[PATCH] x86: make text_poke() atomic using fixmap

Masami Hiramatsu
2009-03-03T16:32:19Z

Masami Hiramatsu wrote:
> Ingo Molnar wrote:
>> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>>
>>> Ingo Molnar wrote:
>>>> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>>>>
>>>>> Ingo Molnar wrote:
>>>>>>>> So perhaps another approach to (re-)consider would be to go back
>>>>>>>> to atomic fixmaps here. It spends 3 slots but that's no big
>>>>>>>> deal.
>>>>>>> Oh, it's a good idea! fixmaps must make it simpler.
>>>>>>>
>>>>>>>> In exchange it will be conceptually simpler, and will also scale
>>>>>>>> much better than a global spinlock. What do you think?
>>>>>>> I think even if I use fixmaps, we have to use a spinlock to protect
>>>>>>> the fixmap area from other threads...
>>>>>> that's why i suggested to use an atomic-kmap, not a fixmap.
>>>>> Even if the mapping is atomic, text_poke() has to protect pte
>>>>> from other text_poke()s while changing code.
>>>>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
>>>> Well, but text_poke() is not a serializing API to begin with.
>>>> It's normally used in code patching sequences when we 'know'
>>>> that there cannot be similar parallel activities. The kprobes
>>>> usage of text_poke() looks unsafe - and that needs to be fixed.
>>> Oh, kprobes already prohibited parallel arming/disarming
>>> by using kprobe_mutex. :-)
>> yeah, but still the API is somewhat unsafe.
>
> Yeah, kprobe_mutex protects text_poke from other kprobes, but
> not from other text_poke() users...
>
>> In any case, you also answered your own question:
>>
>>>>> Even if the mapping is atomic, text_poke() has to protect pte
>>>>> from other text_poke()s while changing code.
>>>>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
>> kprobe_mutex does that.
>
> Anyway, text_edit_lock ensures that.
>
> By the way, I think set_fixmap/clear_fixmap seems simpler than
> kmap_atomic() variant. Would you think improving kmap_atomic_prot()
> is better?

Hi Ingo,

Here is the patch which uses fixmaps instead of vmap in text_poke().
This made the code much simpler than I thought :).

Thanks,

----
Use fixmaps instead of vmap/vunmap in text_poke() for avoiding page allocation
and delayed unmapping.

At the result of above change, text_poke() becomes atomic and can be called
from stop_machine() etc.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---
arch/x86/include/asm/fixmap_32.h | 2 ++
arch/x86/include/asm/fixmap_64.h | 2 ++
arch/x86/kernel/alternative.c | 18 ++++++++++++------
3 files changed, 16 insertions(+), 6 deletions(-)

Index: linux-2.6/arch/x86/include/asm/fixmap_32.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/fixmap_32.h
+++ linux-2.6/arch/x86/include/asm/fixmap_32.h
@@ -81,6 +81,8 @@ enum fixed_addresses {
#ifdef CONFIG_PARAVIRT
FIX_PARAVIRT_BOOTMAP,
#endif
+ FIX_TEXT_POKE0, /* reserve 2 pages for text_poke() */
+ FIX_TEXT_POKE1,
__end_of_permanent_fixed_addresses,
/*
* 256 temporary boot-time mappings, used by early_ioremap(),
Index: linux-2.6/arch/x86/include/asm/fixmap_64.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/fixmap_64.h
+++ linux-2.6/arch/x86/include/asm/fixmap_64.h
@@ -49,6 +49,8 @@ enum fixed_addresses {
#ifdef CONFIG_PARAVIRT
FIX_PARAVIRT_BOOTMAP,
#endif
+ FIX_TEXT_POKE0, /* reserve 2 pages for text_poke() */
+ FIX_TEXT_POKE1,
__end_of_permanent_fixed_addresses,
#ifdef CONFIG_ACPI
FIX_ACPI_BEGIN,
Index: linux-2.6/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/alternative.c
+++ linux-2.6/arch/x86/kernel/alternative.c
@@ -12,7 +12,9 @@
#include <asm/nmi.h>
#include <asm/vsyscall.h>
#include <asm/cacheflush.h>
+#include <asm/tlbflush.h>
#include <asm/io.h>
+#include <asm/fixmap.h>

#define MAX_PATCH_LEN (255-1)

@@ -495,12 +497,13 @@ void *text_poke_early(void *addr, const
* It means the size must be writable atomically and the address must be aligned
* in a way that permits an atomic write. It also makes sure we fit on a single
* page.
+ *
+ * Note: Must be called under text_mutex.
*/
void *__kprobes text_poke(void *addr, const void *opcode, size_t len)
{
unsigned long flags;
char *vaddr;
- int nr_pages = 2;
struct page *pages[2];
int i;

@@ -513,14 +516,17 @@ void *__kprobes text_poke(void *addr, co
pages[1] = virt_to_page(addr + PAGE_SIZE);
}
BUG_ON(!pages[0]);
- if (!pages[1])
- nr_pages = 1;
- vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
- BUG_ON(!vaddr);
+ set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
+ if (pages[1])
+ set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
+ vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
local_irq_save(flags);
memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
local_irq_restore(flags);
- vunmap(vaddr);
+ clear_fixmap(FIX_TEXT_POKE0);
+ if (pages[1])
+ clear_fixmap(FIX_TEXT_POKE1);
+ local_flush_tlb();
sync_core();
/* Could also do a CLFLUSH here to speed up CPU recovery; but
that causes hangs on some VIA CPUs. */

--
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com