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SubjectRe: [PATCH] Check for breakpoint in text_poke to eliminate bug_on
Can you test this new version ? The check was buggy when it fell on a
code boundary : the addr - 1 wan't always a valid address.



Check for breakpoint in text_poke to eliminate bug_on

It's ok to modify an instruction non-atomically (multiple memory accesses to a
large and/or non aligned instruction) *if and only if* we have inserted a
breakpoint at the beginning of the instruction.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---
arch/x86/kernel/alternative.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

Index: linux-2.6-lttng/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6-lttng.orig/arch/x86/kernel/alternative.c 2008-04-20 15:40:30.000000000 -0400
+++ linux-2.6-lttng/arch/x86/kernel/alternative.c 2008-04-20 15:42:08.000000000 -0400
@@ -15,6 +15,7 @@
#include <asm/cacheflush.h>

#define MAX_PATCH_LEN (255-1)
+#define BREAKPOINT_INSTRUCTION 0xcc

#ifdef CONFIG_HOTPLUG_CPU
static int smp_alt_once;
@@ -505,6 +506,11 @@ void *text_poke_early(void *addr, const
* It means the size must be writable atomically and the address must be aligned
* in a way that permits an atomic write. It also makes sure we fit on a single
* page.
+ *
+ * It's ok to modify an instruction non-atomically (multiple memory accesses to
+ * a large and/or non aligned instruction) *if and only if* we have inserted a
+ * breakpoint at the beginning of the instruction and we are modifying the rest
+ * of the instruction.
*/
void *__kprobes text_poke(void *addr, const void *opcode, size_t len)
{
@@ -512,11 +518,9 @@ void *__kprobes text_poke(void *addr, co
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 (is_vmalloc_addr(addr)) {
+ if (!core_kernel_text((unsigned long)addr)) {
pages[0] = vmalloc_to_page(addr);
pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
} else {
@@ -535,5 +539,7 @@ void *__kprobes text_poke(void *addr, co
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;
}
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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