[PATCH] i386: fix file_read_actor() and pipe_read() for original i386 systems

The __copy_to_user_inatomic() calls in file_read_actor() and pipe_read()
are broken on original i386 machines, where WP-works-ok == false, as
__copy_to_user_inatomic() on such systems calls functions which might
sleep and/or contain cond_resched() calls inside of a kmap_atomic()
region.

The original check for WP-works-ok was in access_ok(), but got moved
during the 2.5 series to fix a race vs. swap.

Return the number of bytes to copy in the case where we are in an atomic
region, so the non atomic code pathes in file_read_actor() and
pipe_read() are taken.

This could be optimized to avoid the kmap_atomicby moving the check for
WP-works-ok into fault_in_pages_writeable(), but this is more intrusive
and can be done later.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Acked-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/i386/lib/usercopy.c

@@ -10,6 +10,7 @@
 #include <linux/blkdev.h>
 #include <linux/module.h>
 #include <linux/backing-dev.h>
+#include <linux/interrupt.h>
 #include <asm/uaccess.h>
 #include <asm/mmx.h>
 
@@ -719,6 +720,14 @@ unsigned long __copy_to_user_ll(void __user *to, const void *from,
 #ifndef CONFIG_X86_WP_WORKS_OK
 	if (unlikely(boot_cpu_data.wp_works_ok == 0) &&
 			((unsigned long )to) < TASK_SIZE) {
+		/*
+		 * When we are in an atomic section (see
+		 * mm/filemap.c:file_read_actor), return the full
+		 * length to take the slow path.
+		 */
+		if (in_atomic())
+			return n;
+
 		/* 
 		 * CPU does not honor the WP bit when writing
 		 * from supervisory mode, and due to preemption or SMP,