Subject: [PATCH v3 7/8] Implement kernel live patching for ppc64le (ABIv2)

From: Torsten Duwe <[email-masked]>
Date: Mon, 26 Oct 2015 19:02:16 +0100

* create the appropriate files+functions
    arch/powerpc/include/asm/livepatch.h
        klp_check_compiler_support,
        klp_arch_set_pc
    arch/powerpc/kernel/livepatch.c with a stub for
        klp_write_module_reloc
    This is architecture-independent work in progress.
  * introduce a fixup in arch/powerpc/kernel/entry_64.S
    for local calls that are becoming global due to live patching.
    And of course do the main KLP thing: return to a maybe different
    address, possibly altered by the live patching ftrace op.

Signed-off-by: Torsten Duwe <[email-masked]>
---
 arch/powerpc/include/asm/livepatch.h | 27 ++++++++++++++++++++
 arch/powerpc/kernel/entry_64.S       | 48 +++++++++++++++++++++++++++++++++---
 arch/powerpc/kernel/livepatch.c      | 20 +++++++++++++++
 3 files changed, 91 insertions(+), 4 deletions(-)
 create mode 100644 arch/powerpc/include/asm/livepatch.h
 create mode 100644 arch/powerpc/kernel/livepatch.c

diff --git a/arch/powerpc/include/asm/livepatch.h b/arch/powerpc/include/asm/livepatch.h
new file mode 100644
index 0000000..334eb55
--- /dev/null
+++ b/arch/powerpc/include/asm/livepatch.h
@@ -0,0 +1,27 @@
+#ifndef _ASM_POWERPC64_LIVEPATCH_H
+#define _ASM_POWERPC64_LIVEPATCH_H
+
+#include <linux/module.h>
+#include <linux/ftrace.h>
+
+#ifdef CONFIG_LIVEPATCH
+static inline int klp_check_compiler_support(void)
+{
+#if !defined(_CALL_ELF) || _CALL_ELF != 2
+	return 1;
+#endif
+	return 0;
+}
+
+extern int klp_write_module_reloc(struct module *mod, unsigned long type,
+				   unsigned long loc, unsigned long value);
+
+static inline void klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
+{
+	regs->nip = ip;
+}
+#else
+#error Live patching support is disabled; check CONFIG_LIVEPATCH
+#endif
+
+#endif /* _ASM_POWERPC64_LIVEPATCH_H */
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index b0dfbfe..2681601 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -1264,6 +1264,9 @@ _GLOBAL(ftrace_caller)
 	mflr    r3
 	std     r3, _NIP(r1)
 	std	r3, 16(r1)
+#ifdef CONFIG_LIVEPATCH
+	mr	r14,r3		// remember "old" NIP
+#endif
 	subi    r3, r3, MCOUNT_INSN_SIZE
 	mfmsr   r4
 	std     r4, _MSR(r1)
@@ -1280,7 +1283,10 @@ ftrace_call:
 	nop
 
 	ld	r3, _NIP(r1)
-	mtlr	r3
+	mtctr	r3		// prepare to jump there
+#ifdef CONFIG_LIVEPATCH
+	cmpd	r14,r3		// has NIP been altered?
+#endif
 
 	REST_8GPRS(0,r1)
 	REST_8GPRS(8,r1)
@@ -1293,6 +1299,24 @@ ftrace_call:
 	mtlr	r12
 	mr	r2,r0		// restore callee's TOC
 
+#ifdef CONFIG_LIVEPATCH
+	beq+	4f		// likely(old_NIP == new_NIP)
+
+	// For a local call, restore this TOC after calling the patch function.
+	// For a global call, it does not matter what we restore here,
+	// since the global caller does its own restore right afterwards,
+	// anyway.
+	// Just insert a KLP_return_helper frame in any case,
+	// so a patch function can always count on the changed stack offsets.
+	stdu	r1,-32(r1)	// open new mini stack frame
+	std	r0,24(r1)	// save TOC now, unconditionally.
+	LOAD_REG_IMMEDIATE(r12,KLP_return_helper)
+	std	r12,LRSAVE(r1)
+	mtlr	r12
+	bctr
+4:
+#endif
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	stdu	r1, -112(r1)
 .globl ftrace_graph_call
@@ -1302,15 +1326,31 @@ _GLOBAL(ftrace_graph_stub)
 	addi	r1, r1, 112
 #endif
 
-	mflr	r0		// move this LR to CTR
-	mtctr	r0
-
 	ld	r0,LRSAVE(r1)	// restore callee's lr at _mcount site
 	mtlr	r0
 	bctr			// jump after _mcount site
 #endif /* CC_USING_MPROFILE_KERNEL */
 _GLOBAL(ftrace_stub)
 	blr
+
+#ifdef CONFIG_LIVEPATCH
+/* Helper function for local calls that are becoming global
+   due to live patching.
+   We can't simply patch the NOP after the original call,
+   because, depending on the consistency model, some kernel
+   threads may still have called the original, local function
+   *without* saving their TOC in the respective stack frame slot,
+   so the decision is made per-thread during function return by
+   maybe inserting a KLP_return_helper frame or not.
+*/
+KLP_return_helper:
+	ld	r2,24(r1)	// restore TOC (saved by ftrace_caller)
+	addi r1, r1, 32		// destroy mini stack frame
+	ld	r0,LRSAVE(r1)	// get the real return address
+	mtlr	r0
+	blr
+#endif
+
 #else
 _GLOBAL_TOC(_mcount)
 	/* Taken from output of objdump from lib64/glibc */
diff --git a/arch/powerpc/kernel/livepatch.c b/arch/powerpc/kernel/livepatch.c
new file mode 100644
index 0000000..9dace38
--- /dev/null
+++ b/arch/powerpc/kernel/livepatch.c
@@ -0,0 +1,20 @@
+#include <linux/module.h>
+#include <asm/livepatch.h>
+
+/**
+ * klp_write_module_reloc() - write a relocation in a module
+ * @mod:       module in which the section to be modified is found
+ * @type:      ELF relocation type (see asm/elf.h)
+ * @loc:       address that the relocation should be written to
+ * @value:     relocation value (sym address + addend)
+ *
+ * This function writes a relocation to the specified location for
+ * a particular module.
+ */
+int klp_write_module_reloc(struct module *mod, unsigned long type,
+			    unsigned long loc, unsigned long value)
+{
+	/* This requires infrastructure changes; we need the loadinfos. */
+	pr_err("lpc_write_module_reloc not yet supported\n");
+	return -ENOSYS;
+}
-- 
1.8.5.6