bpf: Remove now-defunct task kfuncs

In commit 22df776a9a86 ("tasks: Extract rcu_users out of union"), the
'refcount_t rcu_users' field was extracted out of a union with the
'struct rcu_head rcu' fie

bpf: Remove now-defunct task kfuncs

In commit 22df776a9a86 ("tasks: Extract rcu_users out of union"), the
'refcount_t rcu_users' field was extracted out of a union with the
'struct rcu_head rcu' field. This allows us to safely perform a
refcount_inc_not_zero() on task->rcu_users when acquiring a reference on
a task struct. A prior patch leveraged this by making struct task_struct
an RCU-protected object in the verifier, and by bpf_task_acquire() to
use the task->rcu_users field for synchronization.

Now that we can use RCU to protect tasks, we no longer need
bpf_task_kptr_get(), or bpf_task_acquire_not_zero(). bpf_task_kptr_get()
is truly completely unnecessary, as we can just use RCU to get the
object. bpf_task_acquire_not_zero() is now equivalent to
bpf_task_acquire().

In addition to these changes, this patch also updates the associated
selftests to no longer use these kfuncs.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230331195733.699708-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

show more ...

bpf: Make struct task_struct an RCU-safe type

struct task_struct objects are a bit interesting in terms of how their
lifetime is protected by refcounts. task structs have two refcount
fields:

1. re

bpf: Make struct task_struct an RCU-safe type

struct task_struct objects are a bit interesting in terms of how their
lifetime is protected by refcounts. task structs have two refcount
fields:

1. refcount_t usage: Protects the memory backing the task struct. When
this refcount drops to 0, the task is immediately freed, without
waiting for an RCU grace period to elapse. This is the field that
most callers in the kernel currently use to ensure that a task
remains valid while it's being referenced, and is what's currently
tracked with bpf_task_acquire() and bpf_task_release().

2. refcount_t rcu_users: A refcount field which, when it drops to 0,
schedules an RCU callback that drops a reference held on the 'usage'
field above (which is acquired when the task is first created). This
field therefore provides a form of RCU protection on the task by
ensuring that at least one 'usage' refcount will be held until an RCU
grace period has elapsed. The qualifier "a form of" is important
here, as a task can remain valid after task->rcu_users has dropped to
0 and the subsequent RCU gp has elapsed.

In terms of BPF, we want to use task->rcu_users to protect tasks that
function as referenced kptrs, and to allow tasks stored as referenced
kptrs in maps to be accessed with RCU protection.

Let's first determine whether we can safely use task->rcu_users to
protect tasks stored in maps. All of the bpf_task* kfuncs can only be
called from tracepoint, struct_ops, or BPF_PROG_TYPE_SCHED_CLS, program
types. For tracepoint and struct_ops programs, the struct task_struct
passed to a program handler will always be trusted, so it will always be
safe to call bpf_task_acquire() with any task passed to a program.
Note, however, that we must update bpf_task_acquire() to be KF_RET_NULL,
as it is possible that the task has exited by the time the program is
invoked, even if the pointer is still currently valid because the main
kernel holds a task->usage refcount. For BPF_PROG_TYPE_SCHED_CLS, tasks
should never be passed as an argument to the any program handlers, so it
should not be relevant.

The second question is whether it's safe to use RCU to access a task
that was acquired with bpf_task_acquire(), and stored in a map. Because
bpf_task_acquire() now uses task->rcu_users, it follows that if the task
is present in the map, that it must have had at least one
task->rcu_users refcount by the time the current RCU cs was started.
Therefore, it's safe to access that task until the end of the current
RCU cs.

With all that said, this patch makes struct task_struct is an
RCU-protected object. In doing so, we also change bpf_task_acquire() to
be KF_ACQUIRE | KF_RCU | KF_RET_NULL, and adjust any selftests as
necessary. A subsequent patch will remove bpf_task_kptr_get(), and
bpf_task_acquire_not_zero() respectively.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230331195733.699708-2-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

show more ...

bpf: Rename __kptr_ref -> __kptr and __kptr -> __kptr_untrusted.

__kptr meant to store PTR_UNTRUSTED kernel pointers inside bpf maps.
The concept felt useful, but didn't get much traction,
since bpf

bpf: Rename __kptr_ref -> __kptr and __kptr -> __kptr_untrusted.

__kptr meant to store PTR_UNTRUSTED kernel pointers inside bpf maps.
The concept felt useful, but didn't get much traction,
since bpf_rdonly_cast() was added soon after and bpf programs received
a simpler way to access PTR_UNTRUSTED kernel pointers
without going through restrictive __kptr usage.

Rename __kptr_ref -> __kptr and __kptr -> __kptr_untrusted to indicate
its intended usage.
The main goal of __kptr_untrusted was to read/write such pointers
directly while bpf_kptr_xchg was a mechanism to access refcnted
kernel pointers. The next patch will allow RCU protected __kptr access
with direct read. At that point __kptr_untrusted will be deprecated.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230303041446.3630-2-alexei.starovoitov@gmail.com

show more ...

selftests/bpf: Add selftests for bpf_task_from_pid()

Add some selftest testcases that validate the expected behavior of the
bpf_task_from_pid() kfunc that was added in the prior patch.

Signed-off-b

selftests/bpf: Add selftests for bpf_task_from_pid()

Add some selftest testcases that validate the expected behavior of the
bpf_task_from_pid() kfunc that was added in the prior patch.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122145300.251210-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

show more ...

bpf/selftests: Add selftests for new task kfuncs

A previous change added a series of kfuncs for storing struct
task_struct objects as referenced kptrs. This patch adds a new
task_kfunc test suite fo

bpf/selftests: Add selftests for new task kfuncs

A previous change added a series of kfuncs for storing struct
task_struct objects as referenced kptrs. This patch adds a new
task_kfunc test suite for validating their expected behavior.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221120051004.3605026-5-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

show more ...