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    SubjectRe: [PATCH 1/1] x86: fix text_poke

    * Linus Torvalds <torvalds@linux-foundation.org> wrote:

    > On Mon, 28 Apr 2008, Jiri Slaby wrote:
    > >
    > > Thanks. Bisected mm down to git-x86.patch, bisected git-x86-latest
    > > down to x86: enhance DEBUG_RODATA support - alternatives The patch
    > > below fixes the problem for me. Comments welcome.
    >
    > You're a hero, Jiri.

    indeed!

    > And that also explains why I didn't see it - I don't do modules.

    neither does my auto-test :-/

    Suspend/resume goes from SMP to UP and then back - and triggers all the
    instrument patching code. I suspect we should/could have seen similar
    problems with a pure CPU hotplug stress-test, on a modular kernel.

    > Thanks a heap.
    >
    > > The 0xf0 pattern comes from alternatives_smp_lock: text_poke(*ptr,
    > > ((unsigned char []){0xf0}), 1);
    >
    > And we should really add a lot more sanity checking there.

    yeah.

    incidentally, this bug was fixed by Mathieu yesterday but the full
    impact of the bug was not realized. Below is that patch from
    sched-devel.

    i'm wondering what the best sanity checking would be. What we want is to
    be sure the patch we modify is truly a kernel or module text page.

    Perhaps we should start marking all kernel/module text pages with
    PageReserved? That way we can not corrupt any userspace/pagecache page.
    (and we'd clear PageReserved on module unload)

    Ingo

    ------------------------->
    Subject: Fix sched-devel text_poke
    From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    Date: Thu, 24 Apr 2008 11:03:33 -0400

    Use core_text_address() instead of kernel_text_address(). Deal with modules in
    the same way used for the core kernel.

    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    arch/x86/kernel/alternative.c | 38 ++++++++++++++++++--------------------
    1 file changed, 18 insertions(+), 20 deletions(-)

    Index: linux/arch/x86/kernel/alternative.c
    ===================================================================
    --- linux.orig/arch/x86/kernel/alternative.c
    +++ linux/arch/x86/kernel/alternative.c
    @@ -511,31 +511,29 @@ void *__kprobes text_poke(void *addr, co
    unsigned long flags;
    char *vaddr;
    int nr_pages = 2;
    + struct page *pages[2];
    + int i;

    - BUG_ON(len > sizeof(long));
    - BUG_ON((((long)addr + len - 1) & ~(sizeof(long) - 1))
    - - ((long)addr & ~(sizeof(long) - 1)));
    - if (kernel_text_address((unsigned long)addr)) {
    - struct page *pages[2] = { virt_to_page(addr),
    - virt_to_page(addr + PAGE_SIZE) };
    - if (!pages[1])
    - nr_pages = 1;
    - vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
    - BUG_ON(!vaddr);
    - local_irq_save(flags);
    - memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
    - local_irq_restore(flags);
    - vunmap(vaddr);
    + if (!core_kernel_text((unsigned long)addr)) {
    + pages[0] = vmalloc_to_page(addr);
    + pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
    } else {
    - /*
    - * modules are in vmalloc'ed memory, always writable.
    - */
    - local_irq_save(flags);
    - memcpy(addr, opcode, len);
    - local_irq_restore(flags);
    + pages[0] = virt_to_page(addr);
    + 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);
    + local_irq_save(flags);
    + memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
    + local_irq_restore(flags);
    + vunmap(vaddr);
    sync_core();
    /* Could also do a CLFLUSH here to speed up CPU recovery; but
    that causes hangs on some VIA CPUs. */
    + for (i = 0; i < len; i++)
    + BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
    return addr;
    }

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