fs/coredump.c: log if a core dump is aborted due to changed file permissions

For obvious security reasons, a core dump is aborted if the filesystem
cannot preserve ownership or permissions of the dump file.

This affects filesystems like e.g.  vfat, but also something like a 9pfs
share in a Qemu test setup, running as a regular user, depending on the
security model used.  In those cases, the result is an empty core file and
a confused user.

To hopefully save other people a lot of time figuring out the cause, this
patch adds a simple log message for those specific cases.

[akpm@linux-foundation.org: s/|%s/%s/ in printk text]

Link: https://lkml.kernel.org/r/20210701233151.102720-1-david.oberhollenzer@sigma-star.atSigned-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -782,10 +782,17 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 		 * filesystem.
 		 */
 		mnt_userns = file_mnt_user_ns(cprm.file);
-		if (!uid_eq(i_uid_into_mnt(mnt_userns, inode), current_fsuid()))
+		if (!uid_eq(i_uid_into_mnt(mnt_userns, inode),
+			    current_fsuid())) {
+			pr_info_ratelimited("Core dump to %s aborted: cannot preserve file owner\n",
+					    cn.corename);
 			goto close_fail;
-		if ((inode->i_mode & 0677) != 0600)
+		}
+		if ((inode->i_mode & 0677) != 0600) {
+			pr_info_ratelimited("Core dump to %s aborted: cannot preserve file permissions\n",
+					    cn.corename);
 			goto close_fail;
+		}
 		if (!(cprm.file->f_mode & FMODE_CAN_WRITE))
 			goto close_fail;
 		if (do_truncate(mnt_userns, cprm.file->f_path.dentry,