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From: Masami Hiramatsu <mhiramat@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Shuah Khan <shuah@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Changbin Du <changbin.du@gmail.com>, Jann Horn <jannh@google.com>,
	Kees Cook <keescook@chromium.org>,
	Andy Lutomirski <luto@kernel.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Nadav Amit <namit@vmware.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	yhs@fb.com
Subject: Re: [RFC PATCH v6 4/6] tracing/probe: Support user-space dereference
Date: Mon, 13 May 2019 21:11:30 +0900	[thread overview]
Message-ID: <20190513211130.24735357a329cfdc25fcecf9@kernel.org> (raw)
In-Reply-To: <20190508112237.76bd0e6b@gandalf.local.home>

On Wed, 8 May 2019 11:22:37 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> > > >  Per-Probe Event Filtering
> > > >  -------------------------
> > > > diff --git a/Documentation/trace/uprobetracer.rst b/Documentation/trace/uprobetracer.rst
> > > > index 4346e23e3ae7..de8812c932bc 100644
> > > > --- a/Documentation/trace/uprobetracer.rst
> > > > +++ b/Documentation/trace/uprobetracer.rst
> > > > @@ -42,16 +42,17 @@ Synopsis of uprobe_tracer
> > > >     @+OFFSET	: Fetch memory at OFFSET (OFFSET from same file as PATH)
> > > >     $stackN	: Fetch Nth entry of stack (N >= 0)
> > > >     $stack	: Fetch stack address.
> > > > -   $retval	: Fetch return value.(*)
> > > > +   $retval	: Fetch return value.(\*1)
> > > >     $comm	: Fetch current task comm.
> > > > -   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
> > > > +   +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3)
> > > >     NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
> > > >     FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
> > > >  		       (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
> > > >  		       (x8/x16/x32/x64), "string" and bitfield are supported.  
> > > 
> > > Hmm, shouldn't uprobes default to userspace. Isn't the purpose mostly
> > > to find out what's going on in userspace. Perhaps we should add a 'k'
> > > annotation to uprobes to denote that it's for kernel space, as that
> > > should be the exception and not the norm.  
> > 
> > No, uprobe can not access kernel space, because it doesn't have the
> > current kernel context. Note that all registers, stacks which
> > can be accessed from uprobe handler are user-space. We can not access
> > kernel context from that. See below
> > 
> > > > -  (*) only for return probe.
> > > > -  (**) this is useful for fetching a field of data structures.
> > > > +  (\*1) only for return probe.
> > > > +  (\*2) this is useful for fetching a field of data structures.
> > > > +  (\*3) Unlike kprobe event, "u" prefix will just be ignored.  
> > 
> > Thus the 'u' is just ignored on uprobe event.
> 
> I totally missed the footnote here. Can we stress this point more up in
> the "User Memory Access" section. Specifically state something like:
> "Uprobes only access userspace memory, thus the 'u' is not required,
> and if it is added to a uprobe, it will simply be ignored".

Sorry, I missed this mail. 

Since the "User Memory Access" section is only in kprobetrace.rst, I think
mentioning uprobe-events in kprobetrace.rst is meaningless. Uprobe user
might read uprobetracer.rst instead of kprobetrace.rst.
So I think it is enough to mention it as a footnote in uprobetracer.rst.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>

  reply	other threads:[~2019-05-13 12:11 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-18  6:42 [RFC PATCH v6 0/6] tracing/probes: uaccess: Add support user-space access Masami Hiramatsu
2019-03-18  6:43 ` [RFC PATCH v6 1/6] x86/uaccess: Allow access_ok() in irq context if pagefault_disabled Masami Hiramatsu
2019-03-22  2:46   ` Steven Rostedt
2019-05-06 15:22     ` Masami Hiramatsu
2019-05-06 15:39       ` Steven Rostedt
2019-03-18  6:43 ` [RFC PATCH v6 2/6] uaccess: Add non-pagefault user-space read functions Masami Hiramatsu
2019-03-18  6:43 ` [RFC PATCH v6 3/6] tracing/probe: Add ustring type for user-space string Masami Hiramatsu
2019-03-18  6:43 ` [RFC PATCH v6 4/6] tracing/probe: Support user-space dereference Masami Hiramatsu
2019-05-06 15:52   ` Steven Rostedt
2019-05-08  4:11     ` Masami Hiramatsu
2019-05-08 15:22       ` Steven Rostedt
2019-05-13 12:11         ` Masami Hiramatsu [this message]
2019-03-18  6:44 ` [RFC PATCH v6 5/6] selftests/ftrace: Add user-memory access syntax testcase Masami Hiramatsu
2019-03-18  6:44 ` [RFC PATCH v6 6/6] perf-probe: Add user memory access attribute support Masami Hiramatsu

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