coredump_filter: permit changing of the default filter

Introduce a new kernel parameter `coredump_filter'.  Setting a value to
this parameter causes the default bitmask of coredump_filter to be
changed.

It is useful for users to change coredump_filter settings for the whole
system at boot time.  Without this parameter, users have to change
coredump_filter settings for each /proc/<pid>/ in an initializing script.
Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -551,6 +551,11 @@ and is between 256 and 4096 characters. It is defined in the file
 			not work reliably with all consoles, but is known
 			to work with serial and VGA consoles.
 
+	coredump_filter=
+			[KNL] Change the default value for
+			/proc/<pid>/coredump_filter.
+			See also Documentation/filesystems/proc.txt.
+
 	cpcihp_generic=	[HW,PCI] Generic port I/O CompactPCI driver
 			Format:
 			<first_slot>,<last_slot>,<port>,<enum_bit>[,<debug>]

kernel/fork.c

@@ -400,6 +400,18 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(mmlist_lock);
 #define allocate_mm()	(kmem_cache_alloc(mm_cachep, GFP_KERNEL))
 #define free_mm(mm)	(kmem_cache_free(mm_cachep, (mm)))
 
+static unsigned long default_dump_filter = MMF_DUMP_FILTER_DEFAULT;
+
+static int __init coredump_filter_setup(char *s)
+{
+	default_dump_filter =
+		(simple_strtoul(s, NULL, 0) << MMF_DUMP_FILTER_SHIFT) &
+		MMF_DUMP_FILTER_MASK;
+	return 1;
+}
+
+__setup("coredump_filter=", coredump_filter_setup);
+
 #include <linux/init_task.h>
 
 static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
@@ -408,8 +420,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
 	atomic_set(&mm->mm_count, 1);
 	init_rwsem(&mm->mmap_sem);
 	INIT_LIST_HEAD(&mm->mmlist);
-	mm->flags = (current->mm) ? current->mm->flags
-				  : MMF_DUMP_FILTER_DEFAULT;
+	mm->flags = (current->mm) ? current->mm->flags : default_dump_filter;
 	mm->core_state = NULL;
 	mm->nr_ptes = 0;
 	set_mm_counter(mm, file_rss, 0);