[RFC PATCH v3 4/6] livepatch: reuse module loader code to write relocations

From: Jessica Yu
Date: Fri Jan 08 2016 - 14:30:15 EST

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@xxxxxxxxxx>
---
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@xxxxxxxxxx>
- * 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
+
#define klp_for_each_object(patch, obj) \
for (obj = patch->objs; obj->funcs; obj++)

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index bc2c85c..64536a4 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -28,6 +28,9 @@
#include <linux/list.h>
#include <linux/kallsyms.h>
#include <linux/livepatch.h>
+#include <linux/elf.h>
+#include <linux/string.h>
+#include <linux/moduleloader.h>
#include <asm/cacheflush.h>

/**
@@ -204,74 +207,70 @@ static int klp_find_object_symbol(const char *objname, const char *name,
return -EINVAL;
}

-/*
- * external symbols are located outside the parent object (where the parent
- * object is either vmlinux or the kmod being patched).
- */
-static int klp_find_external_symbol(struct module *pmod, const char *name,
- unsigned long *addr)
+static int klp_resolve_symbols(Elf_Shdr *relsec, struct module *pmod)
{
- const struct kernel_symbol *sym;
+ int i, len, ret = 0;
+ Elf_Rela *relas;
+ Elf_Sym *sym;
+ char *symname, *sym_objname;

- /* first, check if it's an exported symbol */
- preempt_disable();
- sym = find_symbol(name, NULL, NULL, true, true);
- if (sym) {
- *addr = sym->value;
- preempt_enable();
- return 0;
+ relas = (Elf_Rela *) relsec->sh_addr;
+ /* For each rela in this .klp.rel. section */
+ for (i = 0; i < relsec->sh_size / sizeof(Elf_Rela); i++) {
+ sym = pmod->core_symtab + ELF_R_SYM(relas[i].r_info);
+ symname = pmod->core_strtab + sym->st_name;
+
+ len = strcspn(symname + KLP_TAG_LEN, ".");
+ sym_objname = strncmp(symname + KLP_TAG_LEN, "vmlinux", len) ?
+ kstrndup(symname + KLP_TAG_LEN, len, GFP_KERNEL) : NULL;
+ /* .klp.sym.objname.[symbol_name] */
+ symname += KLP_TAG_LEN + len + 1;
+
+ ret = klp_find_object_symbol(sym_objname, symname,
+ KLP_SYMPOS(sym->st_other),
+ (unsigned long *) &sym->st_value);
+ kfree(sym_objname);
+ if (ret)
+ return ret;
}
- preempt_enable();

- /*
- * Check if it's in another .o within the patch module. This also
- * checks that the external symbol is unique.
- */
- return klp_find_object_symbol(pmod->name, name, 0, addr);
+ return ret;
}

static int klp_write_object_relocations(struct module *pmod,
struct klp_object *obj)
{
- int ret = 0;
- unsigned long val;
- struct klp_reloc *reloc;
+ int i, len, ret = 0;
+ char *secname;
+ const char *objname;

if (WARN_ON(!klp_is_object_loaded(obj)))
return -EINVAL;

- if (WARN_ON(!obj->relocs))
- return -EINVAL;
+ objname = klp_is_module(obj) ? obj->name : "vmlinux";

module_disable_ro(pmod);
+ /* For each klp rela section for this object */
+ for (i = 1; i < pmod->info->hdr->e_shnum; i++) {
+ if (!(pmod->info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH))
+ continue;
+
+ secname = pmod->info->secstrings + pmod->info->sechdrs[i].sh_name;
+ /* .klp.rel.[objname].section_name */
+ len = strcspn(secname + KLP_TAG_LEN, ".");
+
+ if (strncmp(objname, secname + KLP_TAG_LEN, len))
+ continue;

- for (reloc = obj->relocs; reloc->name; reloc++) {
- /* discover the address of the referenced symbol */
- if (reloc->external) {
- if (reloc->sympos > 0) {
- pr_err("non-zero sympos for external reloc symbol '%s' is not supported\n",
- reloc->name);
- ret = -EINVAL;
- goto out;
- }
- ret = klp_find_external_symbol(pmod, reloc->name, &val);
- } else
- ret = klp_find_object_symbol(obj->name,
- reloc->name,
- reloc->sympos,
- &val);
+ ret = klp_resolve_symbols(pmod->info->sechdrs + i, pmod);
if (ret)
goto out;

- ret = klp_write_module_reloc(pmod, reloc->type, reloc->loc,
- val + reloc->addend);
- if (ret) {
- pr_err("relocation failed for symbol '%s' at 0x%016lx (%d)\n",
- reloc->name, val, ret);
+ ret = apply_relocate_add(pmod->info->sechdrs, pmod->core_strtab,
+ pmod->info->index.sym, i, pmod);
+ if (ret)
goto out;
- }
}
-
out:
module_enable_ro(pmod);
return ret;
@@ -703,11 +702,9 @@ static int klp_init_object_loaded(struct klp_patch *patch,
struct klp_func *func;
int ret;

- if (obj->relocs) {
- ret = klp_write_object_relocations(patch->mod, obj);
- if (ret)
- return ret;
- }
+ ret = klp_write_object_relocations(patch->mod, obj);
+ if (ret)
+ return ret;

klp_for_each_func(obj, func) {
ret = klp_find_object_symbol(obj->name, func->old_name,
--
2.4.3