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SubjectRe: livepatch: reuse module loader code to write relocations
+++ Josh Poimboeuf [08/02/16 14:26 -0600]:
>On Wed, Feb 03, 2016 at 08:11:09PM -0500, Jessica Yu wrote:
>> 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)
>> {
>
>I think all the above string parsing code could be replaced with a
>couple of sscanf() calls.
>
>For example:
>
> char objname[64], symname[256];
>
> ret = sscanf(s, ".klp.sym.%63[^.].%255[^,],%u", objname, symname, &sympos);
> if (ret < 3)
> // string doesn't match expected format
>
>That would be much simpler.
>
>Only problem is, the kernel version of sscanf() doesn't seem to support
>the '[' conversion specifier. At least not yet ;-) Adding support for
>that would be a win-win: less code overall, and the addition of a useful
>scanf feature which could be used by other code.

Ah, I had completely forgotten that we have sscanf() in the kernel :-)
That would look *much* nicer, assuming we can get kernel sscanf() to
support the bracket character classes..

Jessica

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