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Subject[RFC PATCH v4 4/6] livepatch: reuse module loader code to write relocations
Date
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 | 30 ++--
kernel/livepatch/core.c | 283 ++++++++++++++++++++++++++++++--------
6 files changed, 238 insertions(+), 155 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 a52b6cc..350a751 100644
--- a/arch/s390/include/asm/livepatch.h
+++ b/arch/s390/include/asm/livepatch.h
@@ -25,13 +25,6 @@ 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 e795f52..d7c2b57 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@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 fdd5f1c..1a40a72 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 +103,16 @@ struct klp_patch {
enum klp_state state;
};

+/*
+ * Livepatch symbol and relocation section prefixes:
+ * ".klp.rela." for relocation sections
+ * ".klp.sym." for livepatch symbols
+ */
+#define KLP_SYM_PREFIX ".klp.sym."
+#define KLP_SYM_PREFIX_LEN 9
+#define KLP_RELASEC_PREFIX ".klp.rela."
+#define KLP_RELASEC_PREFIX_LEN 10
+
#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 7aa975d..c1fe57c 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>

/**
@@ -87,6 +90,166 @@ static bool klp_is_object_loaded(struct klp_object *obj)
return !obj->name || obj->mod;
}

+/*
+ * Check if a livepatch symbol is formatted properly.
+ *
+ * See Documentation/livepatch/module-elf-format.txt for a
+ * detailed outline of requirements.
+ */
+static int klp_check_symbol_format(struct module *pmod, Elf_Sym *sym)
+{
+ size_t len;
+ char *s, *objname, *symname;
+
+ if (sym->st_shndx != SHN_LIVEPATCH)
+ return -EINVAL;
+
+ /*
+ * Livepatch symbol names must follow this format:
+ * .klp.sym.objname.symbol_name,sympos
+ */
+ s = pmod->strtab + sym->st_name;
+ /* [.klp.sym.]objname.symbol_name,sympos */
+ if (!s || strncmp(s, KLP_SYM_PREFIX, KLP_SYM_PREFIX_LEN))
+ return -EINVAL;
+
+ /* .klp.sym.[objname].symbol_name,sympos */
+ objname = s + KLP_SYM_PREFIX_LEN;
+ len = strcspn(objname, ".");
+ if (!(len > 0))
+ return -EINVAL;
+
+ /* .klp.sym.objname.symbol_name,[sympos] */
+ if (!strchr(s, ','))
+ return -EINVAL;
+
+ /* .klp.sym.objname.[symbol_name],sympos */
+ symname = objname + len + 1;
+ len = strcspn(symname, ",");
+ if (!(len > 0))
+ return -EINVAL;
+
+ return 0;
+}
+
+/*
+ * Check if a livepatch relocation section is formatted properly.
+ *
+ * See Documentation/livepatch/module-elf-format.txt for a
+ * detailed outline of requirements.
+ */
+static int klp_check_relasec_format(struct module *pmod, Elf_Shdr *relasec)
+{
+ char *secname;
+ size_t len;
+
+ secname = pmod->klp_info->secstrings + relasec->sh_name;
+ /* [.klp.rela.]objname.section_name */
+ if (!secname || strncmp(secname, KLP_RELASEC_PREFIX,
+ KLP_RELASEC_PREFIX_LEN))
+ return -EINVAL;
+
+ /* .klp.rela.[objname].section_name */
+ len = strcspn(secname + KLP_RELASEC_PREFIX_LEN, ".");
+ if (!(len > 0))
+ return -EINVAL;
+
+ return 0;
+}
+
+/*
+ * Check if obj->name matches the objname encoded in the rela
+ * section name (.klp.rela.[objname].section_name)
+ *
+ * Must pass klp_check_relasec_format() before calling this.
+ */
+static bool klp_relasec_matches_object(struct module *pmod, Elf_Shdr *relasec,
+ struct klp_object *obj)
+{
+ size_t len;
+ const char *obj_objname, *sec_objname, *secname;
+
+ secname = pmod->klp_info->secstrings + relasec->sh_name;
+ /* .klp.rela.[objname].section_name */
+ sec_objname = secname + KLP_RELASEC_PREFIX_LEN;
+ obj_objname = klp_is_module(obj) ? obj->name : "vmlinux";
+
+ /* Get length of the objname encoded in the section name */
+ len = strcspn(sec_objname, ".");
+
+ if (strlen(obj_objname) != len)
+ return false;
+
+ return strncmp(sec_objname, obj_objname, len) ? false : true;
+}
+
+/*
+ * klp_get_* helper functions
+ *
+ * klp_get_* functions extract different components of the name
+ * of a livepatch symbol. The full symbol name from the strtab
+ * is passed in as parameter @s, and @result is filled in with
+ * the extracted component.
+ *
+ * These functions assume a correctly formatted symbol and the
+ * klp_check_symbol_format() test *must* pass before calling any
+ * of these functions.
+ */
+
+/* .klp.sym.[objname].symbol_name,sympos */
+static int klp_get_sym_objname(char *s, char **result)
+{
+ size_t len;
+ char *objname, *objname_start;
+
+ /* .klp.sym.[objname].symbol_name,sympos */
+ objname_start = s + KLP_SYM_PREFIX_LEN;
+ len = strcspn(objname_start, ".");
+ objname = kstrndup(objname_start, len, GFP_KERNEL);
+ if (objname == NULL)
+ return -ENOMEM;
+
+ /* klp_find_object_symbol() treats NULL as vmlinux */
+ if (!strcmp(objname, "vmlinux")) {
+ *result = NULL;
+ kfree(objname);
+ } else
+ *result = objname;
+
+ return 0;
+}
+
+/* .klp.sym.objname.[symbol_name],sympos */
+static int klp_get_symbol_name(char *s, char **result)
+{
+ size_t len;
+ char *objname, *symname;
+
+ /* .klp.sym.[objname].symbol_name,sympos */
+ objname = s + KLP_SYM_PREFIX_LEN;
+ len = strcspn(objname, ".");
+
+ /* .klp.sym.objname.[symbol_name],sympos */
+ symname = objname + len + 1;
+ len = strcspn(symname, ",");
+
+ *result = kstrndup(symname, len, GFP_KERNEL);
+ if (*result == NULL)
+ return -ENOMEM;
+
+ return 0;
+}
+
+/* .klp.sym.objname.symbol_name,[sympos] */
+static int klp_get_sympos(char *s, unsigned long *result)
+{
+ char *sympos;
+
+ /* .klp.sym.symbol_name,[sympos] */
+ sympos = strchr(s, ',') + 1;
+ return kstrtol(sympos, 10, result);
+}
+
/* sets obj->mod if object is not vmlinux and module is found */
static void klp_find_object_module(struct klp_object *obj)
{
@@ -204,74 +367,83 @@ 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 *relasec, struct module *pmod)
{
- 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;
+ int i, ret = 0;
+ Elf_Rela *relas;
+ Elf_Sym *sym;
+ char *s, *symbol_name, *sym_objname;
+ unsigned long sympos;
+
+ relas = (Elf_Rela *) relasec->sh_addr;
+ /* For each rela in this .klp.rela. section */
+ for (i = 0; i < relasec->sh_size / sizeof(Elf_Rela); i++) {
+ sym = pmod->symtab + ELF_R_SYM(relas[i].r_info);
+
+ /* Check if the symbol is formatted correctly */
+ ret = klp_check_symbol_format(pmod, sym);
+ if (ret)
+ goto out;
+ /* Format: .klp.sym.objname.symbol_name,sympos */
+ s = pmod->strtab + sym->st_name;
+ ret = klp_get_symbol_name(s, &symbol_name);
+ if (ret)
+ goto out;
+ ret = klp_get_sym_objname(s, &sym_objname);
+ if (ret)
+ goto free_symbol_name;
+ ret = klp_get_sympos(s, &sympos);
+ if (ret)
+ goto free_objname;
+
+ ret = klp_find_object_symbol(sym_objname, symbol_name, sympos,
+ (unsigned long *) &sym->st_value);
+free_objname:
+ kfree(sym_objname);
+free_symbol_name:
+ kfree(symbol_name);
+ if (ret)
+ goto out;
}
- 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);
+out:
+ 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, ret = 0;
+ Elf_Shdr *sec;

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

- if (WARN_ON(!obj->relocs))
- return -EINVAL;
-
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);
+ /* Check if the klp section is formatted correctly */
+ ret = klp_check_relasec_format(pmod, sec);
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);
+ /* Check if the klp section belongs to obj */
+ if (!klp_relasec_matches_object(pmod, sec, obj))
+ continue;
+
+ /* Resolve all livepatch syms referenced in this rela section */
+ ret = klp_resolve_symbols(sec, pmod);
+ if (ret)
goto out;
- }
- }

+ ret = apply_relocate_add(pmod->klp_info->sechdrs, pmod->strtab,
+ pmod->klp_info->symndx, i, pmod);
+ if (ret)
+ goto out;
+ }
out:
module_enable_ro(pmod);
return ret;
@@ -703,11 +875,9 @@ 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 +1012,9 @@ 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
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