Re: [RFC PATCH] perf, bpf: Retain kernel executable code in memory to aid Intel PT tracing

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 07, 2019 at 01:19:01PM +0200, Adrian Hunter wrote:
> Subject to memory pressure and other limits, retain executable code, such
> as JIT-compiled bpf, in memory instead of freeing it immediately it is no
> longer needed for execution.
> 
> While perf is primarily aimed at statistical analysis, tools like Intel
> PT can aim to provide a trace of exactly what happened. As such, corner
> cases that can be overlooked statistically need to be addressed. For
> example, there is a gap where JIT-compiled bpf can be freed from memory
> before a tracer has a chance to read it out through the bpf syscall.
> While that can be ignored statistically, it contributes to a death by
> 1000 cuts for tracers attempting to assemble exactly what happened. This is
> a bit gratuitous given that retaining the executable code is relatively
> simple, and the amount of memory involved relatively small. The retained
> executable code is then available in memory images such as /proc/kcore.
> 
> This facility could perhaps be extended also to init sections.
> 
> Note that this patch is compile tested only and, at present, is missing
> the ability to retain symbols.

You don't need the symbols; you already have them through
PERF_RECORD_KSYMBOL.

Also; afaict this patch guarantees exactly nothing. It registers a
shrinker which will (given enough memory pressure) happily free your
text before we get around to copying it out.

Did you read this proposal?

  https://lkml.kernel.org/r/20190109101808.GG1900@hirez.programming.kicks-ass.net

(also: s/KCORE_QC/KCORE_QS/ for quiescent state)

That would create an RCU like interface to /proc/kcore and give you the
guarantees you need, while also allowing the memory to get freed once
you've obtained a copy.