linux-kernel - Re: [PATCH 08/12] add trace events for each syscall entry/exit

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Message-ID: <20090825141547.GE6114@nowhere>
Date:	Tue, 25 Aug 2009 16:15:49 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, rostedt@...dmis.org,
	peterz@...radead.org, mathieu.desnoyers@...ymtl.ca,
	jiayingz@...gle.com, mbligh@...gle.com, lizf@...fujitsu.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 08/12] add trace events for each syscall entry/exit

On Tue, Aug 25, 2009 at 02:50:27PM +0200, Hendrik Brueckner wrote:
> On Mon, Aug 10, 2009 at 04:52:47PM -0400, Jason Baron wrote:
> > +void ftrace_syscall_enter(struct pt_regs *regs, long id)
> >  {
> >  	struct syscall_trace_enter *entry;
> >  	struct syscall_metadata *sys_data;
> > @@ -150,6 +105,8 @@ void ftrace_syscall_enter(struct pt_regs *regs)
> >  	int syscall_nr;
> > 
> >  	syscall_nr = syscall_get_nr(current, regs);
> > +	if (!test_bit(syscall_nr, enabled_enter_syscalls))
> > +		return;
> > 
> >  	sys_data = syscall_nr_to_meta(syscall_nr);
> >  	if (!sys_data)
> 
> > +void ftrace_syscall_exit(struct pt_regs *regs, long ret)
> >  {
> >  	struct syscall_trace_exit *entry;
> >  	struct syscall_metadata *sys_data;
> > @@ -178,6 +135,8 @@ void ftrace_syscall_exit(struct pt_regs *regs)
> >  	int syscall_nr;
> > 
> >  	syscall_nr = syscall_get_nr(current, regs);
> > +	if (!test_bit(syscall_nr, enabled_exit_syscalls))
> > +		return;
> Most arch syscall_get_nr() implementations returns -1 if the syscall
> number is not valid.  Accessing the bit field without a check might
> result in a kernel oops (at least I saw it on s390 for ftrace selftest).
> 
> Before this change, this problem did not occur, because the invalid
> syscall number (-1) caused syscall_nr_to_meta() to return NULL.
> 
> There are at least two scenarios where syscall_get_nr() can return -1:
> 
> 1. For example, ptrace stores an invalid syscall number, and thus,
>    tracing code resets it.
>    (see do_syscall_trace_enter in arch/s390/kernel/ptrace.c)
> 
> 2. The syscall_regfunc() (kernel/tracepoint.c) sets the TIF_SYSCALL_FTRACE
>    (now: TIF_SYSCALL_TRACEPOINT) flag for all threads which includes
>    kernel threads.
>    However, the ftrace selftest triggers a kernel oops when testing syscall
>    trace points:
>       - The kernel thread is started as ususal (do_fork()),
>       - tracing code sets TIF_SYSCALL_FTRACE,
>       - the ret_from_fork() function is triggered and starts
> 	ftrace_syscall_exit() with an invalid syscall number.



I wonder if there is any way to identify such situation...?


> 
> To avoid these scenarios, I suggest to check the syscall_nr.
> 
> For instance, the ftrace selftest fails for s390 (with config option
> CONFIG_FTRACE_SYSCALLS set) and produces the following kernel oops.
> 
> Unable to handle kernel pointer dereference at virtual kernel address 2000000000
> 
> Oops: 0038 [#1] PREEMPT SMP
> Modules linked in:
> CPU: 0 Not tainted 2.6.31-rc6-next-20090819-dirty #18
> Process kthreadd (pid: 818, task: 000000003ea207e8, ksp: 000000003e813eb8)
> Krnl PSW : 0704100180000000 00000000000ea54c (ftrace_syscall_exit+0x58/0xdc)
>            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:1 PM:0 EA:3
> Krnl GPRS: 0000000000000000 00000000000e0000 ffffffffffffffff 20000000008c2650
>            0000000000000007 0000000000000000 0000000000000000 0000000000000000
>            0000000000000000 0000000000000000 ffffffffffffffff 000000003e813d78
>            000000003e813f58 0000000000505ba8 000000003e813e18 000000003e813d78
> Krnl Code: 00000000000ea540: e330d0000008       ag      %r3,0(%r13)
>            00000000000ea546: a7480007           lhi     %r4,7
>            00000000000ea54a: 1442               nr      %r4,%r2
>           >00000000000ea54c: e31030000090       llgc    %r1,0(%r3)
>            00000000000ea552: 5410d008           n       %r1,8(%r13)
>            00000000000ea556: 8a104000           sra     %r1,0(%r4)
>            00000000000ea55a: 5410d00c           n       %r1,12(%r13)
>            00000000000ea55e: 1211               ltr     %r1,%r1
> Call Trace:
> ([<0000000000000000>] 0x0)
>  [<000000000001fa22>] do_syscall_trace_exit+0x132/0x18c
>  [<000000000002d0c4>] sysc_return+0x0/0x8
>  [<000000000001c738>] kernel_thread_starter+0x0/0xc
> Last Breaking-Event-Address:
>  [<00000000000ea51e>] ftrace_syscall_exit+0x2a/0xdc
> 
> Signed-off-by: Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>



Yeah, makes sense.
Acked-by: Frederic Weisbecker <fweisbec@...il.com>

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