CVS commit: src/sys/ufs/lfs

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Sat Mar 21 06:11:05 UTC 2020

Modified Files:
        src/sys/ufs/lfs: lfs.h lfs_accessors.h

Log Message:
Avoid misaligned access to lfs64 on-disk records in memory.

lfs64 directory entries are only 32-bit aligned in order to conserve
space in directory blocks, and we had a hack to stuff a 64-bit inode
in them.  This replaces the hack by __aligned(4) __packed, and goes
further:

1. It's not clear that all the other lfs64 data structures are 64-bit
   aligned on disk to begin with.  We can go through these later and
   upgrade them from

        struct foo64 {
                ...
        } __aligned(4) __packed;

        union foo {
                struct foo64 f64;
                ...
        };

   to

        struct foo64 {
                ...
        };

        union foo {
                struct foo64 f64 __aligned(8);
                ...
        } __aligned(4) __packed;

   if we really want to take advantage of 64-bit memory accesses.

   However, the __aligned(4) __packed must remain on the union
   because:

2. We access even the lfs32 data structures via a union that has
   lfs64 members, and it turns out that compilers will assume access
   through a union with 64-bit aligned members implies the whole
   union has 64-bit alignment, even if we're only accessing a 32-bit
   aligned member.


To generate a diff of this commit:
cvs rdiff -u -r1.206 -r1.207 src/sys/ufs/lfs/lfs.h
cvs rdiff -u -r1.48 -r1.49 src/sys/ufs/lfs/lfs_accessors.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.