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SubjectRe: [RFC PATCH v4 2/6] module: preserve Elf information for livepatch modules
On Wed 2016-02-03 20:11:07, Jessica Yu wrote:
> For livepatch modules, copy Elf section, symbol, and string information
> from the load_info struct in the module loader. Persist copies of the
> original symbol table and string table.
>
> Livepatch manages its own relocation sections in order to reuse module
> loader code to write relocations. Livepatch modules must preserve Elf
> information such as section indices in order to apply livepatch relocation
> sections using the module loader's apply_relocate_add() function.
>
> In order to apply livepatch relocation sections, livepatch modules must
> keep a complete copy of their original symbol table in memory. Normally, a
> stripped down copy of a module's symbol table (containing only "core"
> symbols) is made available through module->core_symtab. But for livepatch
> modules, the symbol table copied into memory on module load must be exactly
> the same as the symbol table produced when the patch module was compiled.
> This is because the relocations in each livepatch relocation section refer
> to their respective symbols with their symbol indices, and the original
> symbol indices (and thus the symtab ordering) must be preserved in order
> for apply_relocate_add() to find the right symbol.
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 71c77ed..9c16eb2 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3222,6 +3331,12 @@ static noinline int do_init_module(struct module *mod)
> */
> current->flags &= ~PF_USED_ASYNC;
>
> +#ifdef CONFIG_KALLSYMS
> + /* Make symtab and strtab available prior to module init call */
> + mod->num_symtab = mod->core_num_syms;
> + mod->symtab = mod->core_symtab;
> + mod->strtab = mod->core_strtab;
> +#endif

This should be done with module_mutex. Otherwise, it looks racy
at least against module_kallsyms_on_each_symbol().

BTW: I wonder why even the original code is not racy
for example against module_get_kallsym. It is called
without the mutex. This code sets the number of entries
before the pointer to the entries.

Note that the module is in the list even in the UNFORMED state.


> do_mod_ctors(mod);
> /* Start the module */
> if (mod->init != NULL)
> @@ -3266,11 +3381,6 @@ static noinline int do_init_module(struct module *mod)
> /* Drop initial reference. */
> module_put(mod);
> trim_init_extable(mod);
> -#ifdef CONFIG_KALLSYMS
> - mod->num_symtab = mod->core_num_syms;
> - mod->symtab = mod->core_symtab;
> - mod->strtab = mod->core_strtab;
> -#endif
> mod_tree_remove_init(mod);
> disable_ro_nx(&mod->init_layout);
> module_arch_freeing_init(mod);

In each case, it was called with the mutex here.

Best Regards,
Petr

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