linux-kernel - Re: [PATCH v2 7/7] mm/tracing: introduce max_vma_count

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Message-ID: <CAC_TJvfAQDiL9PydWnKE6TDMcCzw0xrsLMZVZLe6eO0R1LODhA@mail.gmail.com>
Date: Tue, 16 Sep 2025 11:23:03 -0700
From: Kalesh Singh <kaleshsingh@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: akpm@...ux-foundation.org, minchan@...nel.org, lorenzo.stoakes@...cle.com, 
	david@...hat.com, Liam.Howlett@...cle.com, rppt@...nel.org, pfalcato@...e.de, 
	kernel-team@...roid.com, android-mm@...gle.com, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Jann Horn <jannh@...gle.com>, Shuah Khan <shuah@...nel.org>, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 7/7] mm/tracing: introduce max_vma_count_exceeded trace event

On Tue, Sep 16, 2025 at 11:01 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 16 Sep 2025 10:57:43 -0700
> Kalesh Singh <kaleshsingh@...gle.com> wrote:
>
> > > BTW, why the hash of the mm pointer and not the pointer itself? We save
> > > pointers in lots of places, and if it is the pointer, you could use an
> > > eprobe to attache to the trace event to dereference its fields.
> >
> > In Android we try to avoid exposing raw kernel pointers to userspace
> > for security reasons: raising /proc/sys/kernel/kptr_restrict to 2
> > immediately after symbols are resolved for necessary telemetry tooling
> > during early boot. I believe this is also why rss_stat uses the hash
> > and not the raw pointer.
>
> When it comes to tracing, you already lost. If it goes into the ring buffer
> it's a raw pointer. BPF doesn't use the output of the trace event, so you
> are exposing nothing from that. It uses the proto directly.

My understanding is that the BPF tracepoint type uses the trace event
fields from TP_STRUCT__entry(); whereas the raw tracepoint type has
access to the proto arguments. Please CMIW: Isn't what we'd be adding
to the trace buffer is the hashed mm_id value?

>
> Heck, if you enable function tracing, you are exposing every function
> address it traces via the raw data output.

Right, security doesn't allow compiling CONFIG_FUNCTION_TRACER  in
Android production kernels.

Thanks,
Kalesh

>
> -- Steve