Re: [PATCH v3] livepatch: old_name,number scheme in livepatch sysfs directory

From: Josh Poimboeuf
Date: Mon Nov 09 2015 - 15:56:28 EST

I'd recommend splitting this up into two separate patches:

1. introduce old_sympos
2. change the sysfs interface

On Mon, Nov 09, 2015 at 10:16:05AM -0600, Chris J Arges wrote:
> In cases of duplicate symbols in vmlinux, old_sympos will be used to
> disambiguate instead of old_addr. Normally old_sympos will be 0, and
> default to only returning the first found instance of that symbol. If an
> incorrect symbol position is specified then livepatching will fail.

In the case of old_sympos == 0, instead of just returning the first
symbol it finds, I think it should ensure that the symbol is unique. As
Miroslav suggested:

0 - default, preserve more or less current behaviour. If the symbol is
unique there is no problem. If it is not the patching would fail.
1, 2, ... - occurrence of the symbol in kallsyms.

The advantage is that if the user does not care and is certain that the
symbol is unique he doesn't have to do anything. If the symbol is not
unique he still has means how to solve it.

> Finally, old_addr is now an internal structure element and not to be
> specified by the user.
>
> The following directory structure will allow for cases when the same
> function name exists in a single object.
> /sys/kernel/livepatch/<patch>/<object>/<function.number>

Period should be changed to a comma.

> The number corresponds to the nth occurrence of the symbol name in
> kallsyms for the patched object.
>
> An example of patching multiple symbols can be found here:
> https://github.com/dynup/kpatch/issues/493
>
> Signed-off-by: Chris J Arges <chris.j.arges@xxxxxxxxxxxxx>
> ---
> Documentation/ABI/testing/sysfs-kernel-livepatch | 6 ++-
> include/linux/livepatch.h | 20 ++++---
> kernel/livepatch/core.c | 66 ++++++++++++++++--------
> 3 files changed, 61 insertions(+), 31 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch
> index 5bf42a8..21b6bc1 100644
> --- a/Documentation/ABI/testing/sysfs-kernel-livepatch
> +++ b/Documentation/ABI/testing/sysfs-kernel-livepatch
> @@ -33,7 +33,7 @@ Description:
> The object directory contains subdirectories for each function
> that is patched within the object.
>
> -What: /sys/kernel/livepatch/<patch>/<object>/<function>
> +What: /sys/kernel/livepatch/<patch>/<object>/<function,number>
> Date: Nov 2014
> KernelVersion: 3.19.0
> Contact: live-patching@xxxxxxxxxxxxxxx
> @@ -41,4 +41,8 @@ Description:
> The function directory contains attributes regarding the
> properties and state of the patched function.
>
> + The directory name contains the patched function name and a
> + number corresponding to the nth occurrence of the symbol name
> + in kallsyms for the patched object.
> +
> There are currently no such attributes.
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 31db7a0..986e06d 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -37,8 +37,9 @@ enum klp_state {
> * struct klp_func - function structure for live patching
> * @old_name: name of the function to be patched
> * @new_func: pointer to the patched function code
> - * @old_addr: a hint conveying at what address the old function
> + * @old_sympos: a hint indicating which symbol position the old function
> * can be found (optional, vmlinux patches only)

Why is old_sympos only checked for vmlinux patches only? It's a
per-object count, so it should work for modules as well.

> + * @old_addr: the address of the function being patched
> * @kobj: kobject for sysfs resources
> * @state: tracks function-level patch application state
> * @stack_node: list node for klp_ops func_stack list
> @@ -47,17 +48,20 @@ struct klp_func {
> /* external */
> const char *old_name;
> void *new_func;
> +
> /*
> - * The old_addr field is optional and can be used to resolve
> - * duplicate symbol names in the vmlinux object. If this
> - * information is not present, the symbol is located by name
> - * with kallsyms. If the name is not unique and old_addr is
> - * not provided, the patch application fails as there is no
> - * way to resolve the ambiguity.
> + * The old_sympos field is optional and can be used to resolve duplicate
> + * symbol names in the vmlinux object. If this information is not
> + * present, the first symbol located with kallsyms is used. This value
> + * corresponds to the nth occurrence of the symbol name in kallsyms for
> + * the patched object. If the name is not unique and old_sympos is not
> + * provided, the patch application fails as there is no way to resolve
> + * the ambiguity.
> */
> - unsigned long old_addr;
> + unsigned long old_sympos;
>
> /* internal */
> + unsigned long old_addr;
> struct kobject kobj;
> enum klp_state state;
> struct list_head stack_node;
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 6e53441..1dd0d44 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -142,6 +142,7 @@ struct klp_find_arg {
> * name in the same object.
> */
> unsigned long count;
> + unsigned long pos;
> };
>
> static int klp_find_callback(void *data, const char *name,
> @@ -166,28 +167,39 @@ static int klp_find_callback(void *data, const char *name,
> args->addr = addr;
> args->count++;
>
> + /*
> + * ensure count matches the symbol position
> + */
> + if (args->pos == (args->count-1))
> + return 1;
> +

The code can be simplified a bit if args->addr only gets set when
args->pos is a match. Then klp_find_object_symbol() only needs to check
args.addr to see if a match was found.

> return 0;
> }
>
> static int klp_find_object_symbol(const char *objname, const char *name,
> - unsigned long *addr)
> + unsigned long *addr, unsigned long sympos)
> {
> struct klp_find_arg args = {
> .objname = objname,
> .name = name,
> .addr = 0,
> - .count = 0
> + .count = 0,
> + .pos = sympos,
> };
>
> mutex_lock(&module_mutex);
> kallsyms_on_each_symbol(klp_find_callback, &args);
> mutex_unlock(&module_mutex);
>
> - if (args.count == 0)
> + /*
> + * Ensure an address was found, then check that the symbol position
> + * count matches sympos.
> + */
> + if (args.addr == 0)
> pr_err("symbol '%s' not found in symbol table\n", name);
> - else if (args.count > 1)
> - pr_err("unresolvable ambiguity (%lu matches) on symbol '%s' in object '%s'\n",
> - args.count, name, objname);
> + else if (sympos != (args.count - 1))
> + pr_err("symbol position %lu for symbol '%s' in object '%s' not found\n",
> + sympos, name, objname ? objname : "vmlinux");
> else {
> *addr = args.addr;
> return 0;
> @@ -239,20 +251,19 @@ static int klp_verify_vmlinux_symbol(const char *name, unsigned long addr)
> static int klp_find_verify_func_addr(struct klp_object *obj,
> struct klp_func *func)
> {
> + int sympos = 0;
> int ret;
>
> -#if defined(CONFIG_RANDOMIZE_BASE)
> - /* If KASLR has been enabled, adjust old_addr accordingly */
> - if (kaslr_enabled() && func->old_addr)
> - func->old_addr += kaslr_offset();
> -#endif
> + if (func->old_sympos && !klp_is_module(obj))
> + sympos = func->old_sympos;
>
> - if (!func->old_addr || klp_is_module(obj))
> - ret = klp_find_object_symbol(obj->name, func->old_name,
> - &func->old_addr);
> - else
> - ret = klp_verify_vmlinux_symbol(func->old_name,
> - func->old_addr);
> + /*
> + * Verify the symbol, find old_addr, and write it to the structure.
> + * By default sympos will be 0 and thus will only look for the first
> + * occurrence. If another value is specified then that will be used.
> + */
> + ret = klp_find_object_symbol(obj->name, func->old_name,
> + &func->old_addr, sympos);
>
> return ret;
> }
> @@ -277,7 +288,7 @@ static int klp_find_external_symbol(struct module *pmod, const char *name,
> preempt_enable();
>
> /* otherwise check if it's in another .o within the patch module */
> - return klp_find_object_symbol(pmod->name, name, addr);
> + return klp_find_object_symbol(pmod->name, name, addr, 0);
> }
>
> static int klp_write_object_relocations(struct module *pmod,
> @@ -307,7 +318,7 @@ static int klp_write_object_relocations(struct module *pmod,
> else
> ret = klp_find_object_symbol(obj->mod->name,
> reloc->name,
> - &reloc->val);
> + &reloc->val, 0);

I think it would be a good idea to also add old_sympos to klp_reloc so
the relocation code is consistent with the klp_func symbol addressing.

> if (ret)
> return ret;
> }
> @@ -587,7 +598,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch);
> * /sys/kernel/livepatch/<patch>
> * /sys/kernel/livepatch/<patch>/enabled
> * /sys/kernel/livepatch/<patch>/<object>
> - * /sys/kernel/livepatch/<patch>/<object>/<func>
> + * /sys/kernel/livepatch/<patch>/<object>/<func,number>
> */
>
> static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
> @@ -732,8 +743,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func)
> INIT_LIST_HEAD(&func->stack_node);
> func->state = KLP_DISABLED;
>
> - return kobject_init_and_add(&func->kobj, &klp_ktype_func,
> - &obj->kobj, "%s", func->old_name);
> + return 0;
> }

The sysfs entry can still be created here, since the function name and
sympos are both already known.

>
> /* parts of the initialization that is done only when the object is loaded */
> @@ -755,6 +765,18 @@ static int klp_init_object_loaded(struct klp_patch *patch,
> return ret;
> }
>
> + /*
> + * for each function initialize and add, old_sympos will be already
> + * verified at this point
> + */
> + klp_for_each_func(obj, func) {
> + ret = kobject_init_and_add(&func->kobj, &klp_ktype_func,
> + &obj->kobj, "%s,%lu", func->old_name,
> + func->old_sympos ? func->old_sympos : 0);
> + if (ret)
> + return ret;
> + }
> +
> return 0;
> }
>
> --
> 1.9.1
>

--
Josh
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