s3:passdb: smbpasswd reset permissions only if not 0600

s3:passdb: smbpasswd reset permissions only if not 0600

Browsing files or download files from samba server, smbd would check user's
id to decide whether this user could access these files, by lookup user's
information from the password file (e.g. /usr/local/samba/private/smbpasswd).
smbd might goes through startsmbfilepwent(), this api calls [f]chmod() to
make sure the password file has valid permissions 0600.

Consider a scenario: we are doing a read performance benchmark about
downloading a bunch of files (e.g. a thousand files) from a samba server,
monitoring file system i/o activities counters, and expecting that should
be only read operations on file system because this is just downloading, no
uploading is involved. But actually found that still write operations on file
system, because smbd lookup user and always reset 0600 permissions on password
file while access each file, it makes dirty pages (inode modification) in ram,
later triggered a kernel journal daemon to sync dirty pages into back storage
(e.g. ext3 kjournald, or ext4 jbd2).
This looks like not friendly for read performance benchmark if it happened on
an entry-level systems with much less memory and limited computation power,
because dirty pages syncing in the meantime slows down read performance.

This patch adds fstat() before [f]chmod(), it would check whether password
file has valid permissions 0600 or not. If 0600 smbd would bypass [f]chmod()
to avoid making dirty pages on file systems. If not 0600 smbd would warn and
go through [f]chmod() to set valid permissions 0600 to password file as
earlier days.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15555

Signed-off-by: Jones Syue <jonessyue@qnap.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Thu Jan 18 10:28:19 UTC 2024 on atb-devel-224