[V5 Patch 0/2] fix file truncations when both suid and write permissions set

[Amerigo Wang <amwang@redhat.com>]

Andrew, these two patches can replace the -mm patch
"vfs-allow-file-truncations-when-both-suid-and-write-permissions-set.patch".


When suid is set and the non-owner user has write permission,
any writing into this file should be allowed and suid should be
removed after that.

However, current kernel only allows writing without truncations,
when we do truncations on that file, we get EPERM. This is a bug.

Steps to reproduce this bug:

% ls -l rootdir/file1
-rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
% echo h > rootdir/file1
zsh: operation not permitted: rootdir/file1
% ls -l rootdir/file1
-rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
% echo h >> rootdir/file1
% ls -l rootdir/file1
-rwxrwxrwx 1 root root 5 Jun 25 16:34 rootdir/file1

This patch fixes it.

Signed-off-by: WANG Cong <amwang@redhat.com>

Thanks!