CVS commit: src/sys/kern

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

Module Name:    src
Committed By:   riastradh
Date:           Sat Apr 22 11:22:36 UTC 2023

Modified Files:
        src/sys/kern: vfs_vnops.c

Log Message:
readdir(2), lseek(2): Fix races in access to struct file::f_offset.

For non-directory vnodes:
- reading f_offset requires a shared or exclusive vnode lock
- writing f_offset requires an exclusive vnode lock

For directory vnodes, access (read or write) requires either:
- a shared vnode lock AND f_lock, or
- an exclusive vnode lock.

This way, two files for the same underlying directory vnode can still
do VOP_READDIR in parallel, but if two readdir(2) or lseek(2) calls
run in parallel on the same file, the load and store of f_offset is
atomic (otherwise, e.g., on 32-bit systems it might be torn and lead
to corrupt offsets).

There is still a potential problem: the _whole transaction_ of
readdir(2) may not be atomic.  For example, if thread A and thread B
read n bytes of directory content, thread A might get bytes [0,n) and
thread B might get bytes [n,2n) but f_offset might end up at n
instead of 2n once both operations complete.  (However, f_offset
wouldn't be some corrupt garbled number like n & 0xffffffff00000000.)
Fixing this would require either:
(a) using an exclusive vnode lock in vn_readdir,
(b) introducing a new lock that serializes vn_readdir on the same
    file (but ont necessarily the same vnode), or
(c) proving it is safe to hold f_lock across VOP_READDIR, VOP_SEEK,
    and VOP_GETATTR.


To generate a diff of this commit:
cvs rdiff -u -r1.237 -r1.238 src/sys/kern/vfs_vnops.c

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