ext4: allow readdir()'s of large empty directories to be interrupted

If a directory has a large number of empty blocks, iterating over all
of them can take a long time, leading to scheduler warnings and users
getting irritated when they can't kill a process in the middle of one
of these long-running readdir operations.  Fix this by adding checks to
ext4_readdir() and ext4_htree_fill_tree().

This was reverted earlier due to a typo in the original commit where I
experimented with using signal_pending() instead of
fatal_signal_pending().  The test was in the wrong place if we were
going to return signal_pending() since we would end up returning
duplicant entries.  See 9f2394c9be47 for a more detailed explanation.

Added fix as suggested by Linus to check for signal_pending() in
in the filldir() functions.

Reported-by: Benjamin LaHaise <bcrl@kvack.org>
Google-Bug-Id: 27880676
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 4 insertions, 0 deletions

diff --git a/fs/compat.c b/fs/compat.c
index a71936a3f4cb..f940cb20562f 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -936,6 +936,8 @@ static int compat_filldir(struct dir_context *ctx, const char *name, int namlen,
 	}
 	dirent = buf->previous;
 	if (dirent) {
+		if (signal_pending(current))
+			return -EINTR;
 		if (__put_user(offset, &dirent->d_off))
 			goto efault;
 	}
@@ -1020,6 +1022,8 @@ static int compat_filldir64(struct dir_context *ctx, const char *name,
 	dirent = buf->previous;
 
 	if (dirent) {
+		if (signal_pending(current))
+			return -EINTR;
 		if (__put_user_unaligned(offset, &dirent->d_off))
 			goto efault;
 	}