Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions

Mathieu Desnoyers
2009-02-27T18:07:39Z

* 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,
>
[...]
> 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..

> + 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 ?

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 */

--
Mathieu Desnoyers
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