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From: Jiri Olsa <olsajiri@gmail.com>
Date: Tue, 9 Apr 2024 09:57:54 +0200
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jiri Olsa <olsajiri@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, linux-kernel@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Borislav Petkov (AMD)" <bp@alien8.de>, x86@kernel.org,
	linux-api@vger.kernel.org
Subject: Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return
 probe
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On Tue, Apr 09, 2024 at 09:34:39AM +0900, Masami Hiramatsu wrote:

SNIP

> > > 
> > > > this can be fixed by checking the syscall is called from the trampoline
> > > > and prevent handle_trampoline call if it's not
> > > 
> > > Yes, but I still do not think this makes a lot of sense. But I won't argue.
> > > 
> > > And what should sys_uretprobe() do if it is not called from the trampoline?
> > > I'd prefer force_sig(SIGILL) to punish the abuser ;) OK, OK, EINVAL.
> > 
> > so the similar behaviour with int3 ends up with immediate SIGTRAP
> > and not invoking pending uretprobe consumers, like:
> > 
> >   - setup uretprobe for foo
> >   - foo() {
> >       executes int 3 -> sends SIGTRAP
> >     }
> > 
> > because the int3 handler checks if it got executed from the uretprobe's
> > trampoline.. if not it treats that int3 as regular trap
> 
> Yeah, that is consistent behavior. Sounds good to me.
> 
> > 
> > while for uretprobe syscall we have at the moment following behaviour:
> > 
> >   - setup uretprobe for foo
> >   - foo() {
> >      uretprobe_syscall -> executes foo's uretprobe consumers
> >     }
> >   - at some point we get to the 'ret' instruction that jump into uretprobe
> >     trampoline and the uretprobe_syscall won't find pending uretprobe and
> >     will send SIGILL
> > 
> > 
> > so I think we should mimic int3 behaviour and:
> > 
> >   - setup uretprobe for foo
> >   - foo() {
> >      uretprobe_syscall -> check if we got executed from uretprobe's
> >      trampoline and send SIGILL if that's not the case
> 
> OK, this looks good to me.
> 
> > 
> > I think it's better to have the offending process killed right away,
> > rather than having more undefined behaviour, waiting for final 'ret'
> > instruction that jumps to uretprobe trampoline and causes SIGILL
> > 
> > > 
> > > I agree very much with Andrii,
> > > 
> > >        sigreturn()  exists only to allow the implementation of signal handlers.  It should never be
> > >        called directly.  Details of the arguments (if any) passed to sigreturn() vary depending  on
> > >        the architecture.
> > > 
> > > this is how sys_uretprobe() should be treated/documented.
> > 
> > yes, will include man page patch in new version
> 
> And please follow Documentation/process/adding-syscalls.rst in new version,
> then we can avoid repeating the same discussion :-)

yep, will do

thanks,
jirka