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

[Masami Hiramatsu <mhiramat@redhat.com>]

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