livepatch: reuse module loader code to write relocations

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.

Lastly, mark the module as a livepatch module so that the module loader
canappropriately identify and initialize it.

Signed-off-by: Jessica Yu <jeyu@redhat.com>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>   # for s390 changes
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 94 insertions, 54 deletions

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index d68fbf63b083..eb5db6e837aa 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>
 
 /**
@@ -204,75 +206,109 @@ 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;
-        }
-        preempt_enable();
+        int i, cnt, vmlinux, ret;
+        char objname[MODULE_NAME_LEN];
+        char symname[KSYM_NAME_LEN];
+        char *strtab = pmod->core_kallsyms.strtab;
+        Elf_Rela *relas;
+        Elf_Sym *sym;
+        unsigned long sympos, addr;
 
         /*
-         * Check if it's in another .o within the patch module. This also
-         * checks that the external symbol is unique.
+         * Since the field widths for objname and symname in the sscanf()
+         * call are hard-coded and correspond to MODULE_NAME_LEN and
+         * KSYM_NAME_LEN respectively, we must make sure that MODULE_NAME_LEN
+         * and KSYM_NAME_LEN have the values we expect them to have.
+         *
+         * Because the value of MODULE_NAME_LEN can differ among architectures,
+         * we use the smallest/strictest upper bound possible (56, based on
+         * the current definition of MODULE_NAME_LEN) to prevent overflows.
          */
-        return klp_find_object_symbol(pmod->name, name, 0, addr);
+        BUILD_BUG_ON(MODULE_NAME_LEN < 56 || KSYM_NAME_LEN != 128);
+
+        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) {
+                        pr_err("symbol %s is not marked as a livepatch symbol",
+                               strtab + sym->st_name);
+                        return -EINVAL;
+                }
+
+                /* Format: .klp.sym.objname.symname,sympos */
+                cnt = sscanf(strtab + sym->st_name,
+                             ".klp.sym.%55[^.].%127[^,],%lu",
+                             objname, symname, &sympos);
+                if (cnt != 3) {
+                        pr_err("symbol %s has an incorrectly formatted name",
+                               strtab + sym->st_name);
+                        return -EINVAL;
+                }
+
+                /* klp_find_object_symbol() treats a NULL objname as vmlinux */
+                vmlinux = !strcmp(objname, "vmlinux");
+                ret = klp_find_object_symbol(vmlinux ? NULL : objname,
+                                             symname, sympos, &addr);
+                if (ret)
+                        return ret;
+
+                sym->st_value = 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;
+        char sec_objname[MODULE_NAME_LEN];
+        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;
+                secname = pmod->klp_info->secstrings + sec->sh_name;
+                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);
+                /*
+                 * Format: .klp.rela.sec_objname.section_name
+                 * See comment in klp_resolve_symbols() for an explanation
+                 * of the selected field width value.
+                 */
+                cnt = sscanf(secname, ".klp.rela.%55[^.]", sec_objname);
+                if (cnt != 1) {
+                        pr_err("section %s has an incorrectly formatted name",
+                               secname);
+                        ret = -EINVAL;
+                        break;
+                }
+
+                if (strcmp(objname, sec_objname))
+                        continue;
+
+                ret = klp_resolve_symbols(sec, pmod);
                 if (ret)
-                        goto out;
+                        break;
 
-                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;
-                }
+                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 +739,9 @@ 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 +876,12 @@ int klp_register_patch(struct klp_patch *patch)
 {
         int ret;
 
+        if (!is_livepatch_module(patch->mod)) {
+                pr_err("module %s is not marked as a livepatch module",
+                       patch->mod->name);
+                return -EINVAL;
+        }
+
         if (!klp_initialized())
                 return -ENODEV;