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    SubjectRe: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions
    Mathieu Desnoyers wrote:
    > * Masami Hiramatsu (mhiramat@redhat.com) wrote:
    >> Steven Rostedt wrote:
    >>> On Mon, 23 Feb 2009, Mathieu Desnoyers wrote:
    >>>>> Hmm, lets see. I simply set a bit in the PTE mappings. There's not many,
    >>>>> since a lot are 2M pages, for x86_64. Call stop_machine, and now I can
    >>>>> modify 1 or 20,000 locations. Set the PTE bit back. Note, the changing of
    >>>>> the bits are only done when CONFIG_DEBUG_RODATA is set.
    >>>>>
    >>>>> text_poke requires allocating a page. Map the page into memory. Set up a
    >>>>> break point.
    >>>> text_poke does not _require_ a break point. text_poke can work with
    >>>> stop_machine.
    >>> It can? Doesn't text_poke require allocating pages? The code called by
    >>> stop_machine is all atomic. vmap does not give an option to allocate with
    >>> GFP_ATOMIC.
    >> Hi,
    >>
    >> With my patch, text_poke() never allocate pages any more :)
    >>
    >> BTW, IMHO, both of your methods are useful and have trade-off.
    >>
    >> ftrace wants to change massive amount of code at once. If we do
    >> that with text_poke(), we have to map/unmap pages each time and
    >> it will take a long time -- might be longer than one stop_machine_run().
    >>
    >> On the other hand, text_poke() user like as kprobes and tracepoints,
    >> just want to change a few amount of code at once, and it will be
    >> added/removed incrementally. If we do that with stop_machine_run(),
    >> we'll be annoyed by frequent machine stops.(Moreover, kprobes uses
    >> breakpoint, so it doesn't need stop_machine_run())
    >>
    >
    > Hi Masami,
    >
    > Is this text_poke version executable in atomic context ? If yes, then
    > that would be good to add a comment saying it. Please see below for
    > comments.

    Thank you for comments!
    I think it could be. ah, spin_lock might be changed to spin_lock_irqsave()...

    >> Thank you,
    >>
    > [...]
    >> Use map_vm_area() instead of vmap() in text_poke() for avoiding page allocation
    >> and delayed unmapping.
    >>
    >> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
    >> ---
    >> arch/x86/include/asm/alternative.h | 1 +
    >> arch/x86/kernel/alternative.c | 25 ++++++++++++++++++++-----
    >> init/main.c | 3 +++
    >> 3 files changed, 24 insertions(+), 5 deletions(-)
    >>
    >> Index: linux-2.6/arch/x86/include/asm/alternative.h
    >> ===================================================================
    >> --- linux-2.6.orig/arch/x86/include/asm/alternative.h
    >> +++ linux-2.6/arch/x86/include/asm/alternative.h
    >> @@ -177,6 +177,7 @@ extern void add_nops(void *insns, unsign
    >> * The _early version expects the memory to already be RW.
    >> */
    >>
    >> +extern void text_poke_init(void);
    >> extern void *text_poke(void *addr, const void *opcode, size_t len);
    >> extern void *text_poke_early(void *addr, const void *opcode, size_t len);
    >>
    >> 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
    >> @@ -485,6 +485,16 @@ void *text_poke_early(void *addr, const
    >> return addr;
    >> }
    >>
    >> +static struct vm_struct *text_poke_area[2];
    >> +static DEFINE_SPINLOCK(text_poke_lock);
    >> +
    >> +void __init text_poke_init(void)
    >> +{
    >> + text_poke_area[0] = get_vm_area(PAGE_SIZE, VM_ALLOC);
    >> + text_poke_area[1] = get_vm_area(2 * PAGE_SIZE, VM_ALLOC);
    >
    > Why is this text_poke_area[1] 2 * PAGE_SIZE in size ? I would have
    > thought that text_poke_area[0] would be PAGE_SIZE, text_poke_area[1]
    > also be PAGE_SIZE, and that the sum of both would be 2 * PAGE_SIZE..

    Unfortunately, current map_vm_area() tries to map the size of vm_area,
    this means, you can't use 2page-size vm_area for mapping just 1 page...
    (or maybe, we can set pages[1] = pages[0] when 2nd page doesn't exist)


    >> + BUG_ON(!text_poke_area[0] || !text_poke_area[1]);
    >> +}
    >> +
    >> /**
    >> * text_poke - Update instructions on a live kernel
    >> * @addr: address to modify
    >> @@ -501,8 +511,9 @@ void *__kprobes text_poke(void *addr, co
    >> unsigned long flags;
    >> char *vaddr;
    >> int nr_pages = 2;
    >> - struct page *pages[2];
    >> - int i;
    >> + struct page *pages[2], **pgp = pages;
    >
    > Hrm, why do you need **pgp ? Could you simply pass &pages to map_vm_area ?

    As you know, pages means just the address(value) of an array, so you can't
    get the address of the address...(pages and &pages are same.)

    int array[2];
    printf("%p, %p",array, &array);

    please try it :)

    And actually, map_vm_area() requires the address of a pointer.
    ---
    int map_vm_area(struct vm_struct *area, pgprot_t prot, struct page ***pages)
    {
    unsigned long addr = (unsigned long)area->addr;
    unsigned long end = addr + area->size - PAGE_SIZE;
    int err;

    err = vmap_page_range(addr, end, prot, *pages);
    if (err > 0) {
    *pages += err;
    ^^^^^^^^^^^^^^ Here, it tries to add err(=number of mapped pages)
    to the pages pointer!
    err = 0;
    }

    return err;
    }
    ---


    >
    > Thanks,
    >
    > Mathieu
    >
    >> + int i, ret;
    >> + struct vm_struct *vma;
    >>
    >> if (!core_kernel_text((unsigned long)addr)) {
    >> pages[0] = vmalloc_to_page(addr);
    >> @@ -515,12 +526,16 @@ void *__kprobes text_poke(void *addr, co
    >> BUG_ON(!pages[0]);
    >> if (!pages[1])
    >> nr_pages = 1;
    >> - vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
    >> - BUG_ON(!vaddr);
    >> + spin_lock(&text_poke_lock);
    >> + vma = text_poke_area[nr_pages-1];
    >> + ret = map_vm_area(vma, PAGE_KERNEL, &pgp);
    >> + BUG_ON(ret);
    >> + vaddr = vma->addr;
    >> local_irq_save(flags);
    >> memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
    >> local_irq_restore(flags);
    >> - vunmap(vaddr);
    >> + unmap_kernel_range((unsigned long)vma->addr, (unsigned long)vma->size);
    >> + spin_unlock(&text_poke_lock);
    >> sync_core();
    >> /* Could also do a CLFLUSH here to speed up CPU recovery; but
    >> that causes hangs on some VIA CPUs. */
    >> @@ -528,3 +543,4 @@ void *__kprobes text_poke(void *addr, co
    >> BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
    >> return addr;
    >> }
    >> +EXPORT_SYMBOL_GPL(text_poke);
    >> Index: linux-2.6/init/main.c
    >> ===================================================================
    >> --- linux-2.6.orig/init/main.c
    >> +++ linux-2.6/init/main.c
    >> @@ -676,6 +676,9 @@ asmlinkage void __init start_kernel(void
    >> taskstats_init_early();
    >> delayacct_init();
    >>
    >> +#ifdef CONFIG_X86
    >> + text_poke_init();
    >> +#endif
    >> check_bugs();
    >>
    >> acpi_early_init(); /* before LAPIC and SMP init */
    >
    >

    --
    Masami Hiramatsu

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

    e-mail: mhiramat@redhat.com


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