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

Mathieu Desnoyers wrote:
* Masami Hiramatsu (mhiramat@xxxxxxxxxx) 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@xxxxxxxxxx>
---
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@xxxxxxxxxx

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