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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>
This commit is contained in:
Peter Zijlstra 2009-06-15 13:07:24 +02:00 committed by Ingo Molnar
parent 3ff0141aa3
commit 74193ef0ec
1 changed files with 39 additions and 10 deletions
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49
arch/x86/kernel/cpu/perf_counter.c
View File

@ -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);