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Date: Tue, 1 Dec 2015 16:06:01 -0500
From: Jessica Yu <jeyu@...hat.com>
To: Rusty Russell <rusty@...tcorp.com.au>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Seth Jennings <sjenning@...hat.com>,
Jiri Kosina <jikos@...nel.org>,
Vojtech Pavlik <vojtech@...e.com>,
Jonathan Corbet <corbet@....net>,
Miroslav Benes <mbenes@...e.cz>
Cc: linux-api@...r.kernel.org, live-patching@...r.kernel.org,
x86@...nel.org, linux-kernel@...r.kernel.org,
linux-s390@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: module: preserve Elf information for livepatch modules
+++ Jessica Yu [30/11/15 23:21 -0500]:
>For livepatch modules, copy Elf section, symbol, and string information
>from the load_info struct in the module loader.
>
>Livepatch uses special relocation sections in order to be able to patch
>modules that are not yet loaded, as well as apply patches to the kernel
>when the addresses of symbols cannot be determined at compile time (for
>example, when kaslr is enabled). Livepatch modules must preserve Elf
>information such as section indices in order to apply the remaining
>relocation sections at the appropriate time (i.e. when the target module
>loads).
>
>Signed-off-by: Jessica Yu <jeyu@...hat.com>
>---
> include/linux/module.h | 9 +++++
> kernel/module.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 105 insertions(+), 2 deletions(-)
>
>diff --git a/include/linux/module.h b/include/linux/module.h
>index 3a19c79..9b46256 100644
>--- a/include/linux/module.h
>+++ b/include/linux/module.h
>@@ -425,6 +425,14 @@ struct module {
>
> /* Notes attributes */
> struct module_notes_attrs *notes_attrs;
>+
>+ /* Elf information (optionally saved) */
>+ Elf_Ehdr *hdr;
>+ Elf_Shdr *sechdrs;
>+ char *secstrings;
>+ struct {
>+ unsigned int sym, str, mod, vers, info, pcpu;
>+ } index;
> #endif
This particular patch unforunately breaks !CONFIG_KALLSYMS kernel
builds, as I've just discovered. These fields should move out of the
CONFIG_KALLSYMS block. And..
> /* The command line arguments (may be mangled). People like
>@@ -461,6 +469,7 @@ struct module {
> #endif
>
> #ifdef CONFIG_LIVEPATCH
>+ bool klp; /* Is this a livepatch module? */
> bool klp_alive;
> #endif
>
>diff --git a/kernel/module.c b/kernel/module.c
>index 8f051a1..433c2d6 100644
>--- a/kernel/module.c
>+++ b/kernel/module.c
>@@ -1984,6 +1984,13 @@ static void unset_module_core_ro_nx(struct module *mod) { }
> static void unset_module_init_ro_nx(struct module *mod) { }
> #endif
>
>+static void free_module_elf(struct module *mod)
>+{
>+ kfree(mod->hdr);
>+ kfree(mod->sechdrs);
>+ kfree(mod->secstrings);
>+}
>+
> void __weak module_memfree(void *module_region)
> {
> vfree(module_region);
>@@ -2022,6 +2029,9 @@ static void free_module(struct module *mod)
> /* Free any allocated parameters. */
> destroy_params(mod->kp, mod->num_kp);
>
>+ /* Free Elf information if it was saved */
>+ free_module_elf(mod);
>+
> /* Now we can delete it from the lists */
> mutex_lock(&module_mutex);
> /* Unlink carefully: kallsyms could be walking list. */
>@@ -2137,6 +2147,10 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
> (long)sym[i].st_value);
> break;
>
>+ case SHN_LIVEPATCH:
>+ /* klp symbols are resolved by livepatch */
>+ break;
>+
> case SHN_UNDEF:
> ksym = resolve_symbol_wait(mod, info, name);
> /* Ok if resolved. */
>@@ -2185,6 +2199,10 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
> if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC))
> continue;
>
>+ /* klp relocation sections are applied by livepatch */
>+ if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
>+ continue;
>+
> if (info->sechdrs[i].sh_type == SHT_REL)
> err = apply_relocate(info->sechdrs, info->strtab,
> info->index.sym, i, mod);
>@@ -2393,6 +2411,11 @@ static char elf_type(const Elf_Sym *sym, const struct load_info *info)
> {
> const Elf_Shdr *sechdrs = info->sechdrs;
>
>+ if (ELF_ST_BIND(sym->st_info) == STB_LIVEPATCH_EXT)
>+ return 'K';
>+ if (sym->st_shndx == SHN_LIVEPATCH)
>+ return 'k';
>+
> if (ELF_ST_BIND(sym->st_info) == STB_WEAK) {
> if (ELF_ST_TYPE(sym->st_info) == STT_OBJECT)
> return 'v';
>@@ -2475,7 +2498,7 @@ static void layout_symtab(struct module *mod, struct load_info *info)
>
> /* Compute total space required for the core symbols' strtab. */
> for (ndst = i = 0; i < nsrc; i++) {
>- if (i == 0 ||
>+ if (i == 0 || mod->klp ||
> is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum)) {
> strtab_size += strlen(&info->strtab[src[i].st_name])+1;
> ndst++;
>@@ -2517,7 +2540,7 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
> mod->core_strtab = s = mod->module_core + info->stroffs;
> src = mod->symtab;
> for (ndst = i = 0; i < mod->num_symtab; i++) {
>- if (i == 0 ||
>+ if (i == 0 || mod->klp ||
> is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum)) {
> dst[ndst] = src[i];
> dst[ndst++].st_name = s - mod->core_strtab;
>@@ -2638,6 +2661,64 @@ static int elf_header_check(struct load_info *info)
> return 0;
> }
>
>+/*
>+ * copy_module_elf - preserve Elf information about a module
>+ */
>+static int copy_module_elf(struct module *mod, struct load_info *info)
>+{
>+ unsigned int size;
>+ int ret = 0;
>+ Elf_Shdr *symsect;
>+
>+ /* Elf header */
>+ size = sizeof(Elf_Ehdr);
>+ mod->hdr = kzalloc(size, GFP_KERNEL);
>+ if (mod->hdr == NULL) {
>+ ret = -ENOMEM;
>+ goto out;
>+ }
>+ memcpy(mod->hdr, info->hdr, size);
>+
>+ /* Elf section header table */
>+ size = sizeof(Elf_Shdr) * info->hdr->e_shnum;
>+ mod->sechdrs = kzalloc(size, GFP_KERNEL);
>+ if (mod->sechdrs == NULL) {
>+ ret = -ENOMEM;
>+ goto free_hdr;
>+ }
>+ memcpy(mod->sechdrs, info->sechdrs, size);
>+
>+ /* Elf section name string table */
>+ size = info->sechdrs[info->hdr->e_shstrndx].sh_size;
>+ mod->secstrings = kzalloc(size, GFP_KERNEL);
>+ if (mod->secstrings == NULL) {
>+ ret = -ENOMEM;
>+ goto free_sechdrs;
>+ }
>+ memcpy(mod->secstrings, info->secstrings, size);
>+
>+ /* Elf section indices */
>+ memcpy(&mod->index, &info->index, sizeof(info->index));
>+
>+ /*
>+ * Update symtab's sh_addr to point to a valid
>+ * symbol table, as the temporary symtab in module
>+ * init memory will be freed
>+ */
>+ symsect = mod->sechdrs + mod->index.sym;
>+ symsect->sh_addr = (unsigned long)mod->core_symtab;
..instead of relying on CONFIG_KALLSYMS being set, check for
CONFIG_KALLSYMS before the sh_addr assignment above (mod->core_symtab
only exists when CONFIG_KALLSYMS is set). Then we can leave
{free,copy}_module_elf() outside any #ifdef blocks.
Jessica
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