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



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