arm: Add livepatch arch specific code

[Jessica Yu]

+++ Jessica Yu [16/01/17 19:22 -0500]:
>+++ Miroslav Benes [16/01/17 17:47 +0100]:
>>On Tue, 6 Dec 2016, Abel Vesa wrote:
>>
>>>klp_get_ftrace_location is used by ftrace to get the entry for a
>>>specific function from the mcount list. klp_arch_set_pc is used
>>>to set the pc from the regs passed as an argument to the
>>>ftrace_ops_no_ops function to the starting address of the patched
>>>function. klp_write_module_reloc is not doing anything at this
>>>moment.
>>>
>>>Signed-off-by: Abel Vesa <abelvesa at linux.com>
>>>---
>>> MAINTAINERS                      |  3 +++
>>> arch/arm/include/asm/livepatch.h | 46 ++++++++++++++++++++++++++++++++++++++++
>>> arch/arm/kernel/livepatch.c      | 43 +++++++++++++++++++++++++++++++++++++
>>> 3 files changed, 92 insertions(+)
>>> create mode 100644 arch/arm/include/asm/livepatch.h
>>> create mode 100644 arch/arm/kernel/livepatch.c
>>>
>>>diff --git a/MAINTAINERS b/MAINTAINERS
>>>index bd182a1..d43b790 100644
>>>--- a/MAINTAINERS
>>>+++ b/MAINTAINERS
>>>@@ -7466,12 +7466,15 @@ M:	Josh Poimboeuf <jpoimboe at redhat.com>
>>> M:	Jessica Yu <jeyu at redhat.com>
>>> M:	Jiri Kosina <jikos at kernel.org>
>>> M:	Miroslav Benes <mbenes at suse.cz>
>>>+M:	Abel Vesa <abelvesa at linux.com>
>>> R:	Petr Mladek <pmladek at suse.com>
>>> S:	Maintained
>>> F:	kernel/livepatch/
>>> F:	include/linux/livepatch.h
>>> F:	arch/x86/include/asm/livepatch.h
>>> F:	arch/x86/kernel/livepatch.c
>>>+F:	arch/arm/include/asm/livepatch.h
>>>+F:	arch/arm/kernel/livepatch.c
>>> F:	Documentation/livepatch/
>>> F:	Documentation/ABI/testing/sysfs-kernel-livepatch
>>> F:	samples/livepatch/
>>
>>Thanks for the offer but I think we manage quite well ;)
>>
>>>diff --git a/arch/arm/include/asm/livepatch.h b/arch/arm/include/asm/livepatch.h
>>>new file mode 100644
>>>index 0000000..d4e3ff0
>>>--- /dev/null
>>>+++ b/arch/arm/include/asm/livepatch.h
>>>@@ -0,0 +1,46 @@
>>>+/*
>>>+ * livepatch.h - arm specific Kernel Live Patching Core
>>>+ *
>>>+ * Copyright (C) 2016 Abel Vesa <abelvesa at linux.com>
>>>+ *
>>>+ * 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_ARM_LIVEPATCH_H
>>>+#define _ASM_ARM_LIVEPATCH_H
>>>+
>>>+#include <asm/setup.h>
>>>+#include <linux/module.h>
>>>+#include <linux/ftrace.h>
>>>+
>>>+static inline int klp_check_compiler_support(void)
>>>+{
>>>+	return 0;
>>>+}
>>>+
>>>+int klp_write_module_reloc(struct module *mod, unsigned long type,
>>>+			   unsigned long loc, unsigned long value);
>>
>>This is not needed. See below.
>>
>>>+
>>>+static inline void klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
>>>+{
>>>+	regs->uregs[15] = ip;
>>>+}
>>>+
>>>+#define klp_get_ftrace_location klp_get_ftrace_location
>>>+static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
>>>+{
>>>+	return ftrace_location_range(faddr, faddr + 24);
>>
>>Comment here about +24 would be great. See similar in powerpc header file.
>>
>>>+}
>>>+
>>>+#endif /* _ASM_ARM_LIVEPATCH_H */
>>>diff --git a/arch/arm/kernel/livepatch.c b/arch/arm/kernel/livepatch.c
>>>new file mode 100644
>>>index 0000000..0656cd6
>>>--- /dev/null
>>>+++ b/arch/arm/kernel/livepatch.c
>>>@@ -0,0 +1,43 @@
>>>+/*
>>>+ * livepatch.c - arm specific Kernel Live Patching Core
>>>+ *
>>>+ * Copyright (C) 2016 Abel Vesa <abelvesa at linux.com>
>>>+ *
>>>+ * 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/module.h>
>>>+#include <linux/uaccess.h>
>>>+#include <linux/ftrace.h>
>>>+#include <asm/elf.h>
>>>+#include <asm/livepatch.h>
>>>+#include <asm/insn.h>
>>>+#include <asm/ftrace.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)
>>>+{
>>>+	/* Not implemented yet */
>>>+	return 0;
>>>+}
>>
>>This whole file is not needed. Livepatching uses kernel's internal
>>apply_relocate_add() for dealing with relocations. The only thing you need
>>to do is to make sure that all needed arch-specific info is preserved
>>during a module loading. Specifically mod_arch_specific structure needs to
>>be examined in this respect.
>
>Yup. Since we rely on apply_relocate_add() to apply relocations, just
>make sure that you preserve anything (e.g., plt tables) you need to
>make that call post-module init. In the case of s390, we needed to
>make sure we kept mod->arch.syminfo, so that the call to
>apply_relocate_add() would succeed.
>
>>But apply_relocate_add() is not implemented on arm yet. I guess it would
>>nice to have it... otherwise we could get to an unpleasant situation.
>>Livepatch module can rely on its livepatching relocations (that is, there
>>are some). apply_relocate_add() returns 0 on arm, so everything seems to
>>be nice and then boom some time later.
>
>Doesn't it return -ENOEXEC? MODULES_USE_ELF_RELA is not defined
>on arm, I think (see moduleloader.h).

...And I should have looked at the whole patchset first before
replying :-) I see now that a placeholder was added in patch 3/7.

Jessica