
> > >  4 files changed, 61 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> > > index 643809cc78c3..5ae438fdf567 100644
> > > --- a/include/linux/bpf_lsm.h
> > > +++ b/include/linux/bpf_lsm.h
> > > @@ -12,6 +12,9 @@
> > >  #include <linux/bpf_verifier.h>
> > >  #include <linux/lsm_hooks.h>
> > >  
> > > +struct ns_common;
> > > +struct nsset;
> > > +
> > >  #ifdef CONFIG_BPF_LSM
> > >  
> > >  #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
> > > @@ -48,6 +51,11 @@ void bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog, bpf_func_t *bpf_func)
> > >  
> > >  int bpf_lsm_get_retval_range(const struct bpf_prog *prog,
> > >  			     struct bpf_retval_range *range);
> > > +
> > > +int bpf_lsm_namespace_alloc(struct ns_common *ns);
> > > +void bpf_lsm_namespace_free(struct ns_common *ns);
> > > +int bpf_lsm_namespace_install(struct nsset *nsset, struct ns_common *ns);
> > > +
> > >  int bpf_set_dentry_xattr_locked(struct dentry *dentry, const char *name__str,
> > >  				const struct bpf_dynptr *value_p, int flags);
> > >  int bpf_remove_dentry_xattr_locked(struct dentry *dentry, const char *name__str);
> > > @@ -104,6 +112,19 @@ static inline bool bpf_lsm_has_d_inode_locked(const struct bpf_prog *prog)
> > >  {
> > >  	return false;
> > >  }
> > > +
> > > +static inline int bpf_lsm_namespace_alloc(struct ns_common *ns)
> > > +{
> > > +	return 0;
> > > +}
> > > +static inline void bpf_lsm_namespace_free(struct ns_common *ns)
> > > +{
> > > +}
> > > +static inline int bpf_lsm_namespace_install(struct nsset *nsset,
> > > +					    struct ns_common *ns)
> > > +{
> > > +	return 0;
> > > +}
> > >  #endif /* CONFIG_BPF_LSM */
> > >  
> > >  #endif /* _LINUX_BPF_LSM_H */
> > > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> > > index 0c4a0c8e6f70..f6378db46220 100644
> > > --- a/kernel/bpf/bpf_lsm.c
> > > +++ b/kernel/bpf/bpf_lsm.c
> > > @@ -30,10 +30,32 @@ __weak noinline RET bpf_lsm_##NAME(__VA_ARGS__)	\
> > >  #include <linux/lsm_hook_defs.h>
> > >  #undef LSM_HOOK
> > >  
> > > +__bpf_hook_start();
> > > +
> > > +__weak noinline int bpf_lsm_namespace_alloc(struct ns_common *ns)
> > > +{
> > > +	return 0;
> > > +}
> > > +
> > > +__weak noinline void bpf_lsm_namespace_free(struct ns_common *ns)
> > > +{
> > > +}
> > > +
> > > +__weak noinline int bpf_lsm_namespace_install(struct nsset *nsset,
> > > +					  struct ns_common *ns)
> > > +{
> > > +	return 0;
> > > +}
> > > +
> > > +__bpf_hook_end();
> > 
> > Is the usage of __bpf_hook_start()/__bpf_hook_end() strictly necessary
> > here? If so, why is that? My understanding was that they're only
> > needed in situations where public function prototypes don't exist
> > (e.g., BPF kfuncs).
> 
> I don't know. I just went by other sites that added bpf specific
> functions. Seems like bpf specific functions I'm adding so I used the
> hook annotation. If unneeded I happily drop it. I just need someone to
> tell whether that's right and I can't infer from your "my understanding
> [...]" phrasing whether that's an authoritative statement or an
> expression of doubt.

Truly apologies about the delay here Christian, I've been out of
office the last few weeks.

Initially an expression of doubt, but now an authoritative
statement. You do not need your new BPF LSM specific hooks wrapped
within __bpf_hook_start() and __bpf_hook_end(). Those are technically
for BPF kfuncs which are global functions, but are often only called
from a BPF program. The default BPF LSM hook definitions provided by
the LSM_HOOK() macro also aren't wrapped in __bpf_hook_start() and
__bpf_hook_end().