Oded Gabbay <oded.gabbay@xxxxxxx> writes:
> On 12/24/2014 01:01 AM, Rusty Russell wrote:
>> Oded Gabbay <oded.gabbay@xxxxxxx> writes:
>>> I didn't say it doesn't always work.
>>> The actual thing that doesn't work is the define symbol_get and only in a
>>> specific case of 32bit kernel AND CONFIG_MODULES is unset AND
>>> CONFIG_RANDOMIZE_BASE is set.
>>> The define in that case is:
>>> #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak)); &(x); })
>>>
>>> Why it doesn't work (doesn't return NULL when symbol doesn't exists) ?
>>
>> Hmm, I'd guess CONFIG_RANDOMIZE_BASE is relocating NULL symbols...
>>
>> No, I can't reproduce this. Please send your .config privately.
>>
>> Here's my test case:
>>
>> diff --git a/init/main.c b/init/main.c
>> index 61b993767db5..a3ee1ec97ec3 100644
>> --- a/init/main.c
>> +++ b/init/main.c
>> @@ -683,6 +683,12 @@ asmlinkage __visible void __init start_kernel(void)
>>
>> ftrace_init();
>>
>> + {
>> + extern void nonexistent_fn(void);
>> + printk("symbol_get(nonexistent_fn) = %p\n",
>> + symbol_get(nonexistent_fn));
>> + }
>> +
>> /* Do the rest non-__init'ed, we're now alive */
>> rest_init();
>> }
>>
>> Thanks,
>> Rusty.
>>
> Hi Rusty,
>
> Attached is the bad config file. (config-bad)
> I have narrowed the changes you need to do to the config file in order to
> reproduce this bug.
> The base assumption is a 32-bit kernel and without modules support. Rest of the
> config file is pretty standard, IMO.
> Then, its not enough to enable CONFIG_RANDOMIZE_BASE like I wrote in my original
> post. You need also to unset CONFIG_HIBERNATION.
Indeed, thanks; your config breaks as reported. With CONFIG_HIBERNATION
the kernel offset is 0, so we don't see this.
Kees, as far as I can tell you need another 0-terminated vmlinux.relocs
section for weak symbols. These should not be relocated if already 0.
Put this somewhere to test. It fails for x86_64, too:
diff --git a/init/main.c b/init/main.c
index 61b993767db5..c9e0195c792a 100644
--- a/init/main.c
+++ b/init/main.c
@@ -683,6 +683,12 @@ asmlinkage __visible void __init start_kernel(void)
ftrace_init();
+ {
+ extern void __attribute__((weak)) nonexistent_fn(void);
+ printk("nonexistent_fn = %p\n", nonexistent_fn);
+ BUG_ON(nonexistent_fn != NULL);
+ }
+
/* Do the rest non-__init'ed, we're now alive */
rest_init();
}
Cheers,
Rusty.
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