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Subject: Re: [PATCH v5] tracepoint: Do not fail unregistering a probe due to
 memory failure
To:     Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matt Mullins <mmullins@mmlx.us>, paulmck <paulmck@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
References: <20210127170721.58bce7cc@gandalf.local.home>
From:   Alexey Kardashevskiy <aik@ozlabs.ru>
Message-ID: <ac000ed4-3eeb-60df-f896-c05cabb4c1c9@ozlabs.ru>
Date:   Sat, 30 Jan 2021 23:30:18 +1100
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On 28/01/2021 09:07, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> The list of tracepoint callbacks is managed by an array that is protected
> by RCU. To update this array, a new array is allocated, the updates are
> copied over to the new array, and then the list of functions for the
> tracepoint is switched over to the new array. After a completion of an RCU
> grace period, the old array is freed.
> 
> This process happens for both adding a callback as well as removing one.
> But on removing a callback, if the new array fails to be allocated, the
> callback is not removed, and may be used after it is freed by the clients
> of the tracepoint.
> 
> The handling of a failed allocation for removing an event can break use
> cases as the error report is not propagated up to the original callers. To
> make matters worse, there's some paths that can not handle error cases.
> 
> Instead of allocating a new array for removing a tracepoint, allocate twice
> the needed size when adding tracepoints to the array. On removing, use the
> second half of the allocated array. This removes the need to allocate memory
> for removing a tracepoint, as the allocation for removals will already have
> been done.
> 
> Link: https://lkml.kernel.org/r/20201115055256.65625-1-mmullins@mmlx.us
> Link: https://lkml.kernel.org/r/20201116175107.02db396d@gandalf.local.home
> Link: https://lkml.kennel.org/r/20201118093405.7a6d2290@gandalf.local.home
> 
> Reported-by: Matt Mullins <mmullins@mmlx.us>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>


I still need the following chunk (same "if (it_func_ptr)" as in the v2's 
reply) in order to stop crashes:


diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 82eba6a05a1c..b7cf7a5a4f43 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -311,6 +311,7 @@ static inline struct tracepoint 
*tracepoint_ptr_deref(tracepoint_ptr_t *p)
                                                                         \
                 it_func_ptr =                                           \
 
rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
+               if (it_func_ptr) \
                 do {                                                    \
                         it_func = (it_func_ptr)->func;                  \
                         __data = (it_func_ptr)->data;                   \


-- 
Alexey