Extend the KPI to lock and unlock f_offset member of struct file. It

now fully encapsulates all accesses to f_offset, and extends f_offset
locking to other consumers that need it, in particular, to lseek() and
variants of getdirentries().

Ensure that on 32bit architectures f_offset, which is 64bit quantity,
always read and written under the mtxpool protection. This fixes
apparently easy to trigger race when parallel lseek()s or lseek() and
read/write could destroy file offset.

The already broken ABI emulations, including iBCS and SysV, are not
converted (yet).

Tested by:	pho
No objections from:	jhb
MFC after:    3 weeks

1 files changed, 3 insertions, 2 deletions

diff --git a/sys/compat/linux/linux_file.c b/sys/compat/linux/linux_file.c
index 67b1b38..b0c2e4a 100644
--- a/sys/compat/linux/linux_file.c
+++ b/sys/compat/linux/linux_file.c
@@ -357,15 +357,16 @@ getdents_common(struct thread *td, struct linux_getdents64_args *args,
 		return (EBADF);
 	}
 
+	off = foffset_lock(fp, 0);
 	vp = fp->f_vnode;
 	vfslocked = VFS_LOCK_GIANT(vp->v_mount);
 	if (vp->v_type != VDIR) {
 		VFS_UNLOCK_GIANT(vfslocked);
+		foffset_unlock(fp, off, 0);
 		fdrop(fp, td);
 		return (EINVAL);
 	}
 
-	off = fp->f_offset;
 
 	buflen = max(LINUX_DIRBLKSIZ, nbytes);
 	buflen = min(buflen, MAXBSIZE);
@@ -514,7 +515,6 @@ getdents_common(struct thread *td, struct linux_getdents64_args *args,
 		goto eof;
 	}
 
-	fp->f_offset = off;
 	if (justone)
 		nbytes = resid + linuxreclen;
 
@@ -527,6 +527,7 @@ out:
 
 	VOP_UNLOCK(vp, 0);
 	VFS_UNLOCK_GIANT(vfslocked);
+	foffset_unlock(fp, off, 0);
 	fdrop(fp, td);
 	free(buf, M_TEMP);
 	free(lbuf, M_TEMP);