fsioctl.c: make generic_block_fiemap() signal-tolerant

__generic_block_fiemap may spin very long time for large sparse files.

Without this patch an unprivileged user may abuse system resources simply
by spawning a vast number of unkilable busyloops (works on ext2/ext3):

  truncate --size 1T test
  for ((i=0;i<1024;i++))
  do
         filefrag test > /dev/null &
  done

Signed-off-by: Dmitry Monakhov <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Dmitry Monakhov

Documentation/filesystems/fiemap.txt

@@ -196,7 +196,8 @@ struct fiemap_extent_info {
 };
 
 It is intended that the file system should not need to access any of this
-structure directly.
+structure directly. Filesystem handlers should be tolerant to signals and return
+EINTR once fatal signal received.
 
 
 Flag checking should be done at the beginning of the ->fiemap callback via the

fs/ioctl.c

@@ -379,6 +379,11 @@ int __generic_block_fiemap(struct inode *inode,
 				past_eof = true;
 		}
 		cond_resched();
+		if (fatal_signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
+
 	} while (1);
 
 	/* If ret is 1 then we just hit the end of the extent array */