linux-kernel - Re: test-case (Was: [PATCH 12/X] uprobes: x86: introduce abort

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Date:	Tue, 25 Oct 2011 19:36:13 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: test-case (Was: [PATCH 12/X] uprobes: x86: introduce
 abort_xol())

> 
> Ananth, Srikar, I'd suggest this test-case:
> 
> 	#include <stdio.h>
> 	#include <signal.h>
> 	#include <ucontext.h>
> 
> 	void *fault_insn;
> 
> 	static inline void *uc_ip(struct ucontext *ctxt)
> 	{
> 		return (void*)ctxt->uc_mcontext.gregs[16];
> 	}
> 
> 	void segv(int sig, siginfo_t *info, void *ctxt)
> 	{
> 		static int cnt;
> 
> 		printf("SIGSEGV! ip=%p addr=%p\n", uc_ip(ctxt), info->si_addr);
> 
> 		if (uc_ip(ctxt) != fault_insn)
> 			printf("ERR!! wrong ip\n");
> 		if (info->si_addr != (void*)0x12345678)
> 			printf("ERR!! wrong addr\n");
> 
> 		if (++cnt == 3)
> 			signal(SIGSEGV, SIG_DFL);
> 	}
> 
> 	int main(void)
> 	{
> 		struct sigaction sa = {
> 			.sa_sigaction	= segv,
> 			.sa_flags	= SA_SIGINFO,
> 		};
> 
> 		sigaction(SIGSEGV, &sa, NULL);
> 
> 		fault_insn = &&label;
> 
> 	label:
> 		asm volatile ("movl $0x0,0x12345678");
> 
> 		return 0;
> 	}
> 
> result:
> 
> 	$ ulimit -c unlimited
> 
> 	$ ./segv
> 	SIGSEGV! ip=0x4006eb addr=0x12345678
> 	SIGSEGV! ip=0x4006eb addr=0x12345678
> 	SIGSEGV! ip=0x4006eb addr=0x12345678
> 	Segmentation fault (core dumped)
> 
> 	$ gdb -c ./core.1826
> 	...
> 	Program terminated with signal 11, Segmentation fault.
> 	#0  0x00000000004006eb in ?? ()
> 
> Now. If you insert uprobe at asm("movl") insn, result should be the same
> or the patches I sent are wrong. In particular, the addr in the coredump
> should be correct too. And consumer->handler() should be called 3 times
> too. This insn is really executed 3 times.
> 
> I have no idea how can I test this.
> 

I have tested this on both x86_32 and x86_64 and can confirm that the
behaviour is same with or without uprobes placed at that instruction.
This is on the uprobes code with your changes.

However on x86_32; the output is different from x86_64. 

On x86_32 (I have additionally printed the uc_ip and fault_insn.

SIGSEGV! ip=0x10246 addr=0x12345678
ERR!! wrong ip uc_ip(ctxt) = 10246 fault_insn = 804856c
SIGSEGV! ip=0x10246 addr=0x12345678
ERR!! wrong ip uc_ip(ctxt) = 10246 fault_insn = 804856c
SIGSEGV! ip=0x10246 addr=0x12345678
ERR!! wrong ip uc_ip(ctxt) = 10246 fault_insn = 804856c
Segmentation fault

the fault_insn matches the address shown in disassemble of gdb.
I still trying to dig up what uc_ip is and why its different on x86_32.

On x86_64 the result is what you pasted above.


Also I was thinking on your suggestion of making abort_xol a weak
function.  In which case we could have architecture independent function
in kernel/uprobes.c which is just a wrapper for set_instruction_pointer. 

void __weak abort_xol(struct pt_regs *regs, struct uprobe_task *utask)
{
	set_instruction_pointer(regs, utask->vaddr);	
}

where it would called  from uprobe_notify_resume() as 

	abort_xol(regs, utask);

If other archs would want to do something else, they could override
abort_xol definition.

-- 
Thanks and Regards
Srikar

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