sound: Improve hw.snd.compat_linux_mmap

- Reject PROT_EXEC in all cases when Linux support is not compiled in.
- Define sysctl only when Linux support is compiled in.
- Document better.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	emaste
Pull Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/29

share/man/man4/pcm.4

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd February 15, 2025
+.Dd May 15, 2026
 .Dt SOUND 4
 .Os
 .Sh NAME
@@ -254,21 +254,29 @@ are global settings and
 are device specific.
 .Bl -tag -width indent
 .It Va hw.snd.compat_linux_mmap
-Linux
+This
+.Xr sysctl 8
+variable is available only when Linux application support is compiled into the
+kernel, and affects Linux binaries only.
+It is a hack to get around the fact that, for i386 emulation,
+.Xr linux 4
+historically set PROT_EXEC automatically when PROT_READ or PROT_WRITE was set
+during an
 .Xr mmap 2
-compatibility.
+call, which
+.Fx
+does not.
+.Pp
 The following values are supported (default is 0):
 .Bl -tag -width 2n
 .It -1
-Force disabling/denying PROT_EXEC
+Force-disable PROT_EXEC
 .Xr mmap 2
-requests.
+requests, even for Linux applications.
 .It 0
-Auto detect proc/ABI type, allow
+Allow PROT_EXEC
 .Xr mmap 2
-for Linux applications, and deny for everything else.
-.It 1
-Always allow PROT_EXEC page mappings.
+requests for Linux applications only.
 .El
 .It Va hw.snd.default_auto
 Automatically assign the default sound unit.

sys/dev/sound/pcm/dsp.c

@@ -54,10 +54,12 @@ struct dsp_cdevpriv {
 	struct pcm_channel *wrch;
 };
 
+#ifdef SV_ABI_LINUX
 static int dsp_mmap_allow_prot_exec = 0;
 SYSCTL_INT(_hw_snd, OID_AUTO, compat_linux_mmap, CTLFLAG_RWTUN,
     &dsp_mmap_allow_prot_exec, 0,
-    "linux mmap compatibility (-1=force disable 0=auto 1=force enable)");
+    "linux mmap compatibility (-1=force-disable 0=auto)");
+#endif
 
 static int dsp_basename_clone = 1;
 SYSCTL_INT(_hw_snd, OID_AUTO, basename_clone, CTLFLAG_RWTUN,
@@ -1923,20 +1925,15 @@ dsp_mmap_single(struct cdev *i_dev, vm_ooffset_t *offset,
 	struct pcm_channel *wrch, *rdch, *c;
 	int err;
 
-	/*
-	 * Reject PROT_EXEC by default. It just doesn't makes sense.
-	 * Unfortunately, we have to give up this one due to linux_mmap
-	 * changes.
-	 *
-	 * https://lists.freebsd.org/pipermail/freebsd-emulation/2007-June/003698.html
-	 *
-	 */
 #ifdef SV_ABI_LINUX
+	/*
+	 * https://lists.freebsd.org/pipermail/freebsd-emulation/2007-June/003698.html
+	 */
 	if ((nprot & PROT_EXEC) && (dsp_mmap_allow_prot_exec < 0 ||
 	    (dsp_mmap_allow_prot_exec == 0 &&
 	    SV_CURPROC_ABI() != SV_ABI_LINUX)))
 #else
-	if ((nprot & PROT_EXEC) && dsp_mmap_allow_prot_exec < 1)
+	if (nprot & PROT_EXEC)
 #endif
 		return (EINVAL);