powerpc/kgdb: add kgdb_arch_set/remove_breakpoint()

Generic implementation fails to remove breakpoints after init
when CONFIG_STRICT_KERNEL_RWX is selected:

[   13.251285] KGDB: BP remove failed: c001c338
[   13.259587] kgdbts: ERROR PUT: end of test buffer on 'do_fork_test' line 8 expected OK got $E14#aa
[   13.268969] KGDB: re-enter exception: ALL breakpoints killed
[   13.275099] CPU: 0 PID: 1 Comm: init Not tainted 4.18.0-g82bbb913ffd8 #860
[   13.282836] Call Trace:
[   13.285313] [c60e1ba0] [c0080ef0] kgdb_handle_exception+0x6f4/0x720 (unreliable)
[   13.292618] [c60e1c30] [c000e97c] kgdb_handle_breakpoint+0x3c/0x98
[   13.298709] [c60e1c40] [c000af54] program_check_exception+0x104/0x700
[   13.305083] [c60e1c60] [c000e45c] ret_from_except_full+0x0/0x4
[   13.310845] [c60e1d20] [c02a22ac] run_simple_test+0x2b4/0x2d4
[   13.316532] [c60e1d30] [c0081698] put_packet+0xb8/0x158
[   13.321694] [c60e1d60] [c00820b4] gdb_serial_stub+0x230/0xc4c
[   13.327374] [c60e1dc0] [c0080af8] kgdb_handle_exception+0x2fc/0x720
[   13.333573] [c60e1e50] [c000e928] kgdb_singlestep+0xb4/0xcc
[   13.339068] [c60e1e70] [c000ae1c] single_step_exception+0x90/0xac
[   13.345100] [c60e1e80] [c000e45c] ret_from_except_full+0x0/0x4
[   13.350865] [c60e1f40] [c000e11c] ret_from_syscall+0x0/0x38
[   13.356346] Kernel panic - not syncing: Recursive entry to debugger

This patch creates powerpc specific version of
kgdb_arch_set_breakpoint() and kgdb_arch_remove_breakpoint()
using patch_instruction()

Fixes: 1e0fc9d1eb2b ("powerpc/Kconfig: Enable STRICT_KERNEL_RWX for some configs")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/kgdb.h

@@ -26,9 +26,12 @@
 #define BREAK_INSTR_SIZE	4
 #define BUFMAX			((NUMREGBYTES * 2) + 512)
 #define OUTBUFMAX		((NUMREGBYTES * 2) + 512)
+
+#define BREAK_INSTR		0x7d821008	/* twge r2, r2 */
+
 static inline void arch_kgdb_breakpoint(void)
 {
-	asm(".long 0x7d821008"); /* twge r2, r2 */
+	asm(stringify_in_c(.long BREAK_INSTR));
 }
 #define CACHE_FLUSH_IS_SAFE	1
 #define DBG_MAX_REG_NUM     70

arch/powerpc/kernel/kgdb.c

@@ -24,6 +24,7 @@
 #include <asm/processor.h>
 #include <asm/machdep.h>
 #include <asm/debug.h>
+#include <asm/code-patching.h>
 #include <linux/slab.h>
 
 /*
@@ -144,7 +145,7 @@ static int kgdb_handle_breakpoint(struct pt_regs *regs)
 	if (kgdb_handle_exception(1, SIGTRAP, 0, regs) != 0)
 		return 0;
 
-	if (*(u32 *) (regs->nip) == *(u32 *) (&arch_kgdb_ops.gdb_bpt_instr))
+	if (*(u32 *)regs->nip == BREAK_INSTR)
 		regs->nip += BREAK_INSTR_SIZE;
 
 	return 1;
@@ -441,16 +442,42 @@ int kgdb_arch_handle_exception(int vector, int signo, int err_code,
 	return -1;
 }
 
+int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
+{
+	int err;
+	unsigned int instr;
+	unsigned int *addr = (unsigned int *)bpt->bpt_addr;
+
+	err = probe_kernel_address(addr, instr);
+	if (err)
+		return err;
+
+	err = patch_instruction(addr, BREAK_INSTR);
+	if (err)
+		return -EFAULT;
+
+	*(unsigned int *)bpt->saved_instr = instr;
+
+	return 0;
+}
+
+int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
+{
+	int err;
+	unsigned int instr = *(unsigned int *)bpt->saved_instr;
+	unsigned int *addr = (unsigned int *)bpt->bpt_addr;
+
+	err = patch_instruction(addr, instr);
+	if (err)
+		return -EFAULT;
+
+	return 0;
+}
+
 /*
  * Global data
  */
-struct kgdb_arch arch_kgdb_ops = {
-#ifdef __LITTLE_ENDIAN__
-	.gdb_bpt_instr = {0x08, 0x10, 0x82, 0x7d},
-#else
-	.gdb_bpt_instr = {0x7d, 0x82, 0x10, 0x08},
-#endif
-};
+struct kgdb_arch arch_kgdb_ops;
 
 static int kgdb_not_implemented(struct pt_regs *regs)
 {