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SubjectRe: [PATCHv2] x86 trace: Fix page fault tracing bug
Vince, you probably missed my other emails, as I sent from mutt, and
not my normal email client. I see that mutt uses my own box to send,
and I got these error messages (need to put back sending via my ISP
instead of my local mail server):

>>> vincent.weaver@maine.edu (after MAIL FROM): 550 5.5.4 <rostedt@home.goodmis.org>... Domain of sender address rostedt@home.goodmis.org refused. MX points to hosts without valid addresses. This violates RFC 1035/2181.

-- Steve


On Fri, 28 Feb 2014 17:05:26 +0100
Jiri Olsa <jolsa@redhat.com> wrote:

> On Fri, Feb 28, 2014 at 04:47:08PM +0100, Peter Zijlstra wrote:
> > On Fri, Feb 28, 2014 at 04:33:40PM +0100, Jiri Olsa wrote:
> >
> > While I like the idea of just pushing up the CR2 read; the below does
> > the read too late still, exception_enter() also has a tracepoint in.
>
> please check v2, thanks
>
> jirka
>
>
> ---
> The trace_do_page_fault function trigger tracepoint
> and then handles the actual page fault.
>
> This could lead to error if the tracepoint caused page
> fault. The original cr2 value gets lost and the original
> page fault handler kills current process with SIGSEGV.
>
> This happens if you record page faults with callchain
> data, the user part of it will cause tracepoint handler
> to page fault:
>
> # perf record -g -e exceptions:page_fault_user ls
>
> Fixing this by saving the original cr2 value
> and using it after tracepoint handler is done.
>
> v2: Moving the cr2 read before exception_enter, because
> it could trigger tracepoint as well.
>
> Reported-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Seiji Aguchi <seiji.aguchi@hds.com>
> Cc: Vince Weaver <vincent.weaver@maine.edu>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> ---
> arch/x86/mm/fault.c | 20 +++++++++++++-------
> 1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 9d591c8..dd59031 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1016,11 +1016,11 @@ static inline bool smap_violation(int error_code, struct pt_regs *regs)
> * routines.
> */
> static void __kprobes
> -__do_page_fault(struct pt_regs *regs, unsigned long error_code)
> +__do_page_fault(struct pt_regs *regs, unsigned long error_code,
> + unsigned long address)
> {
> struct vm_area_struct *vma;
> struct task_struct *tsk;
> - unsigned long address;
> struct mm_struct *mm;
> int fault;
> unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
> @@ -1028,9 +1028,6 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
> tsk = current;
> mm = tsk->mm;
>
> - /* Get the faulting address: */
> - address = read_cr2();
> -
> /*
> * Detect and handle instructions that would cause a page fault for
> * both a tracked kernel page and a userspace page.
> @@ -1248,9 +1245,11 @@ dotraplinkage void __kprobes
> do_page_fault(struct pt_regs *regs, unsigned long error_code)
> {
> enum ctx_state prev_state;
> + /* Get the faulting address: */
> + unsigned long address = read_cr2();
>
> prev_state = exception_enter();
> - __do_page_fault(regs, error_code);
> + __do_page_fault(regs, error_code, address);
> exception_exit(prev_state);
> }
>
> @@ -1267,9 +1266,16 @@ dotraplinkage void __kprobes
> trace_do_page_fault(struct pt_regs *regs, unsigned long error_code)
> {
> enum ctx_state prev_state;
> + /*
> + * The exception_enter and tracepoint processing could
> + * trigger another page faults (user space callchain
> + * reading) and destroy the original cr2 value, so read
> + * the faulting address now.
> + */
> + unsigned long address = read_cr2();
>
> prev_state = exception_enter();
> trace_page_fault_entries(regs, error_code);
> - __do_page_fault(regs, error_code);
> + __do_page_fault(regs, error_code, address);
> exception_exit(prev_state);
> }



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