Date: Wed, 9 Dec 2020 14:05:08 +0000 (UTC) From: Mark Johnston <markj@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r368486 - head/sys/kern Message-ID: <202012091405.0B9E58Yq050582@repo.freebsd.org>
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Author: markj Date: Wed Dec 9 14:05:08 2020 New Revision: 368486 URL: https://svnweb.freebsd.org/changeset/base/368486 Log: Plug a race between fd table teardown and several loops To export information from fd tables we have several loops which do this: FILDESC_SLOCK(fdp); for (i = 0; fdp->fd_refcount > 0 && i <= lastfile; i++) <export info for fd i>; FILDESC_SUNLOCK(fdp); Before r367777, fdescfree() acquired the fd table exclusive lock between decrementing fdp->fd_refcount and freeing table entries. This serialized with the loop above, so the file at descriptor i would remain valid until the lock is dropped. Now there is no serialization, so the loops may race with teardown of file descriptor tables. Acquire the exclusive fdtable lock after releasing the final table reference to provide a barrier synchronizing with these loops. Reported by: pho Reviewed by: kib (previous version), mjg Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D27513 Modified: head/sys/kern/kern_descrip.c Modified: head/sys/kern/kern_descrip.c ============================================================================== --- head/sys/kern/kern_descrip.c Wed Dec 9 14:04:54 2020 (r368485) +++ head/sys/kern/kern_descrip.c Wed Dec 9 14:05:08 2020 (r368486) @@ -2463,6 +2463,13 @@ fdescfree_fds(struct thread *td, struct filedesc *fdp, struct file *fp; int i, lastfile; + KASSERT(refcount_load(&fdp->fd_refcnt) == 0, + ("%s: fd table %p carries references", __func__, fdp)); + + /* Serialize with threads iterating over the table. */ + FILEDESC_XLOCK(fdp); + FILEDESC_XUNLOCK(fdp); + lastfile = fdlastfile_single(fdp); for (i = 0; i <= lastfile; i++) { fde = &fdp->fd_ofiles[i]; @@ -2536,6 +2543,11 @@ pdescfree(struct thread *td) void fdescfree_remapped(struct filedesc *fdp) { +#ifdef INVARIANTS + /* fdescfree_fds() asserts that fd_refcnt == 0. */ + if (!refcount_release(&fdp->fd_refcnt)) + panic("%s: fd table %p has extra references", __func__, fdp); +#endif fdescfree_fds(curthread, fdp, 0); }
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