loader: set boot_safe when safe mode is selected

This may be used to disable or limit functionality of some services
when safe mode is selected. The particular value of boot_safe should not
be considered significant, only its presence in the environment.

Reviewed by:	imp, tsoome
Differential Revision:	https://reviews.freebsd.org/D32172

stand/forth/menu-commands.4th

@@ -128,6 +128,7 @@ also menu-namespace also menu-command-helpers
 	s" set hw.ata.atapi_dma=0" evaluate
 	s" set kern.eventtimer.periodic=1" evaluate
 	s" set kern.geom.part.check_integrity=0" evaluate
+	s" set boot_safe=YES" evaluate
 ;
 
 : safemode_disable ( -- )
@@ -136,6 +137,7 @@ also menu-namespace also menu-command-helpers
 	s" hw.ata.atapi_dma" unsetenv
 	s" kern.eventtimer.periodic" unsetenv
 	s" kern.geom.part.check_integrity" unsetenv
+	s" boot_safe" unsetenv
 ;
 
 : init_safemode ( N -- N )

stand/lua/core.lua

@@ -179,12 +179,14 @@ function core.setSafeMode(safe_mode)
 		loader.setenv("hw.ata.atapi_dma", "0")
 		loader.setenv("kern.eventtimer.periodic", "1")
 		loader.setenv("kern.geom.part.check_integrity", "0")
+		loader.setenv("boot_safe", "YES")
 	else
 		loader.unsetenv("kern.smp.disabled")
 		loader.unsetenv("hw.ata.ata_dma")
 		loader.unsetenv("hw.ata.atapi_dma")
 		loader.unsetenv("kern.eventtimer.periodic")
 		loader.unsetenv("kern.geom.part.check_integrity")
+		loader.unsetenv("boot_safe")
 	end
 	core.sm = safe_mode
 end

stand/man/loader_simp.8

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 29, 2021
+.Dd September 30, 2021
 .Dt LOADER_SIMP 8
 .Os
 .Sh NAME
@@ -339,6 +339,21 @@ In a running system, the state of console muting can be manipulated by the
 utility.
 .It Va boot_pause
 During the device probe, pause after each line is printed.
+.It Va boot_safe
+Force userland to boot in
+.Dq safe mode ,
+which may disable or limit the functionality of some services that may not be
+desired in safe mode.
+This is typically set by selecting
+.Dq safe mode
+in the loader menu, which also sets some other hints for the kernel.
+Applications wishing to respect safe mode should
+.Sy only
+test for the presence of
+.Va boot_safe
+in
+.Xr kenv 1 ,
+not for any particular value.
 .It Va boot_serial
 Force the use of a serial console even when an internal console
 is present.