The patch adds support for /proc/sys/kernel/osrelease.
XXX /proc/sys is visible when not mounted with -o linux.
XXX Heavily based on copy/paste, some bugs may sneak in.
XXX Probably need further work.

Index: sys/miscfs/procfs/procfs.h
===================================================================
RCS file: /cvsroot/src/sys/miscfs/procfs/procfs.h,v
retrieving revision 1.70
diff -p -u -r1.70 procfs.h
--- sys/miscfs/procfs/procfs.h	27 Jul 2014 16:47:26 -0000	1.70
+++ sys/miscfs/procfs/procfs.h	4 Dec 2014 09:42:34 -0000
@@ -110,6 +110,9 @@ typedef enum {
 	PFSstatm,	/* process memory info (if -o linux) */
 	PFSversion,	/* kernel version (if -o linux) */
 	PFStask,	/* task subdirector (if -o linux) */
+	PFSsys,		/* sys subdirectory (if -o linux) */
+	PFSsyskern,	/* sys/kernel subdirectory (if -o linux) */
+	PFSosrelease,	/* sys/kernel/osrelease (if -o linux) */
 #ifdef __HAVE_PROCFS_MACHDEP
 	PROCFS_MACHDEP_NODE_TYPES
 #endif
@@ -233,6 +236,8 @@ int procfs_doemul(struct lwp *, struct p
     struct uio *);
 int procfs_doversion(struct lwp *, struct proc *, struct pfsnode *,
     struct uio *);
+int procfs_doosrelease(struct lwp *, struct proc *, struct pfsnode *,
+    struct uio *);
 
 void procfs_revoke_vnodes(struct proc *, void *);
 int procfs_getfp(struct pfsnode *, struct proc *, struct file **);
Index: sys/miscfs/procfs/procfs_linux.c
===================================================================
RCS file: /cvsroot/src/sys/miscfs/procfs/procfs_linux.c,v
retrieving revision 1.70
diff -p -u -r1.70 procfs_linux.c
--- sys/miscfs/procfs/procfs_linux.c	10 Aug 2014 06:22:06 -0000	1.70
+++ sys/miscfs/procfs/procfs_linux.c	4 Dec 2014 09:42:35 -0000
@@ -732,3 +732,68 @@ out:
 	sysctl_unlock();
 	return error;
 }
+
+/*
+ * Linux compatible /proc/sys/kernel/osrelease. Only active
+ * when the -o linux mountflag is used.
+ */
+int
+procfs_doosrelease(struct lwp *curl, struct proc *p,
+    struct pfsnode *pfs, struct uio *uio)
+{
+	char losrelease[20];
+	int nm[4];
+	char *bf;
+	const char *posrelease;
+	const char *emulname = curlwp->l_proc->p_emul->e_name;
+	size_t buflen;
+	int len;
+	int error = 0;
+
+	CTASSERT(EMUL_LINUX_KERN_OSRELEASE == EMUL_LINUX32_KERN_OSRELEASE);
+
+	bf = malloc(LBFSZ, M_TEMP, M_WAITOK);
+
+	sysctl_lock(false);
+
+	if (strncmp(emulname, "linux", 5) == 0) {
+		/*
+		 * Lookup the emulation osrelease.
+		 * Since compat_linux and compat_linux32 can be built as
+		 * modules, we use sysctl to obtain the values instead of
+		 * using the symbols directly.
+		 */
+
+		if (strcmp(emulname, "linux32") == 0) {
+			nm[0] = CTL_EMUL;
+			nm[1] = EMUL_LINUX32;
+			nm[2] = EMUL_LINUX32_KERN;
+		} else {
+			nm[0] = CTL_EMUL;
+			nm[1] = EMUL_LINUX;
+			nm[2] = EMUL_LINUX_KERN;
+		}
+
+		nm[3] = EMUL_LINUX_KERN_OSRELEASE;
+		buflen = sizeof(losrelease);
+		error = sysctl_dispatch(nm, __arraycount(nm),
+		    losrelease, &buflen,
+		    NULL, 0, NULL, NULL, NULL);
+		if (error)
+			goto out;
+
+		posrelease = losrelease;
+	} else {
+		posrelease = osrelease;
+	}
+
+	len = snprintf(bf, LBFSZ, "%s\n", posrelease);
+	if (len == 0)
+		goto out;
+
+	error = uiomove_frombuf(bf, len, uio);
+out:
+	free(bf, M_TEMP);
+	sysctl_unlock();
+	return error;
+}
Index: sys/miscfs/procfs/procfs_subr.c
===================================================================
RCS file: /cvsroot/src/sys/miscfs/procfs/procfs_subr.c,v
retrieving revision 1.106
diff -p -u -r1.106 procfs_subr.c
--- sys/miscfs/procfs/procfs_subr.c	10 Nov 2014 18:46:34 -0000	1.106
+++ sys/miscfs/procfs/procfs_subr.c	4 Dec 2014 09:42:35 -0000
@@ -272,6 +272,10 @@ procfs_rw(void *v)
 		error = procfs_doversion(curl, p, pfs, uio);
 		break;
 
+	case PFSosrelease:
+		error = procfs_doosrelease(curl, p, pfs, uio);
+		break;
+
 #ifdef __HAVE_PROCFS_MACHDEP
 	PROCFS_MACHDEP_NODETYPE_CASES
 		error = procfs_machdep_rw(curl, l, pfs, uio);
Index: sys/miscfs/procfs/procfs_vfsops.c
===================================================================
RCS file: /cvsroot/src/sys/miscfs/procfs/procfs_vfsops.c,v
retrieving revision 1.94
diff -p -u -r1.94 procfs_vfsops.c
--- sys/miscfs/procfs/procfs_vfsops.c	10 Nov 2014 18:46:34 -0000	1.94
+++ sys/miscfs/procfs/procfs_vfsops.c	4 Dec 2014 09:42:35 -0000
@@ -386,6 +386,12 @@ procfs_loadvnode(struct mount *mp, struc
 		vp->v_type = VREG;
 		break;
 
+	case PFSsys:		/* /proc/sys = dr-xr-xr-x */
+	case PFSsyskern:	/* /proc/sys/kernel = dr-xr-xr-x */
+		pfs->pfs_mode = S_IRUSR|S_IXUSR|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH;
+		vp->v_type = VDIR;
+		break;
+
 	case PFSmap:	/* /proc/N/map = -r--r--r-- */
 	case PFSmaps:	/* /proc/N/maps = -r--r--r-- */
 	case PFSstatus:	/* /proc/N/status = -r--r--r-- */
@@ -401,6 +407,7 @@ procfs_loadvnode(struct mount *mp, struc
 	case PFSloadavg:	/* /proc/loadavg = -r--r--r-- */
 	case PFSstatm:	/* /proc/N/statm = -r--r--r-- */
 	case PFSversion:	/* /proc/version = -r--r--r-- */
+	case PFSosrelease:	/* /proc/sys/kernel/osrelease = -r--r--r-- */
 		pfs->pfs_mode = S_IRUSR|S_IRGRP|S_IROTH;
 		vp->v_type = VREG;
 		break;
Index: sys/miscfs/procfs/procfs_vnops.c
===================================================================
RCS file: /cvsroot/src/sys/miscfs/procfs/procfs_vnops.c,v
retrieving revision 1.192
diff -p -u -r1.192 procfs_vnops.c
--- sys/miscfs/procfs/procfs_vnops.c	5 Sep 2014 09:26:16 -0000	1.192
+++ sys/miscfs/procfs/procfs_vnops.c	4 Dec 2014 09:42:35 -0000
@@ -198,6 +198,7 @@ static const struct proc_target proc_roo
 	{ DT_REG, N("stat"),	    PFScpustat,        procfs_validfile_linux },
 	{ DT_REG, N("loadavg"),	    PFSloadavg,        procfs_validfile_linux },
 	{ DT_REG, N("version"),     PFSversion,        procfs_validfile_linux },
+	{ DT_DIR, N("sys"),	    PFSsys,            NULL },
 #undef N
 };
 static const int nproc_root_targets =
@@ -642,6 +643,9 @@ procfs_getattr(void *v)
 	case PFSroot:
 	case PFScurproc:
 	case PFSself:
+	case PFSsys:
+	case PFSsyskern:
+	case PFSosrelease:
 		procp = NULL;
 		break;
 
@@ -837,6 +841,19 @@ procfs_getattr(void *v)
 		vap->va_gid = kauth_cred_getegid(procp->p_cred);
 		vap->va_bytes = vap->va_size = DEV_BSIZE;
 		break;
+	case PFSsys:
+	case PFSsyskern:
+		vap->va_nlink = 2;
+		vap->va_uid = 0;
+		vap->va_gid = 0;
+		vap->va_bytes = vap->va_size = DEV_BSIZE;
+		break;
+	case PFSosrelease:
+		vap->va_nlink = 1;
+		vap->va_uid = 0;
+		vap->va_gid = 0;
+		vap->va_bytes = vap->va_size = sizeof("osrelease") - 1;
+		break;
 
 	case PFSfile:
 		error = EOPNOTSUPP;
@@ -1186,6 +1203,38 @@ procfs_lookup(void *v)
 		procfs_proc_unlock(p);
 		return error;
 	}
+	case PFSsys:
+		if (cnp->cn_flags & ISDOTDOT) {
+			error = procfs_allocvp(dvp->v_mount,
+			    vpp, 0, PFSroot, -1);
+			return error;
+		}
+
+		if (cnp->cn_namelen == 6 &&
+		    memcmp(pname, "kernel", 6) == 0) {
+			error = procfs_allocvp(dvp->v_mount,
+			    vpp, 0, PFSsyskern, -1);
+			return error;
+		}
+
+		return ENOENT;
+
+	case PFSsyskern:
+		if (cnp->cn_flags & ISDOTDOT) {
+			error = procfs_allocvp(dvp->v_mount,
+			    vpp, 0, PFSsys, -1);
+			return error;
+		}
+
+		if (cnp->cn_namelen == 9 &&
+		    memcmp(pname, "osrelease", 9) == 0) {
+			error = procfs_allocvp(dvp->v_mount,
+			    vpp, 0, PFSosrelease, -1);
+			return error;
+		}
+
+		return ENOENT;
+
 	default:
 		return (ENOTDIR);
 	}
@@ -1578,6 +1627,88 @@ procfs_readdir(void *v)
 		break;
 	}
 
+	case PFSsys: {
+		int nc = 0;
+
+		if (ap->a_ncookies) {
+			/*
+			 * XXX Potentially allocating too much space here,
+			 * but I'm lazy. This loop needs some work.
+			 */
+			cookies = malloc(ncookies * sizeof (off_t),
+			    M_TEMP, M_WAITOK);
+			*ap->a_cookies = cookies;
+		}
+		error = 0;
+		/* 0 ... 2 are static entries. */
+		for (; i <= 2 && uio->uio_resid >= UIO_MX; i++) {
+			switch (i) {
+			case 0:		/* `.' */
+			case 1:		/* `..' */
+				d.d_fileno = PROCFS_FILENO(0, PFSsys, -1);
+				d.d_namlen = i + 1;
+				memcpy(d.d_name, "..", d.d_namlen);
+				d.d_name[i + 1] = '\0';
+				d.d_type = DT_DIR;
+				break;
+			case 2:
+				d.d_fileno = PROCFS_FILENO(0, PFSsyskern, -1);
+				d.d_namlen = sizeof("kernel") - 1;
+				memcpy(d.d_name, "kernel", d.d_namlen + 1);
+				d.d_type = DT_DIR;
+				break;
+			}
+
+			if ((error = uiomove(&d, UIO_MX, uio)) != 0)
+				break;
+			nc++;
+			if (cookies)
+				*cookies++ = i + 1;
+		}
+		break;
+	}
+
+	case PFSsyskern: {
+		int nc = 0;
+
+		if (ap->a_ncookies) {
+			/*
+			 * XXX Potentially allocating too much space here,
+			 * but I'm lazy. This loop needs some work.
+			 */
+			cookies = malloc(ncookies * sizeof (off_t),
+			    M_TEMP, M_WAITOK);
+			*ap->a_cookies = cookies;
+		}
+		error = 0;
+		/* 0 ... 2 are static entries. */
+		for (; i <= 2 && uio->uio_resid >= UIO_MX; i++) {
+			switch (i) {
+			case 0:		/* `.' */
+			case 1:		/* `..' */
+				d.d_fileno = PROCFS_FILENO(0, PFSsyskern, -1);
+				d.d_namlen = i + 1;
+				memcpy(d.d_name, "..", d.d_namlen);
+				d.d_name[i + 1] = '\0';
+				d.d_type = DT_DIR;
+				break;
+			case 2:
+				d.d_fileno = PROCFS_FILENO(0, PFSosrelease, -1);
+				d.d_namlen = sizeof("osrelease") - 1;
+				memcpy(d.d_name, "osrelease", d.d_namlen + 1);
+				d.d_type = DT_REG;
+				break;
+			}
+
+			if ((error = uiomove(&d, UIO_MX, uio)) != 0)
+				break;
+			nc++;
+			if (cookies)
+				*cookies++ = i + 1;
+		}
+		break;
+	}
+
 	default:
 		error = ENOTDIR;
 		break;