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

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Message-ID: <CAC_TJvc2MwQHvY_ry=a4CGA_zEw2TE8R_K520Eizjt83At0N_Q@mail.gmail.com>
Date: Tue, 16 Sep 2025 13:08:29 -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 11:51 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 16 Sep 2025 11:23:03 -0700
> Kalesh Singh <kaleshsingh@...gle.com> wrote:
>
> > > 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?
>
> Ah, right. Can't the BPF infrastructure protect against it?
>
> >
> > >
> > > 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.
>
> Does it block all the other trace events that share pointers?
>
> Like nmi handler tracepoints, x86_fpu tracepoints, page_fault kernel
> tracepoint, tasklet tracepoints, cpu hot plug tracepoints, timer
> tracepoints, work queue tracepoints, ipi tracepoints, scheduling
> tracepoints, locking tracepoints, rcu tracepoints, dma tracepoints,
> module tracepoints, xdp tracepoints, filemap tracepoints, page map
> tracepoints, vmscan tracepoints, percpu tracepoints, kmem_cache
> tracepoints, mmap lock tracepoints, file lock tracepoints, and many
> more! (I got tired of looking them up).

Hi Steve,

I see your point :)  I'll use the raw pointer and handle not exposing
it from the BPF side.

Thanks for discussing.

--Kalesh

>
> Again, are you really protecting anything if this one trace point
> hashes the pointer? Most other tracepoints expose this. If BPF can
> access a tracepoint entry struct, it can access the raw address and
> break KASLR.

Thanks,
Kalesh
>
> -- Steve