Re: musl-based toolchain for J2/SH-2 - kernel patches

[Rich Felker <dalias@libc.org>]

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On Wed, Aug 26, 2015 at 01:42:12PM -0400, Rich Felker wrote:
> I've packaged up a small cross-compiler-toolchain build script that's
> especially geared towards use with the Open Processor Foundation's J2
> Core, which uses the SH-2 ISA, and in the future other NOMMU archs:
> 
> https://github.com/richfelker/musl-cross-make

One more thing: A couple kernel-side patches are needed to support the
binaries produced by this toolchain on SH-2/J2 and other NOMMU
systems. One adds the unified syscall trap number for SH-2/3/4; the
other enables loading of ELF files on NOMMU. Early versions of both
were included in this email to the musl list:

http://www.openwall.com/lists/musl/2015/06/16/5

The version of the SH unified syscall trap patch that's been proposed
upstream is available here as well:

http://lkml.iu.edu/hypermail/linux/kernel/1508.3/01702.html

It touches some of the same code touched by the 0pf/J2 patches:

http://lkml.iu.edu/hypermail/linux/kernel/1506.2/02538.html

so some minor work to merge them is needed for using this on the real
J2 hardware.

I'm attaching both patches to this email as well.

Rich

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diff -urp ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2/entry.S ./arch/sh/kernel/cpu/sh2/entry.S
--- ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2/entry.S	2015-08-09 19:54:30.000000000 +0000
+++ ./arch/sh/kernel/cpu/sh2/entry.S	2015-08-24 03:23:34.159387924 +0000
@@ -144,9 +144,9 @@ ENTRY(exception_handler)
 	mov	#64,r8
 	cmp/hs	r8,r9
 	bt	interrupt_entry	! vec >= 64 is interrupt
-	mov	#32,r8
+	mov	#31,r8
 	cmp/hs	r8,r9
-	bt	trap_entry	! 64 > vec >= 32  is trap
+	bt	trap_entry	! 64 > vec >= 31  is trap
 
 	mov.l	4f,r8
 	mov	r9,r4
@@ -178,9 +178,9 @@ interrupt_entry:
 
 trap_entry:
 	mov	#0x30,r8
-	cmp/ge	r8,r9		! vector 0x20-0x2f is systemcall
+	cmp/ge	r8,r9		! vector 0x1f-0x2f is systemcall
 	bt	1f
-	add	#-0x10,r9	! convert SH2 to SH3/4 ABI
+	mov     #0x1f,r9	! convert to unified SH2/3/4 trap number
 1:	
 	shll2	r9			! TRA
 	bra	system_call	! jump common systemcall entry
diff -urp ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2a/entry.S ./arch/sh/kernel/cpu/sh2a/entry.S
--- ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2a/entry.S	2015-08-09 19:54:30.000000000 +0000
+++ ./arch/sh/kernel/cpu/sh2a/entry.S	2015-08-24 03:23:58.849386418 +0000
@@ -109,9 +109,9 @@ ENTRY(exception_handler)
 	mov	#64,r8
 	cmp/hs	r8,r9
 	bt	interrupt_entry	! vec >= 64 is interrupt
-	mov	#32,r8
+	mov	#31,r8
 	cmp/hs	r8,r9
-	bt	trap_entry	! 64 > vec >= 32  is trap
+	bt	trap_entry	! 64 > vec >= 31  is trap
 
 	mov.l	4f,r8
 	mov	r9,r4
@@ -143,9 +143,9 @@ interrupt_entry:
 
 trap_entry:
 	mov	#0x30,r8
-	cmp/ge	r8,r9		! vector 0x20-0x2f is systemcall
+	cmp/ge	r8,r9		! vector 0x1f-0x2f is systemcall
 	bt	1f
-	add	#-0x10,r9	! convert SH2 to SH3/4 ABI
+	mov     #0x1f,r9	! convert to unified SH2/3/4 trap number
 1:	
 	shll2	r9			! TRA
 	bra	system_call	! jump common systemcall entry
diff -urp ../linux-4.2-rc6.orig/arch/sh/kernel/entry-common.S ./arch/sh/kernel/entry-common.S
--- ../linux-4.2-rc6.orig/arch/sh/kernel/entry-common.S	2015-08-09 19:54:30.000000000 +0000
+++ ./arch/sh/kernel/entry-common.S	2015-08-25 21:08:15.835463227 +0000
@@ -268,20 +268,29 @@ debug_trap:
  *	Syscall #: R3
  *	Arguments #0 to #3: R4--R7
  *	Arguments #4 to #6: R0, R1, R2
- *	TRA: (number of arguments + ABI revision) x 4
+ *	TRA: See following table.
  *
- * This code also handles delegating other traps to the BIOS/gdb stub
- * according to:
- *
- * Trap number
  * (TRA>>2)	Purpose
  * --------	-------
  * 0x00-0x0f	original SH-3/4 syscall ABI (not in general use).
  * 0x10-0x1f	general SH-3/4 syscall ABI.
- * 0x20-0x2f	syscall ABI for SH-2 parts.
+ *      0x1f	unified SH-2/3/4 syscall ABI (preferred).
+ * 0x20-0x2f	original SH-2 syscall ABI.
  * 0x30-0x3f	debug traps used by the kernel.
  * 0x40-0xff	Not supported by all parts, so left unhandled.
  *
+ * For making system calls, any trap number in the range for the
+ * given cpu model may be used, but the unified trap number 0x1f is
+ * preferred for compatibility with all models.
+ *
+ * The low bits of the trap number were once documented as matching
+ * the number of arguments, but they were never actually used as such
+ * by the kernel. SH-2 originally used its own separate trap range
+ * because several hardware exceptions fell in the range used for the
+ * SH-3/4 syscall ABI.
+ *
+ * This code also handles delegating other traps to the BIOS/gdb stub.
+ *
  * Note: When we're first called, the TRA value must be shifted
  * right 2 bits in order to get the value that was used as the "trapa"
  * argument.

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--- fs/binfmt_elf_fdpic.c.orig
+++ fs/binfmt_elf_fdpic.c
@@ -103,14 +103,27 @@
 core_initcall(init_elf_fdpic_binfmt);
 module_exit(exit_elf_fdpic_binfmt);
 
+static int is_fdpic(struct elfhdr *hdr)
+{
+#ifdef CONFIG_MMU
+	return 1;
+#else
+	return elf_check_fdpic(hdr);
+#endif
+}
+
 static int is_elf_fdpic(struct elfhdr *hdr, struct file *file)
 {
 	if (memcmp(hdr->e_ident, ELFMAG, SELFMAG) != 0)
 		return 0;
 	if (hdr->e_type != ET_EXEC && hdr->e_type != ET_DYN)
 		return 0;
-	if (!elf_check_arch(hdr) || !elf_check_fdpic(hdr))
+	if (!elf_check_arch(hdr))
 		return 0;
+#ifdef CONFIG_MMU
+	if (!elf_check_fdpic(hdr))
+		return 0;
+#endif
 	if (!file->f_op->mmap)
 		return 0;
 	return 1;
@@ -269,7 +282,7 @@
 
 	}
 
-	if (elf_check_const_displacement(&exec_params.hdr))
+	if (elf_check_const_displacement(&exec_params.hdr) || !is_fdpic(&exec_params.hdr))
 		exec_params.flags |= ELF_FDPIC_FLAG_CONSTDISP;
 
 	/* perform insanity checks on the interpreter */
@@ -306,9 +319,9 @@
 
 	retval = -ENOEXEC;
 	if (stack_size == 0)
-		goto error;
+		stack_size = 131072;
 
-	if (elf_check_const_displacement(&interp_params.hdr))
+	if (elf_check_const_displacement(&interp_params.hdr) || !is_fdpic(&interp_params.hdr))
 		interp_params.flags |= ELF_FDPIC_FLAG_CONSTDISP;
 
 	/* flush all traces of the currently running executable */
@@ -319,7 +332,8 @@
 	/* there's now no turning back... the old userspace image is dead,
 	 * defunct, deceased, etc.
 	 */
-	set_personality(PER_LINUX_FDPIC);
+	if (is_fdpic(&exec_params.hdr))
+		set_personality(PER_LINUX_FDPIC);
 	if (elf_read_implies_exec(&exec_params.hdr, executable_stack))
 		current->personality |= READ_IMPLIES_EXEC;
 
@@ -400,8 +414,6 @@
 
 	current->mm->brk = current->mm->start_brk;
 	current->mm->context.end_brk = current->mm->start_brk;
-	current->mm->context.end_brk +=
-		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
 	current->mm->start_stack = current->mm->start_brk + stack_size;
 #endif