Re: [PATCH] binfmt_elf: core dump masking support

["Kawai, Hidehiro" <hidehiro.kawai.ez@hitachi.com>]

Hi,

>>>>>(run echo 1 > coremask, echo 0 > coremask in a loop while dumping
>>>>>core. Do you have enough locking to make it work as expected?)
>>>>
>>>>Currently, any lock isn't acquired.  But I think the kernel only
>>>>have to preserve the coremask setting in a local variable at the
>>>>begining of core dumping.  I'm going to do this in the next version.
>>>
>>>No, I do not think that is enough. At minimum, you'd need atomic_t
>>>variable. But I'd recomend against it. Playing with locking is tricky.
>>
>>Why do you think it is not enough?  I think that any locking is not
>>needed.
> 
> You are wrong.
> 
>>  unsigned int mask = current->mm->coremask;
> 
> Not a valid C; coremask can change from another cpu.

Sure.  Although I thought it was no problem because coremask uses only
1 bit, it will be problematic when coremask grows beyond byte alignment.


>>>No, I do not think that is enough. At minimum, you'd need atomic_t
>>>variable. But I'd recomend against it. Playing with locking is tricky.

But Andrew proposed:

> Regarding the implementation: if we add
> 
> 	unsigned char coredump_omit_anon_memory:1;
> 
> into the mm_struct right next to `dumpable' then we avoid increasing the
> size of the mm_struct, and the code gets neater.

So I'm going to use a bit field to store the bitmask. Because of this,
I can't use an atomic_t variable.
Instead, I'll use a spin lock to read/write atomically.

Best regards,
-- 
Hidehiro Kawai
Hitachi, Ltd., Systems Development Laboratory