[PATCH v5 4/6] livepatch: reuse module loader code to write relocations

Jessica Yu
2016-03-16T19:47:06Z

Reuse module loader code to write relocations, thereby eliminating the need
for architecture specific relocation code in livepatch. Specifically, reuse
the apply_relocate_add() function in the module loader to write relocations
instead of duplicating functionality in livepatch's arch-dependent
klp_write_module_reloc() function.

In order to accomplish this, livepatch modules manage their own relocation
sections (marked with the SHF_RELA_LIVEPATCH section flag) and
livepatch-specific symbols (marked with SHN_LIVEPATCH symbol section
index). To apply livepatch relocation sections, 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 and the s390
klp_write_module_reloc() function stub. They are no longer needed since
relocation work has been offloaded to module loader.

Signed-off-by: Jessica Yu <jeyu@redhat.com>
---
arch/s390/include/asm/livepatch.h | 7 --
arch/x86/include/asm/livepatch.h | 2 -
arch/x86/kernel/Makefile | 1 -
arch/x86/kernel/livepatch.c | 70 -------------------
include/linux/livepatch.h | 20 ------
kernel/livepatch/core.c | 140 +++++++++++++++++++++++---------------
6 files changed, 84 insertions(+), 156 deletions(-)
delete mode 100644 arch/x86/kernel/livepatch.c

diff --git a/arch/s390/include/asm/livepatch.h b/arch/s390/include/asm/livepatch.h
index d5427c7..2c12137 100644
--- a/arch/s390/include/asm/livepatch.h
+++ b/arch/s390/include/asm/livepatch.h
@@ -24,13 +24,6 @@ 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)
-{
- /* not supported yet */
- return -ENOSYS;
-}
-
static inline void klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
{
regs->psw.addr = ip;
diff --git a/arch/x86/include/asm/livepatch.h b/arch/x86/include/asm/livepatch.h
index 7e68f95..a7f9181 100644
--- a/arch/x86/include/asm/livepatch.h
+++ b/arch/x86/include/asm/livepatch.h
@@ -32,8 +32,6 @@ 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 616ebd2..89f8ade 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -79,7 +79,6 @@ 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@redhat.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 c056797..8df60a2 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -63,27 +63,8 @@ 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
@@ -93,7 +74,6 @@ struct klp_reloc {
struct klp_object {
/* external */
const char *name;
- struct klp_reloc *relocs;
struct klp_func *funcs;

/* internal */
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 780f00c..2aa20fa 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -28,6 +28,8 @@
#include <linux/list.h>
#include <linux/kallsyms.h>
#include <linux/livepatch.h>
+#include <linux/elf.h>
+#include <linux/moduleloader.h>
#include <asm/cacheflush.h>

/**
@@ -50,6 +52,14 @@ struct klp_ops {
};

/*
+ * Buffers for storing temporary object and symbol names.
+ */
+struct klp_buf {
+ char symname[KSYM_SYMBOL_LEN];
+ char objname[MODULE_NAME_LEN];
+};
+
+/*
* The klp_mutex protects the global lists and state transitions of any
* structure reachable from them. References to any structure must be obtained
* under mutex protection (except in klp_ftrace_handler(), which uses RCU to
@@ -62,6 +72,11 @@ static LIST_HEAD(klp_ops);

static struct kobject *klp_root_kobj;

+static inline void klp_clear_buf(struct klp_buf *buf)
+{
+ memset(buf, 0, sizeof(*buf));
+}
+
static struct klp_ops *klp_find_ops(unsigned long old_addr)
{
struct klp_ops *ops;
@@ -204,75 +219,87 @@ 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)
-{
- const struct kernel_symbol *sym;
-
- /* 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;
+static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod)
+{
+ int i, cnt, vmlinux, ret;
+ struct klp_buf bufs = {0};
+ Elf_Rela *relas;
+ Elf_Sym *sym;
+ char *symname;
+ unsigned long sympos;
+
+ relas = (Elf_Rela *) relasec->sh_addr;
+ /* For each rela in this klp relocation section */
+ for (i = 0; i < relasec->sh_size / sizeof(Elf_Rela); i++) {
+ sym = pmod->core_kallsyms.symtab + ELF_R_SYM(relas[i].r_info);
+ if (sym->st_shndx != SHN_LIVEPATCH)
+ return -EINVAL;
+
+ klp_clear_buf(&bufs);
+
+ /* Format: .klp.sym.objname.symbol_name,sympos */
+ symname = pmod->core_kallsyms.strtab + sym->st_name;
+ cnt = sscanf(symname, ".klp.sym.%64[^.].%128[^,],%lu",
+ bufs.objname, bufs.symname, &sympos);
+ if (cnt != 3)
+ return -EINVAL;
+
+ /* klp_find_object_symbol() treats a NULL objname as vmlinux */
+ vmlinux = !strcmp(bufs.objname, "vmlinux");
+ ret = klp_find_object_symbol(vmlinux ? NULL : bufs.objname,
+ bufs.symname, sympos,
+ (unsigned long *) &sym->st_value);
+ 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 0;
}

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, cnt, ret = 0;
+ const char *objname, *secname;
+ struct klp_buf bufs = {0};
+ Elf_Shdr *sec;

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 relocation section */
+ for (i = 1; i < pmod->klp_info->hdr.e_shnum; i++) {
+ sec = pmod->klp_info->sechdrs + i;
+ if (!(sec->sh_flags & SHF_RELA_LIVEPATCH))
+ 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);
- if (ret)
- goto out;
+ klp_clear_buf(&bufs);

- 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);
- goto out;
+ /* Check if this klp relocation section belongs to obj */
+ secname = pmod->klp_info->secstrings + sec->sh_name;
+ cnt = sscanf(secname, ".klp.rela.%64[^.]", bufs.objname);
+ if (cnt != 1) {
+ ret = -EINVAL;
+ break;
}
+
+ if (strcmp(bufs.objname, objname))
+ continue;
+
+ ret = klp_resolve_symbols(sec, pmod);
+ if (ret)
+ break;
+
+ ret = apply_relocate_add(pmod->klp_info->sechdrs,
+ pmod->core_kallsyms.strtab,
+ pmod->klp_info->symndx, i, pmod);
+ if (ret)
+ break;
}

-out:
module_enable_ro(pmod);
return ret;
}
@@ -703,11 +730,9 @@ 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,
@@ -842,6 +867,9 @@ int klp_register_patch(struct klp_patch *patch)
{
int ret;

+ if (!is_livepatch_module(patch->mod))
+ return -EINVAL;
+
if (!klp_initialized())
return -ENODEV;

--
2.4.3