Re: Make kernel self-relocatable [PATCH]

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

On Tue, May 02, 2023 at 12:39:44AM +0000, Emmanuel Dreyfus wrote:
> Here is an additional change to add a SELFRELOC kernel option, enabled
> by defaut, for the sake of documentation clarity. 
> 
> Instead of telling that x86/boot(8) reloc command needs a kernel able 
> to self relocate, we can tell it needs a kernel built with the
> SELFRELOC option. This keeps the reader from wondering what could
> make a kernel able to self relocate.

And here is the missing patch

Index: share/man/man4/options.4
===================================================================
RCS file: /cvsroot/src/share/man/man4/options.4,v
retrieving revision 1.523
diff -U4 -r1.523 options.4
--- share/man/man4/options.4	28 Aug 2022 14:29:05 -0000	1.523
+++ share/man/man4/options.4	2 May 2023 00:24:48 -0000
@@ -1306,8 +1306,16 @@
 .Xr vnd 4
 and
 .Xr vnconfig 8
 for more information.
+.It Cd options SELFRELOC
+Make the kernel able to self relocate at bootstrap, so that it can
+run whatever its load address is. 
+This is intented to be used withe the 
+.Ic reloc
+boostrap command documented in
+.Xr x86/boot 8 ,
+to workaround UEFI bugs, and is only available on amd64.
 .It Cd options SPLDEBUG
 Help the kernel programmer find bugs related to the interrupt priority
 level.
 When
Index: sys/arch/amd64/amd64/locore.S
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/amd64/locore.S,v
retrieving revision 1.219
diff -U4 -r1.219 locore.S
--- sys/arch/amd64/amd64/locore.S	20 Apr 2023 00:42:23 -0000	1.219
+++ sys/arch/amd64/amd64/locore.S	2 May 2023 00:24:48 -0000
@@ -155,8 +155,9 @@
 #include "opt_ddb.h"
 #include "opt_ddbparam.h"
 #include "opt_modular.h"
 #include "opt_realmem.h"
+#include "opt_selfreloc.h"
 
 #include "opt_compat_netbsd.h"
 #include "opt_compat_netbsd32.h"
 #include "opt_xen.h"
@@ -455,15 +456,17 @@
 ENTRY(start)
 #ifndef XENPV
 	.code32
 
+#ifdef SELFRELOC
 	call	next
 next:	pop	%edi
 	sub     $(next - kernel_text), %edi 
 
 	/* If not KERNBASE, reloc ourselves to KERNBASE */
 	cmpl	$(KERNTEXTOFF_LO - KERNBASE_LO), %edi
 	jne	selfreloc_start
+#endif /* SELFRELOC */
 
 	/* Warm boot */
 	movw	$0x1234,0x472
 
@@ -1765,8 +1768,9 @@
 LABEL(nomds_leave)
 	NOMDS_LEAVE
 LABEL(nomds_leave_end)
 
+#ifdef SELFRELOC
 /*
  * selfreloc(loadddr edi)
  * This is adapted from sys/arch/i386/i386/locore.S
  */
@@ -1899,4 +1903,5 @@
 	.word	gdtr - gdt
 gdtrr:
 	.quad
 END(selfreloc_start)
+#endif /* SELFRELOC */
Index: sys/arch/amd64/conf/files.amd64
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/conf/files.amd64,v
retrieving revision 1.120
diff -U4 -r1.120 files.amd64
--- sys/arch/amd64/conf/files.amd64	21 Oct 2020 13:31:51 -0000	1.120
+++ sys/arch/amd64/conf/files.amd64	2 May 2023 00:24:48 -0000
@@ -19,8 +19,11 @@
 
 # Enable GCC spectre V2 mitigation options
 defflag opt_spectre.h	SPECTRE_V2_GCC_MITIGATION
 
+# Enable kernel self-relocation at bootstrap
+defflag opt_selfreloc.h SELFRELOC
+
 #
 # XXX these are just here at the moment so that we can share files
 # with the i386 (they include the opt_*.h for these)
 #
Index: sys/arch/amd64/conf/std.amd64
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/conf/std.amd64,v
retrieving revision 1.12
diff -U4 -r1.12 std.amd64
--- sys/arch/amd64/conf/std.amd64	25 Apr 2020 15:26:16 -0000	1.12
+++ sys/arch/amd64/conf/std.amd64	2 May 2023 00:24:48 -0000
@@ -14,8 +14,10 @@
 
 options 	CHILD_MAX=1024	# 160 is too few
 options 	OPEN_MAX=1024	# 128 is too few
 
+options 	SELFRELOC
+
 mainbus0 at root
 cpu* at mainbus?
 ioapic* at mainbus? apid ?
 
Index: share/man/man8/man8.x86/boot.8
===================================================================
RCS file: /cvsroot/src/share/man/man8/man8.x86/boot.8,v
retrieving revision 1.27
diff -U4 -r1.27 boot.8
--- share/man/man8/man8.x86/boot.8	24 Apr 2023 13:55:45 -0000	1.27
+++ share/man/man8/man8.x86/boot.8	2 May 2023 00:24:48 -0000
@@ -654,10 +654,12 @@
 Reboot the system.
 .It Ic reloc Op Va default No \(or Va none No \(or Va address
 [Only UEFI boot] Sets where the kernel is copied by bootstrap
 before it is started. Values other than default require a kernel
-that can relocate itself at the right address, otherwise a crash
-occurs at boot time.
+built with the
+.Cd SELFRELOC
+option, so that can relocate itself at the right address,
+otherwise a crash occurs at boot time.
 .Bl -tag -width default
 .It Va default
 Copy the kernel at ELF header load address, this is the historical
 behavior.

-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost