Date:	Tue, 12 Jan 2016 10:12:07 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Jessica Yu <jeyu@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Jonathan Corbet <corbet@....net>,
	Miroslav Benes <mbenes@...e.cz>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: livepatch: reuse module loader code to write relocations

On Mon 2016-01-11 21:05:52, Josh Poimboeuf wrote:
> On Mon, Jan 11, 2016 at 05:35:13PM -0500, Jessica Yu wrote:
> > +++ Josh Poimboeuf [11/01/16 15:33 -0600]:
> > >On Fri, Jan 08, 2016 at 02:28:22PM -0500, Jessica Yu wrote:
> > >>Reuse module loader code to write relocations, thereby eliminating the need
> > >>for architecture specific relocation code in livepatch. Namely, we reuse
> > >>apply_relocate_add() in the module loader to write relocations instead of
> > >>duplicating functionality in livepatch's klp_write_module_reloc(). To apply
> > >>relocation sections, remaining SHN_LIVEPATCH symbols referenced by relocs
> > >>are resolved and then apply_relocate_add() is called to apply those
> > >>relocations.
> > >>
> > >>In addition, remove x86 livepatch relocation code. It is no longer needed
> > >>since symbol resolution and relocation work have been offloaded to module
> > >>loader.
> > >>
> > >>Signed-off-by: Jessica Yu <jeyu@...hat.com>
> > >>---
> > >> arch/x86/include/asm/livepatch.h |   2 -
> > >> arch/x86/kernel/Makefile         |   1 -
> > >> arch/x86/kernel/livepatch.c      |  70 ---------------------------
> > >> include/linux/livepatch.h        |  33 +++++--------
> > >> kernel/livepatch/core.c          | 101 +++++++++++++++++++--------------------
> > >> 5 files changed, 62 insertions(+), 145 deletions(-)
> > >> delete mode 100644 arch/x86/kernel/livepatch.c
> > >>
> > >>diff --git a/arch/x86/include/asm/livepatch.h b/arch/x86/include/asm/livepatch.h
> > >>index 19c099a..7312e25 100644
> > >>--- a/arch/x86/include/asm/livepatch.h
> > >>+++ b/arch/x86/include/asm/livepatch.h
> > >>@@ -33,8 +33,6 @@ static inline int klp_check_compiler_support(void)
> > >> #endif
> > >> 	return 0;
> > >> }
> > >>-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)
> > >> {
> > >>diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> > >>index b1b78ff..c5e9a5c 100644
> > >>--- a/arch/x86/kernel/Makefile
> > >>+++ b/arch/x86/kernel/Makefile
> > >>@@ -67,7 +67,6 @@ obj-$(CONFIG_X86_MPPARSE)	+= mpparse.o
> > >> obj-y				+= apic/
> > >> obj-$(CONFIG_X86_REBOOTFIXUPS)	+= reboot_fixups_32.o
> > >> obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
> > >>-obj-$(CONFIG_LIVEPATCH)		+= livepatch.o
> > >> obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += ftrace.o
> > >> obj-$(CONFIG_FTRACE_SYSCALLS)	+= ftrace.o
> > >> obj-$(CONFIG_X86_TSC)		+= trace_clock.o
> > >>diff --git a/arch/x86/kernel/livepatch.c b/arch/x86/kernel/livepatch.c
> > >>deleted file mode 100644
> > >>index 92fc1a5..0000000
> > >>--- a/arch/x86/kernel/livepatch.c
> > >>+++ /dev/null
> > >>@@ -1,70 +0,0 @@
> > >>-/*
> > >>- * livepatch.c - x86-specific Kernel Live Patching Core
> > >>- *
> > >>- * Copyright (C) 2014 Seth Jennings <sjenning@...hat.com>
> > >>- * Copyright (C) 2014 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/module.h>
> > >>-#include <linux/uaccess.h>
> > >>-#include <asm/elf.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)
> > >>-{
> > >>-	size_t size = 4;
> > >>-	unsigned long val;
> > >>-	unsigned long core = (unsigned long)mod->core_layout.base;
> > >>-	unsigned long core_size = mod->core_layout.size;
> > >>-
> > >>-	switch (type) {
> > >>-	case R_X86_64_NONE:
> > >>-		return 0;
> > >>-	case R_X86_64_64:
> > >>-		val = value;
> > >>-		size = 8;
> > >>-		break;
> > >>-	case R_X86_64_32:
> > >>-		val = (u32)value;
> > >>-		break;
> > >>-	case R_X86_64_32S:
> > >>-		val = (s32)value;
> > >>-		break;
> > >>-	case R_X86_64_PC32:
> > >>-		val = (u32)(value - loc);
> > >>-		break;
> > >>-	default:
> > >>-		/* unsupported relocation type */
> > >>-		return -EINVAL;
> > >>-	}
> > >>-
> > >>-	if (loc < core || loc >= core + core_size)
> > >>-		/* loc does not point to any symbol inside the module */
> > >>-		return -EINVAL;
> > >>-
> > >>-	return probe_kernel_write((void *)loc, &val, size);
> > >>-}
> > >>diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > >>index a882865..2f12ce7 100644
> > >>--- a/include/linux/livepatch.h
> > >>+++ b/include/linux/livepatch.h
> > >>@@ -65,27 +65,8 @@ struct klp_func {
> > >> };
> > >>
> > >> /**
> > >>- * struct klp_reloc - relocation structure for live patching
> > >>- * @loc:	address where the relocation will be written
> > >>- * @sympos:	position in kallsyms to disambiguate symbols (optional)
> > >>- * @type:	ELF relocation type
> > >>- * @name:	name of the referenced symbol (for lookup/verification)
> > >>- * @addend:	offset from the referenced symbol
> > >>- * @external:	symbol is either exported or within the live patch module itself
> > >>- */
> > >>-struct klp_reloc {
> > >>-	unsigned long loc;
> > >>-	unsigned long sympos;
> > >>-	unsigned long type;
> > >>-	const char *name;
> > >>-	int addend;
> > >>-	int external;
> > >>-};
> > >>-
> > >>-/**
> > >>  * struct klp_object - kernel object structure for live patching
> > >>  * @name:	module name (or NULL for vmlinux)
> > >>- * @relocs:	relocation entries to be applied at load time
> > >>  * @funcs:	function entries for functions to be patched in the object
> > >>  * @kobj:	kobject for sysfs resources
> > >>  * @mod:	kernel module associated with the patched object
> > >>@@ -95,7 +76,6 @@ struct klp_reloc {
> > >> struct klp_object {
> > >> 	/* external */
> > >> 	const char *name;
> > >>-	struct klp_reloc *relocs;
> > >> 	struct klp_func *funcs;
> > >>
> > >> 	/* internal */
> > >>@@ -123,6 +103,19 @@ struct klp_patch {
> > >> 	enum klp_state state;
> > >> };
> > >>
> > >>+/*
> > >>+ * Livepatch-specific symbols and relocation
> > >>+ * sections are prefixed with a tag:
> > >>+ * .klp.rel. for relocation sections
> > >>+ * .klp.sym. for livepatch symbols
> > >>+ */
> > >>+#define KLP_TAG_LEN 9
> > >>+/*
> > >>+ * Livepatch-specific bits for specifying symbol
> > >>+ * positions in the Elf_Sym st_other field
> > >>+ */
> > >>+#define KLP_SYMPOS(o) (o >> 2) & 0xff
> > >>+
> > >Can st_value be used instead?  I think we ended up deciding that would
> > >be better:
>
> Maybe encoding it in the sym name is the safest bet.  Why not, we've
> already got a bunch of other stuff there anyway :-)  One idea would be
> to append it with ',<sympos>' to be consistent with the sysfs entries:
>
>   .klp.sym.vmlinux.symname,1

This looks like the most safe solution to me as well.

Best Regards,
Petr

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