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Subject: [PATCH 2/2] ext4: don't fail GETFSUUID when the caller provides a
 long buffer
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     djwong@kernel.org, tytso@mit.edu
Cc:     catherine.hoang@oracle.com, linux-ext4@vger.kernel.org
Date:   Thu, 10 Nov 2022 12:16:34 -0800
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From: Darrick J. Wong <djwong@kernel.org>

If userspace provides a longer UUID buffer than is required, we
shouldn't fail the call with EINVAL -- rather, we can fill the caller's
buffer with the bytes we /can/ fill, and update the length field to
reflect what we copied.  This doesn't break the UAPI since we're
enabling a case that currently fails, and so far Ted hasn't released a
version of e2fsprogs that uses the new ext4 ioctl.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/ext4/ioctl.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)


diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 5f91f3ad3e50..31e643795016 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1159,14 +1159,16 @@ static int ext4_ioctl_getuuid(struct ext4_sb_info *sbi,
 		return 0;
 	}
 
-	if (fsuuid.fsu_len != UUID_SIZE || fsuuid.fsu_flags != 0)
+	if (fsuuid.fsu_len < UUID_SIZE || fsuuid.fsu_flags != 0)
 		return -EINVAL;
 
 	lock_buffer(sbi->s_sbh);
 	memcpy(uuid, sbi->s_es->s_uuid, UUID_SIZE);
 	unlock_buffer(sbi->s_sbh);
 
-	if (copy_to_user(&ufsuuid->fsu_uuid[0], uuid, UUID_SIZE))
+	fsuuid.fsu_len = UUID_SIZE;
+	if (copy_to_user(ufsuuid, &fsuuid, sizeof(fsuuid)) ||
+	    copy_to_user(&ufsuuid->fsu_uuid[0], uuid, UUID_SIZE))
 		return -EFAULT;
 	return 0;
 }