On 03/20/2012 04:31 PM, Tim Bird wrote:
> On 03/20/2012 11:32 AM, Jason Wessel wrote:
>> On 09/21/2011 03:07 PM, Tim Bird wrote:
>>> On x86, if CONFIG_DEBUG_RODATA is set, one cannot set breakpoints
>>> via KDB. Apparently this is a well-known problem, as at least one
>>> distribution
>>> now ships with both KDB enabled and CONFIG_DEBUG_RODATA=y for security
>>> reasons.
>>>
>>> This patch just adds an extra printk message to the breakpoint failure case,
>>> in order to provide some useful diagnostics to the user.
>>>
>> The patch is definitely the right idea. I believe we should try and tell
>> the whole story and only print the message for the type of breakpoint that
>> fails. It is absolutely the case that you can still use kdb/kdb without
>> recompiling the kernel.
>>
>> I propose a slightly different implementation below.
> The new patch looks fine.
Thanks. Because you posed the question a while back about potentially changing
this such that it works, I thought I might re-open the discussion around
further integration with kgdb/kdb and kprobes. Back in 2008 I explained you
can "hack it" so as to use copy on write patches, just for the debugger in
order to use software breakpoints.
Something horrible like:
@@ -42,11 +43,24 @@ EXPORT_SYMBOL_GPL(probe_kernel_read);
long probe_kernel_write(void *dst, void *src, size_t size)
{
long ret;
+#ifdef CONFIG_X86
+ unsigned int level;
+ pte_t *pte = lookup_address((unsigned long)dst, &level);
+ int unprotect = !pte_write(*pte);
+#endif
mm_segment_t old_fs = get_fs();
[...clipped rest of original hack...]
Obviously we are not going to go down the horrible hack route, so I thought I
might look at how the kprobe implementation works, because they do deal with
read-only pages. They created a function called text_poke() that is called
from arch_arm_probe().
It might be possible to take a short cut and use the arch_arm_probe() directly.
Ultimately I would like to consider just using kprobes instead for things like
single stepping etc..., but it still has a few problems with atomic
reservations. This work had been shelved in back 2010, in favor of merging
kdb. I am not sure if anyone wants to take a look at it, but I do think it
might warrant some experimentation.
Jason.
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