[v5][PATCH] livepatch/ppc: Enable livepatching on powerpc

Changelog v5:
	1. Removed the mini-stack frame created for klp_return_helper.
	   As a result of the mini-stack frame, function with > 8
	   arguments could not be patched
	2. Removed camel casing in the comments
Changelog v4:
	1. Renamed klp_matchaddr() to klp_get_ftrace_location()
	   and used it just to convert the function address.
	2. Synced klp_write_module_reloc() with s390(); made it
	   inline, no error message, return -ENOSYS
	3. Added an error message when including
	   powerpc/include/asm/livepatch.h without HAVE_LIVEPATCH
	4. Update some comments.
Changelog v3:
	1. Moved -ENOSYS to -EINVAL in klp_write_module_reloc
	2. Moved klp_matchaddr to use ftrace_location_range
Changelog v2:
	1. Implement review comments by Michael
	2. The previous version compared _NIP from the
	   wrong location to check for whether we
	   are going to a patched location

This patch enables live patching for powerpc. The current patch
is applied on top of topic/mprofile-kernel at
https://git.kernel.org/cgit/linux/kernel/git/powerpc/linux.git/

This patch builds on top of ftrace with regs changes and the
-mprofile-kernel changes. It detects a change in NIP after
the klp subsystem has potentially changes the NIP as a result
of a livepatch. In that case it saves the TOC in the parents
stack and the offset of the return address from the TOC in
the reserved (CR+4) space. This hack allows us to provide the
complete frame of the calling function as is to the caller
without having to create a mini-frame

Upon return from the patched function, the TOC and correct
LR is restored.

I tested the sample in the livepatch and an additional sample
that patches int_to_scsilun. I'll post out that sample if there
is interest later. I also tested ftrace functionality on the
command line to check for breakage

Signed-off-by: Torsten Duwe <redacted>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 arch/powerpc/Kconfig                 |  3 ++
 arch/powerpc/include/asm/livepatch.h | 47 ++++++++++++++++++++++++++++
 arch/powerpc/kernel/Makefile         |  1 +
 arch/powerpc/kernel/entry_64.S       | 60 ++++++++++++++++++++++++++++++++++++
 arch/powerpc/kernel/livepatch.c      | 29 +++++++++++++++++
 include/linux/ftrace.h               |  1 +
 include/linux/livepatch.h            |  2 ++
 kernel/livepatch/core.c              | 28 +++++++++++++++--
 kernel/trace/ftrace.c                | 14 ++++++++-
 9 files changed, 181 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/Kconfig b/arch/powerpc/Kconfig
index 91da283..926c0ea 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -159,6 +159,7 @@ config PPC
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select HAVE_ARCH_SECCOMP_FILTER
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
+	select HAVE_LIVEPATCH if HAVE_DYNAMIC_FTRACE_WITH_REGS
 
 config GENERIC_CSUM
 	def_bool CPU_LITTLE_ENDIAN

@@ -1110,3 +1111,5 @@ config PPC_LIB_RHEAP
 	bool
 
 source "arch/powerpc/kvm/Kconfig"
+
+source "kernel/livepatch/Kconfig"

diff --git a/arch/powerpc/include/asm/livepatch.h b/arch/powerpc/include/asm/livepatch.h
new file mode 100644
index 0000000..b9856ce
--- /dev/null
+++ b/arch/powerpc/include/asm/livepatch.h

@@ -0,0 +1,47 @@
+/*
+ * livepatch.h - powerpc-specific Kernel Live Patching Core
+ *
+ * Copyright (C) 2015 SUSE
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef _ASM_POWERPC64_LIVEPATCH_H
+#define _ASM_POWERPC64_LIVEPATCH_H
+
+#include <linux/module.h>
+
+#ifdef CONFIG_LIVEPATCH
+
+static inline int klp_check_compiler_support(void)
+{
+	return 0;
+}
+
+static inline 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. */
+	return -ENOSYS;
+}
+
+static inline void klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
+{
+	regs->nip = ip;
+}
+
+#else /* CONFIG_LIVEPATCH */
+#error Include linux/livepatch.h, not asm/livepatch.h
+#endif /* CONFIG_LIVEPATCH */
+
+#endif /* _ASM_POWERPC64_LIVEPATCH_H */

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 2da380f..b767e14 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile

@@ -119,6 +119,7 @@ obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
 obj-$(CONFIG_FUNCTION_GRAPH_TRACER)	+= ftrace.o
 obj-$(CONFIG_FTRACE_SYSCALLS)	+= ftrace.o
 obj-$(CONFIG_TRACING)		+= trace_clock.o
+obj-$(CONFIG_LIVEPATCH)		+= livepatch.o
 
 ifneq ($(CONFIG_PPC_INDIRECT_PIO),y)
 obj-y				+= iomap.o

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index ec7f8aa..e0406e6 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S

@@ -1224,6 +1224,9 @@ _GLOBAL(ftrace_caller)
 	addi	r3,r3,function_trace_op@toc@l
 	ld	r5,0(r3)
 
+#ifdef CONFIG_LIVEPATCH
+	mr	r14,r7		/* remember old NIP */
+#endif
 	/* Calculate ip from nip-4 into r3 for call below */
 	subi    r3, r7, MCOUNT_INSN_SIZE
 

@@ -1248,6 +1251,9 @@ ftrace_call:
 	/* Load ctr with the possibly modified NIP */
 	ld	r3, _NIP(r1)
 	mtctr	r3
+#ifdef CONFIG_LIVEPATCH
+	cmpd	r14,r3		/* has NIP been altered? */
+#endif
 
 	/* Restore gprs */
 	REST_8GPRS(0,r1)

@@ -1265,6 +1271,39 @@ ftrace_call:
 	ld	r0, LRSAVE(r1)
 	mtlr	r0
 
+#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.
+	 * The patch introduces a frame such that from the patched function
+	 * we return back to klp_return helper. For ABI compliance r12,
+	 * lr and LRSAVE(r1) contain the address of klp_return_helper.
+	 * We loaded ctr with the address of the patched function earlier
+	 * We use the reserved space in the parent stack frame at CR+4.
+	 * This space is stored with the contents of LR-TOC, where LR
+	 * refers to the address to return to.
+	 * Why do we need this?
+	 * After patching we need to return to a trampoline return function
+	 * that guarantees that we restore the TOC and return to the correct
+	 * caller back
+	 */
+	std	r2, 24(r1)	/* save TOC now, unconditionally. */
+	subf	r0, r2, r0	/* Calculate offset from current TOC */
+	stw	r0, 12(r1)	/* Of the final LR and save it in CR+4 */
+	bl	5f
+5:	mflr	r12
+	addi	r12, r12, (klp_return_helper + 4 - .)@l
+	std	r12, LRSAVE(r1)
+	mtlr	r12
+	mfctr	r12		/* allow for TOC calculation in newfunc */
+	bctr
+4:
+#endif
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	stdu	r1, -112(r1)
 .globl ftrace_graph_call

@@ -1281,6 +1320,27 @@ _GLOBAL(ftrace_graph_stub)
 
 _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) */
+	lwa	r0, 12(r1)	/* Load from CR+4, offset of LR w.r.t TOC */
+	add	r0, r0, r2	/* Add the offset to current TOC */
+	std	r0, LRSAVE(r1)	/* save 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..17f1a14
--- /dev/null
+++ b/arch/powerpc/kernel/livepatch.c

@@ -0,0 +1,29 @@
+/*
+ * livepatch.c - powerpc-specific Kernel Live Patching Core
+ *
+ * Copyright (C) 2015 SUSE
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+#include <linux/ftrace.h>
+#include <asm/livepatch.h>
+
+/*
+ * Live patch works only with -mprofile-kernel on PPC. In this case,
+ * the ftrace location is always within the first 16 bytes.
+ */
+unsigned long klp_get_ftrace_location(unsigned long faddr)
+{
+	return ftrace_location_range(faddr, faddr + 16);
+}

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 81de712..3481a8e 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h

@@ -455,6 +455,7 @@ int ftrace_update_record(struct dyn_ftrace *rec, int enable);
 int ftrace_test_record(struct dyn_ftrace *rec, int enable);
 void ftrace_run_stop_machine(int command);
 unsigned long ftrace_location(unsigned long ip);
+unsigned long ftrace_location_range(unsigned long start, unsigned long end);
 unsigned long ftrace_get_addr_new(struct dyn_ftrace *rec);
 unsigned long ftrace_get_addr_curr(struct dyn_ftrace *rec);
 

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index a882865..25a267b 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h

@@ -134,6 +134,8 @@ int klp_unregister_patch(struct klp_patch *);
 int klp_enable_patch(struct klp_patch *);
 int klp_disable_patch(struct klp_patch *);
 
+unsigned long klp_get_ftrace_location(unsigned long faddr);
+
 #endif /* CONFIG_LIVEPATCH */
 
 #endif /* _LINUX_LIVEPATCH_H_ */

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index bc2c85c..e2dbf01 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c

@@ -298,22 +298,39 @@ unlock:
 	rcu_read_unlock();
 }
 
+/*
+ * Convert a function address into the appropriate ftrace location.
+ *
+ * The given address is returned on most architectures. Live patching
+ * usually works only when the ftrace location is the first instruction
+ * in the function.
+ */
+unsigned long __weak klp_get_ftrace_location(unsigned long faddr)
+{
+	return faddr;
+}
+
 static void klp_disable_func(struct klp_func *func)
 {
 	struct klp_ops *ops;
+	unsigned long ftrace_loc;
 
 	if (WARN_ON(func->state != KLP_ENABLED))
 		return;
 	if (WARN_ON(!func->old_addr))
 		return;
 
+	ftrace_loc = klp_get_ftrace_location(func->old_addr);
+	if (WARN_ON(!ftrace_loc))
+		return;
+
 	ops = klp_find_ops(func->old_addr);
 	if (WARN_ON(!ops))
 		return;
 
 	if (list_is_singular(&ops->func_stack)) {
 		WARN_ON(unregister_ftrace_function(&ops->fops));
-		WARN_ON(ftrace_set_filter_ip(&ops->fops, func->old_addr, 1, 0));
+		WARN_ON(ftrace_set_filter_ip(&ops->fops, ftrace_loc, 1, 0));
 
 		list_del_rcu(&func->stack_node);
 		list_del(&ops->node);

@@ -328,6 +345,7 @@ static void klp_disable_func(struct klp_func *func)
 static int klp_enable_func(struct klp_func *func)
 {
 	struct klp_ops *ops;
+	unsigned long ftrace_loc;
 	int ret;
 
 	if (WARN_ON(!func->old_addr))

@@ -336,6 +354,10 @@ static int klp_enable_func(struct klp_func *func)
 	if (WARN_ON(func->state != KLP_DISABLED))
 		return -EINVAL;
 
+	ftrace_loc = klp_get_ftrace_location(func->old_addr);
+	if (WARN_ON(!ftrace_loc))
+		return -EINVAL;
+
 	ops = klp_find_ops(func->old_addr);
 	if (!ops) {
 		ops = kzalloc(sizeof(*ops), GFP_KERNEL);

@@ -352,7 +374,7 @@ static int klp_enable_func(struct klp_func *func)
 		INIT_LIST_HEAD(&ops->func_stack);
 		list_add_rcu(&func->stack_node, &ops->func_stack);
 
-		ret = ftrace_set_filter_ip(&ops->fops, func->old_addr, 0, 0);
+		ret = ftrace_set_filter_ip(&ops->fops, ftrace_loc, 0, 0);
 		if (ret) {
 			pr_err("failed to set ftrace filter for function '%s' (%d)\n",
 			       func->old_name, ret);

@@ -363,7 +385,7 @@ static int klp_enable_func(struct klp_func *func)
 		if (ret) {
 			pr_err("failed to register ftrace handler for function '%s' (%d)\n",
 			       func->old_name, ret);
-			ftrace_set_filter_ip(&ops->fops, func->old_addr, 1, 0);
+			ftrace_set_filter_ip(&ops->fops, ftrace_loc, 1, 0);
 			goto err;
 		}
 

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index eca592f..e1b3f23 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c

@@ -1533,7 +1533,19 @@ static int ftrace_cmp_recs(const void *a, const void *b)
 	return 0;
 }
 
-static unsigned long ftrace_location_range(unsigned long start, unsigned long end)
+/**
+ * ftrace_location_range - return the first address of a traced location
+ *	if it touches the given ip range
+ * @start: start of range to search.
+ * @end: end of range to search (inclusive). @end points to the last byte
+ *	to check.
+ *
+ * Returns rec->ip if the related ftrace location is a least partly within
+ * the given address range. That is, the first address of the instruction
+ * that is either a NOP or call to the function tracer. It checks the ftrace
+ * internal tables to determine if the address belongs or not.
+ */
+unsigned long ftrace_location_range(unsigned long start, unsigned long end)
 {
 	struct ftrace_page *pg;
 	struct dyn_ftrace *rec;

-- 
2.5.0

On Tue, Mar 08, 2016 at 06:33:57PM +1100, Balbir Singh wrote:

Changelog v5:
	1. Removed the mini-stack frame created for klp_return_helper.
	   As a result of the mini-stack frame, function with > 8
	   arguments could not be patched

Did you get my previous mails? Those functions only require special
care, the _can_ be patched. In general, writing replacement functions
always requires attention!

Have you *tested* this patch? Replacing a function in the kernel?
Replacing a function in a module? For local calls? For global calls?
I strongly doubt so because it does not work this way.

To be fair, my last mail still was not 100% correct, but the conclusion
that the mini frame is not needed at all is invalid. Please leave it as it
was, I'm working on a test / demonstrator for how to handle these.

+	 * Why do we need this?
+	 * After patching we need to return to a trampoline return function
+	 * that guarantees that we restore the TOC and return to the correct
+	 * caller back
+	 */
+	std	r2, 24(r1)	/* save TOC now, unconditionally. */
+	subf	r0, r2, r0	/* Calculate offset from current TOC */
+	stw	r0, 12(r1)	/* Of the final LR and save it in CR+4 */
+	bl	5f
+5:	mflr	r12
+	addi	r12, r12, (klp_return_helper + 4 - .)@l
+	std	r12, LRSAVE(r1)

[...]

+ * maybe inserting a klp_return_helper frame or not.
+*/
+klp_return_helper:
+	ld	r2, 24(r1)	/* restore TOC (saved by ftrace_caller) */
+	lwa	r0, 12(r1)	/* Load from CR+4, offset of LR w.r.t TOC */
+	add	r0, r0, r2	/* Add the offset to current TOC */
+	std	r0, LRSAVE(r1)	/* save the real return address */
+	mtlr	r0
+	blr
+#endif

NAKed-by: Torsten Duwe [off-list ref]

	Torsten

On 08/03/16 21:45, Torsten Duwe wrote:

On Tue, Mar 08, 2016 at 06:33:57PM +1100, Balbir Singh wrote:

quoted

Changelog v5:
	1. Removed the mini-stack frame created for klp_return_helper.
	   As a result of the mini-stack frame, function with > 8
	   arguments could not be patched

Did you get my previous mails? Those functions only require special
care, the _can_ be patched. In general, writing replacement functions
always requires attention!

Yes, I did. We did think about documenting that limitation, but the big concern
was that the system can be panic'd with a simple test case. We expect live patches
to be tested and signed so we should be OK, but there still is a window

Have you *tested* this patch? Replacing a function in the kernel?
Replacing a function in a module? For local calls? For global calls?
I strongly doubt so because it does not work this way.

Yes, if you care to read the changelog
"

I tested the sample in the livepatch and an additional sample
that patches int_to_scsilun. I'll post out that sample if there
is interest later. I also tested ftrace functionality on the
command line to check for breakage"

I've tested patching calls from module to module
(ibmvscsi to scsi_mod), within the kernel (livepatch-sample/
proc_cmdline_show). I must admit there is more testing to
be done

To be fair, my last mail still was not 100% correct, but the conclusion
that the mini frame is not needed at all is invalid. Please leave it as it
was, I'm working on a test / demonstrator for how to handle these.

Why, the magic will be in the patched function? Please share the test/demonstrator

quoted

+	 * Why do we need this?
+	 * After patching we need to return to a trampoline return function
+	 * that guarantees that we restore the TOC and return to the correct
+	 * caller back
+	 */
+	std	r2, 24(r1)	/* save TOC now, unconditionally. */
+	subf	r0, r2, r0	/* Calculate offset from current TOC */
+	stw	r0, 12(r1)	/* Of the final LR and save it in CR+4 */
+	bl	5f
+5:	mflr	r12
+	addi	r12, r12, (klp_return_helper + 4 - .)@l
+	std	r12, LRSAVE(r1)

[...]

quoted

+ * maybe inserting a klp_return_helper frame or not.
+*/
+klp_return_helper:
+	ld	r2, 24(r1)	/* restore TOC (saved by ftrace_caller) */
+	lwa	r0, 12(r1)	/* Load from CR+4, offset of LR w.r.t TOC */
+	add	r0, r0, r2	/* Add the offset to current TOC */
+	std	r0, LRSAVE(r1)	/* save the real return address */
+	mtlr	r0
+	blr
+#endif

NAKed-by: Torsten Duwe [off-list ref]

Why? For using CR+4 or removing the frame? Or you believe there is a better way to
handle this that work, IOW what is broken?

	Torsten

Balbir Singh

On Wed, Mar 09, 2016 at 12:52:03AM +1100, Balbir Singh wrote:

On 08/03/16 21:45, Torsten Duwe wrote:

quoted

To be fair, my last mail still was not 100% correct, but the conclusion

Wrote a correction to the correction. It should be clear now. Please nag me
if it isn't clear why klp_return_helper and its stack frame is needed.

quoted

that the mini frame is not needed at all is invalid. Please leave it as it
was, I'm working on a test / demonstrator for how to handle these.

Why, the magic will be in the patched function? Please share the test/demonstrator

Here it comes...

quoted

NAKed-by: Torsten Duwe [off-list ref]

Why? For using CR+4 or removing the frame? Or you believe there is a better way to
handle this that work, IOW what is broken?

The stack frame removal. You're risking a memory access or jump into nirvana
or and endless loop.

klp_return_helper will do the right thing, and functions like e.g. printk
I would live patch like this (untested :-) :

------------------------8<----------------------
#include <stdarg.h>

/* compile using "-ffixed-r14"! */
register unsigned long pass_TOC asm("r14");

/*
 * Function pre-prologue to pop the klp_return_helper
 * mini stack frame. The saved r2 TOC value is read and
 * passed in pass_TOC (r14), the original LR is passed
 * in r0 and the LR itself. R12 is updated appropriately
 * for local TOC recalculation.
 */
extern void caller(void) asm("printk");
void caller(void)
{
  asm("ld %0,24(1)" : "=r" (pass_TOC));
  asm("addi 1,1,32");
  asm("addi 12,12,(real_printk-printk)@l");
  asm("ld 0,16(1)\n\tmtlr 0");
  asm("b real_printk");
}

extern int vprintk_default(const char *fmt, va_list args);

extern int printk(const char *fmt, ...) asm("real_printk");
int printk(const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);

        r = vprintk_default(fmt, args);

        va_end(args);

	asm("mr 2,%0" : : "r" (pass_TOC));
        return r;
}
------------------------8<----------------------
Signed-off-by: Torsten Duwe <redacted>

As you can see, the extra effort for args on the stack is limited.

	Torsten

On Tue 2016-03-08 18:33:57, Balbir Singh wrote:

Changelog v5:
	1. Removed the mini-stack frame created for klp_return_helper.
	   As a result of the mini-stack frame, function with > 8
	   arguments could not be patched
	2. Removed camel casing in the comments

I tested this patch and it fails when I call a patched printk()
from a module.

You might try it with the test patch below. It is a bit twisted
because it calls the patched printk from livepatch_cmdline_proc_show()
that it added by the same patch module. Please, look at
livepatch_cmdline_proc_show(), it does:

	static int count;

	if (!count++)
		trace_printk("%s\n", "this has been live patched");
	else
		printk("%s\n", "this has been live patched");


It means that calls only trace_printk() when called first time.
It calls the patched printk when called second time.


I have tested it the following way:


# booted kernel with the changes below
# applied the patch:
$> modprobe livepatch-sample

# trigger the pached printk()
$>cat /sys/kernel/livepatch/livepatch_sample/enabled
1

# look into both dmesg and trace buffer
$> dmesg | tail -n 1
[  727.537307] patch enabled: 1
$> cat /sys/kernel/debug/tracing/trace | tail -n 1
             cat-3588  [003] ....   727.537448: livepatch_printk: patch enabled: 1

# trigger livepatch_cmdline_proc_show() 1st time
c79:~ # cat /proc/cmdline 
this has been live patched

# the message appeared only in trace buffer
$> dmesg | tail -n 1
[  727.537307] patch enabled: 1
c79:~ # cat /sys/kernel/debug/tracing/trace | tail -n 1
             cat-3511  [000] ....    862.958383:	     livepatch_cmdline_proc_show: this has been live patched


# trigger livepatch_cmdline_proc_show() 2nd time
c79:~ # cat /proc/cmdline 

!!! KABOOM !!!

It is becaused it tried to call the patched printk()?

Unable to handle kernel paging request for instruction fetch
Faulting instruction address: 0xc0000000023f014c
Oops: Kernel access of bad area, sig: 11 [#1]
SMP NR_CPUS=2048 NUMA pSeries
Modules linked in: livepatch_sample af_packet dm_mod rtc_generic e1000 ext4 crc16 mbcache jbd2 sr_mod cdrom sd_mod ibmvscsi scsi_transport_srp sg scsi_mod autofs4
CPU: 1 PID: 3514 Comm: cat Tainted: G              K 4.5.0-rc7-11-default+ #110
task: c000000003e60e20 ti: c000000003d38000 task.ti: c000000003d38000
NIP: c0000000023f014c LR: c0000000023f014c CTR: c0000000001a72c0
REGS: c000000003d3b930 TRAP: 0400   Tainted: G              K  (4.5.0-rc7-11-default+)
MSR: 8000000010009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 28222022  XER: 20000000
CFAR: c000000000009e9c SOFTE: 1 
GPR00: c0000000023f014c c000000003d3bbb0 c000000000fae100 0000000000000000 
GPR04: c0000000fea60038 000000000000000c 0000000068637461 0000000000000068 
GPR08: 0000000000000000 c000000003e627cc c000000003e60e20 d0000000023f0308 
GPR12: 0000000000002200 c000000007e80300 0000000010020360 0000000000010000 
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
GPR20: 0000010000300000 c0000000035bf540 0000000000010000 c000000003d3be00 
GPR24: c0000000035b8500 0000000000000000 fffffffffffff000 c000000003d3bc58 
GPR28: 0000010000300000 c0000000035bf500 d0000000023f0578 d0000000023f0558 
NIP [c0000000023f014c] 0xc0000000023f014c
LR [c0000000023f014c] 0xc0000000023f014c
Call Trace:
[c000000003d3bbb0] [c0000000023f014c] 0xc0000000023f014c (unreliable)
[c000000003d3bc30] [c000000000009e88] klp_return_helper+0x0/0x18
[c000000003d3bcd0] [c00000000034798c] proc_reg_read+0x8c/0xd0
[c000000003d3bd00] [c0000000002b7fbc] __vfs_read+0x4c/0x160
[c000000003d3bd90] [c0000000002b9318] vfs_read+0xa8/0x1c0
[c000000003d3bde0] [c0000000002ba61c] SyS_read+0x6c/0x110
[c000000003d3be30] [c000000000009204] system_call+0x38/0xb4
Instruction dump:
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX 
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX 
---[ end trace 17a32fcaa99f5af5 ]---



Here is the patch that I used:

From 313627cab861dd53d3325efc3d4d364eee77f9db Mon Sep 17 00:00:00 2001

From: Petr Mladek <pmladek@suse.com>
Date: Tue, 8 Mar 2016 13:51:02 +0100
Subject: [PATCH] livepatch: test_printk() patch

!!!!IMPORTANT!!!

The patch is a bit ugly because cmdline_proc_show() can be called
also by some other code. So, you might get the crash earlier than
you expect.
---
 include/linux/printk.h               |  3 +++
 kernel/livepatch/core.c              |  1 +
 kernel/printk/printk.c               | 23 +++++++++++++++++++++
 samples/livepatch/livepatch-sample.c | 39 ++++++++++++++++++++++++++++++++++++
 4 files changed, 66 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9ccbdf2c1453..ffe0ceb56972 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h

@@ -125,6 +125,9 @@ void early_printk(const char *s, ...) { }
 typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
 
 #ifdef CONFIG_PRINTK
+int vprintk_default(const char *fmt, va_list args);
+int test_printk(const char *fmt, ...);
+
 asmlinkage __printf(5, 0)
 int vprintk_emit(int facility, int level,
 		 const char *dict, size_t dictlen,

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index e2dbf0127f0f..7d0a6029043c 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c

@@ -615,6 +615,7 @@ static ssize_t enabled_show(struct kobject *kobj,
 	struct klp_patch *patch;
 
 	patch = container_of(kobj, struct klp_patch, kobj);
+	printk("patch enabled: %d\n", patch->state);
 	return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->state);
 }
 

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c963ba534a78..9f785bfbb3fd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c

@@ -1920,6 +1920,29 @@ asmlinkage __visible int printk(const char *fmt, ...)
 }
 EXPORT_SYMBOL(printk);
 
+int test_printk(const char *fmt, ...)
+{
+	printk_func_t vprintk_func;
+	va_list args;
+	int r;
+
+	va_start(args, fmt);
+
+	/*
+	 * If a caller overrides the per_cpu printk_func, then it needs
+	 * to disable preemption when calling printk(). Otherwise
+	 * the printk_func should be set to the default. No need to
+	 * disable preemption here.
+	 */
+	vprintk_func = this_cpu_read(printk_func);
+	r = vprintk_func(fmt, args);
+
+	va_end(args);
+
+	return r;
+}
+EXPORT_SYMBOL(test_printk);
+
 #else /* CONFIG_PRINTK */
 
 #define LOG_LINE_MAX		0

diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c
index fb8c8614e728..d5c09bc629e8 100644
--- a/samples/livepatch/livepatch-sample.c
+++ b/samples/livepatch/livepatch-sample.c

@@ -40,16 +40,53 @@
  */
 
 #include <linux/seq_file.h>
+#include <linux/printk.h>
 static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
 {
+	static int count;
+
+	if (!count++)
+		trace_printk("%s\n", "this has been live patched");
+	else
+		printk("%s\n", "this has been live patched");
+
 	seq_printf(m, "%s\n", "this has been live patched");
 	return 0;
 }
 
+asmlinkage static int livepatch_printk(const char *fmt, ...)
+{
+	va_list args, args2;
+	int r = 0;
+
+	va_start(args, fmt);
+
+	/*
+	 * If a caller overrides the per_cpu printk_func, then it needs
+	 * to disable preemption when calling printk(). Otherwise
+	 * the printk_func should be set to the default. No need to
+	 * disable preemption here.
+	 */
+	vprintk_default(fmt, args);
+
+	va_end(args);
+
+	va_start(args2, fmt);
+	ftrace_vprintk(fmt, args2);
+	va_end(args2);
+
+
+	return r;
+}
+
 static struct klp_func funcs[] = {
 	{
 		.old_name = "cmdline_proc_show",
 		.new_func = livepatch_cmdline_proc_show,
+	},
+	{
+		.old_name = "printk",
+		.new_func = livepatch_printk,
 	}, { }
 };
 

@@ -77,6 +114,8 @@ static int livepatch_init(void)
 		WARN_ON(klp_unregister_patch(&patch));
 		return ret;
 	}
+	/* Make sure that trace_printk buffers are allocated. */
+	trace_printk("LivePatch sample loaded\n");
 	return 0;
 }
 

-- 
1.8.5.6

On 09/03/16 03:02, Petr Mladek wrote:

On Tue 2016-03-08 18:33:57, Balbir Singh wrote:

quoted

Changelog v5:
	1. Removed the mini-stack frame created for klp_return_helper.
	   As a result of the mini-stack frame, function with > 8
	   arguments could not be patched
	2. Removed camel casing in the comments

I tested this patch and it fails when I call a patched printk()
from a module.

You might try it with the test patch below. It is a bit twisted
because it calls the patched printk from livepatch_cmdline_proc_show()
that it added by the same patch module. Please, look at
livepatch_cmdline_proc_show(), it does:

	static int count;

	if (!count++)
		trace_printk("%s\n", "this has been live patched");
	else
		printk("%s\n", "this has been live patched");


It means that calls only trace_printk() when called first time.
It calls the patched printk when called second time.


I have tested it the following way:


# booted kernel with the changes below
# applied the patch:
$> modprobe livepatch-sample

# trigger the pached printk()
$>cat /sys/kernel/livepatch/livepatch_sample/enabled
1

# look into both dmesg and trace buffer
$> dmesg | tail -n 1
[  727.537307] patch enabled: 1
$> cat /sys/kernel/debug/tracing/trace | tail -n 1
             cat-3588  [003] ....   727.537448: livepatch_printk: patch enabled: 1

# trigger livepatch_cmdline_proc_show() 1st time
c79:~ # cat /proc/cmdline 
this has been live patched

# the message appeared only in trace buffer
$> dmesg | tail -n 1
[  727.537307] patch enabled: 1
c79:~ # cat /sys/kernel/debug/tracing/trace | tail -n 1
             cat-3511  [000] ....    862.958383:	     livepatch_cmdline_proc_show: this has been live patched


# trigger livepatch_cmdline_proc_show() 2nd time
c79:~ # cat /proc/cmdline 

!!! KABOOM !!!

It is becaused it tried to call the patched printk()?

Yes, the situation is that we restored the r2 for the kernel (from ftrace_caller, it is now kernel_toc),
whereas the LR points to the module. The difference between r2 and r0 > 4GB.

Very good test case. Did it work with v4? I presume it did because we have enough space to save both

Thanks,
Balbir Singh

On 09/03/16 02:34, Torsten Duwe wrote:

On Wed, Mar 09, 2016 at 12:52:03AM +1100, Balbir Singh wrote:

quoted

On 08/03/16 21:45, Torsten Duwe wrote:

quoted

To be fair, my last mail still was not 100% correct, but the conclusion

Wrote a correction to the correction. It should be clear now. Please nag me
if it isn't clear why klp_return_helper and its stack frame is needed.

quoted

quoted

that the mini frame is not needed at all is invalid. Please leave it as it
was, I'm working on a test / demonstrator for how to handle these.

Why, the magic will be in the patched function? Please share the test/demonstrator

Here it comes...

quoted

quoted

NAKed-by: Torsten Duwe [off-list ref]

Why? For using CR+4 or removing the frame? Or you believe there is a better way to
handle this that work, IOW what is broken?

The stack frame removal. You're risking a memory access or jump into nirvana
or and endless loop.

klp_return_helper will do the right thing, and functions like e.g. printk
I would live patch like this (untested :-) :

------------------------8<----------------------
#include <stdarg.h>

/* compile using "-ffixed-r14"! */
register unsigned long pass_TOC asm("r14");

/*
 * Function pre-prologue to pop the klp_return_helper
 * mini stack frame. The saved r2 TOC value is read and
 * passed in pass_TOC (r14), the original LR is passed
 * in r0 and the LR itself. R12 is updated appropriately
 * for local TOC recalculation.
 */
extern void caller(void) asm("printk");
void caller(void)
{
  asm("ld %0,24(1)" : "=r" (pass_TOC));
  asm("addi 1,1,32");
  asm("addi 12,12,(real_printk-printk)@l");
  asm("ld 0,16(1)\n\tmtlr 0");
  asm("b real_printk");
}

extern int vprintk_default(const char *fmt, va_list args);

extern int printk(const char *fmt, ...) asm("real_printk");
int printk(const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);

        r = vprintk_default(fmt, args);

        va_end(args);

	asm("mr 2,%0" : : "r" (pass_TOC));
        return r;
}
------------------------8<----------------------
Signed-off-by: Torsten Duwe <redacted>

As you can see, the extra effort for args on the stack is limited.

Leaving it to the patch to do the right thing I think makes it more
complex and each livepatch hardware dependent to a large extent.
I find it hard to read the patch, let alone audit it and apply it or
worse create one

	Torsten

Balbir Singh.

On Tue 2016-03-08 16:34:35, Torsten Duwe wrote:

On Wed, Mar 09, 2016 at 12:52:03AM +1100, Balbir Singh wrote:

quoted

On 08/03/16 21:45, Torsten Duwe wrote:

quoted

To be fair, my last mail still was not 100% correct, but the conclusion

Wrote a correction to the correction. It should be clear now. Please nag me
if it isn't clear why klp_return_helper and its stack frame is needed.

quoted

quoted

that the mini frame is not needed at all is invalid. Please leave it as it
was, I'm working on a test / demonstrator for how to handle these.

Why, the magic will be in the patched function? Please share the test/demonstrator

Here it comes...

quoted

quoted

NAKed-by: Torsten Duwe [off-list ref]

Why? For using CR+4 or removing the frame? Or you believe there is a better way to
handle this that work, IOW what is broken?

The stack frame removal. You're risking a memory access or jump into nirvana
or and endless loop.

klp_return_helper will do the right thing, and functions like e.g. printk
I would live patch like this (untested :-) :

------------------------8<----------------------
#include <stdarg.h>

/* compile using "-ffixed-r14"! */
register unsigned long pass_TOC asm("r14");

BTW: Is this reentrant, please? I mean, is it possible to use this
hack for two functions? Could the functions call each other?

Sigh, I still need to sort all the information. I am not sure
what are the exact limitations of each proposed solution.

Thanks,
Petr

/*
 * Function pre-prologue to pop the klp_return_helper
 * mini stack frame. The saved r2 TOC value is read and
 * passed in pass_TOC (r14), the original LR is passed
 * in r0 and the LR itself. R12 is updated appropriately
 * for local TOC recalculation.
 */
extern void caller(void) asm("printk");
void caller(void)
{
  asm("ld %0,24(1)" : "=r" (pass_TOC));
  asm("addi 1,1,32");
  asm("addi 12,12,(real_printk-printk)@l");
  asm("ld 0,16(1)\n\tmtlr 0");
  asm("b real_printk");
}

extern int vprintk_default(const char *fmt, va_list args);

extern int printk(const char *fmt, ...) asm("real_printk");
int printk(const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);

        r = vprintk_default(fmt, args);

        va_end(args);

	asm("mr 2,%0" : : "r" (pass_TOC));
        return r;
}
------------------------8<----------------------
Signed-off-by: Torsten Duwe <redacted>

As you can see, the extra effort for args on the stack is limited.

	Torsten

On Wed, Mar 09, 2016 at 05:10:25PM +0100, Petr Mladek wrote:

On Tue 2016-03-08 16:34:35, Torsten Duwe wrote:

quoted

/* compile using "-ffixed-r14"! */
register unsigned long pass_TOC asm("r14");

BTW: Is this reentrant, please? I mean, is it possible to use this
hack for two functions? Could the functions call each other?

Yes, it is. pass_TOC isn't really a global variable. It only lives
between the caller and the real function's return. With this notation
the task can easily be shifted to any other register.

	Torsten