Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

Mathieu Desnoyers
2009-06-15T16:15:17Z

* tip-bot for Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:
> Commit-ID: 74193ef0ecab92535c8517f082f1f50504526c9b
> Gitweb: http://git.kernel.org/tip/74193ef0ecab92535c8517f082f1f50504526c9b
> Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> AuthorDate: Mon, 15 Jun 2009 13:07:24 +0200
> Committer: Ingo Molnar <mingo@elte.hu>
> CommitDate: Mon, 15 Jun 2009 15:57:53 +0200
>
> perf_counter: x86: Fix call-chain support to use NMI-safe methods
>
> __copy_from_user_inatomic() isn't NMI safe in that it can trigger
> the page fault handler which is another trap and its return path
> invokes IRET which will also close the NMI context.
>
> Therefore use a GUP based approach to copy the stack frames over.
>
> We tried an alternative solution as well: we used a forward ported
> version of Mathieu Desnoyers's "NMI safe INT3 and Page Fault" patch
> that modifies the exception return path to use an open-coded IRET with
> explicit stack unrolling and TF checking.
>
> This didnt work as it interacted with faulting user-space instructions,
> causing them not to restart properly, which corrupts user-space
> registers.
>
> Solving that would probably involve disassembling those instructions
> and backtracing the RIP. But even without that, the code was deemed
> rather complex to the already non-trivial x86 entry assembly code,
> so instead we went for this GUP based method that does a
> software-walk of the pagetables.
>

Hrm, I'm probably missing something. Normally, you should test for
"in_nmi()" upon return from exception, and only in these cases go for
the open-coded IRET with stack unrolling and ret. I really don't see how
you end up messing up the page fault return to userspace path, as it's
impossible to have in_nmi() set.

Mathieu

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Nick Piggin <npiggin@suse.de>
> Cc: Pekka Enberg <penberg@cs.helsinki.fi>
> Cc: Vegard Nossum <vegard.nossum@gmail.com>
> Cc: Jeremy Fitzhardinge <jeremy@goop.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> LKML-Reference: <new-submission>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
>
> ---
> arch/x86/kernel/cpu/perf_counter.c | 49 ++++++++++++++++++++++++++++-------
> 1 files changed, 39 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_counter.c b/arch/x86/kernel/cpu/perf_counter.c
> index 6d5e7cf..e8c68a5 100644
> --- a/arch/x86/kernel/cpu/perf_counter.c
> +++ b/arch/x86/kernel/cpu/perf_counter.c
> @@ -19,6 +19,7 @@
> #include <linux/kdebug.h>
> #include <linux/sched.h>
> #include <linux/uaccess.h>
> +#include <linux/highmem.h>
>
> #include <asm/apic.h>
> #include <asm/stacktrace.h>
> @@ -1617,20 +1618,48 @@ perf_callchain_kernel(struct pt_regs *regs, struct perf_callchain_entry *entry)
> entry->kernel = entry->nr - nr;
> }
>
> -static int copy_stack_frame(const void __user *fp, struct stack_frame *frame)
> +/*
> + * best effort, GUP based copy_from_user() that assumes IRQ or NMI context
> + */
> +static unsigned long
> +copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
> {
> + unsigned long offset, addr = (unsigned long)from;
> + int type = in_nmi() ? KM_NMI : KM_IRQ0;
> + unsigned long size, len = 0;
> + struct page *page;
> + void *map;
> int ret;
>
> - if (!access_ok(VERIFY_READ, fp, sizeof(*frame)))
> - return 0;
> + do {
> + ret = __get_user_pages_fast(addr, 1, 0, &page);
> + if (!ret)
> + break;
>
> - ret = 1;
> - pagefault_disable();
> - if (__copy_from_user_inatomic(frame, fp, sizeof(*frame)))
> - ret = 0;
> - pagefault_enable();
> + offset = addr & (PAGE_SIZE - 1);
> + size = min(PAGE_SIZE - offset, n - len);
>
> - return ret;
> + map = kmap_atomic(page, type);
> + memcpy(to, map+offset, size);
> + kunmap_atomic(map, type);
> + put_page(page);
> +
> + len += size;
> + to += size;
> + addr += size;
> +
> + } while (len < n);
> +
> + return len;
> +}
> +
> +static int copy_stack_frame(const void __user *fp, struct stack_frame *frame)
> +{
> + unsigned long bytes;
> +
> + bytes = copy_from_user_nmi(frame, fp, sizeof(*frame));
> +
> + return bytes == sizeof(*frame);
> }
>
> static void
> @@ -1643,7 +1672,7 @@ perf_callchain_user(struct pt_regs *regs, struct perf_callchain_entry *entry)
> if (!user_mode(regs))
> regs = task_pt_regs(current);
>
> - fp = (void __user *)regs->bp;
> + fp = (void __user *)regs->bp;
>
> callchain_store(entry, regs->ip);
>

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68