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

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);


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