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SubjectRe: [RFC PATCH v4 4/6] livepatch: reuse module loader code to write relocations
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.

> @@ -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;

IMO whitespace after all the gotos would help with readability.

Also, since the "out" label is just a return, the "goto out"'s can all
just be replaced with returns.

> +
> + 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);

I think this code is self-evident and the comment isn't needed.

> 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
>

--
Josh

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