Re: [PATCH 2/2] kernel: add support for live patching

[Jiri Kosina <jkosina@suse.cz>]

On Thu, 6 Nov 2014, Seth Jennings wrote:

> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> new file mode 100644
> index 0000000..b32dbb5
> --- /dev/null
> +++ b/kernel/livepatch/core.c

[ ... snip ... ]

> +/****************************************
> + * dynamic relocations (load-time linker)
> + ****************************************/
> +
> +/*
> + * external symbols are located outside the parent object (where the parent
> + * object is either vmlinux or the kmod being patched).
> + */
> +static int lpc_find_external_symbol(struct module *pmod, const char *name,
> +					unsigned long *addr)
> +{
> +	const struct kernel_symbol *sym;
> +
> +	/* first, check if it's an exported symbol */
> +	preempt_disable();
> +	sym = find_symbol(name, NULL, NULL, true, true);
> +	preempt_enable();
> +	if (sym) {
> +		*addr = sym->value;
> +		return 0;
> +	}
> +
> +	/* otherwise check if it's in another .o within the patch module */
> +	return lpc_find_symbol(pmod->name, name, addr);
> +}
> +
> +static int lpc_write_object_relocations(struct module *pmod,
> +					struct lpc_object *obj)
> +{
> +	int ret, size, readonly = 0, numpages;
> +	struct lp_dynrela *dynrela;
> +	u64 loc, val;
> +	unsigned long core = (unsigned long)pmod->module_core;
> +	unsigned long core_ro_size = pmod->core_ro_size;
> +	unsigned long core_size = pmod->core_size;
> +
> +	for (dynrela = obj->dynrelas; dynrela->name; dynrela++) {
> +		if (!strcmp(obj->name, "vmlinux")) {
> +			ret = lpc_verify_vmlinux_symbol(dynrela->name,
> +							dynrela->src);
> +			if (ret)
> +				return ret;
> +		} else {
> +			/* module, dynrela->src needs to be discovered */
> +			if (dynrela->external)
> +				ret = lpc_find_external_symbol(pmod,
> +							       dynrela->name,
> +							       &dynrela->src);
> +			else
> +				ret = lpc_find_symbol(obj->mod->name,
> +						      dynrela->name,
> +						      &dynrela->src);
> +			if (ret)
> +				return -EINVAL;
> +		}
> +
> +		switch (dynrela->type) {
> +		case R_X86_64_NONE:
> +			continue;
> +		case R_X86_64_PC32:
> +			loc = dynrela->dest;
> +			val = (u32)(dynrela->src + dynrela->addend -
> +				    dynrela->dest);
> +			size = 4;
> +			break;
> +		case R_X86_64_32S:
> +			loc = dynrela->dest;
> +			val = (s32)dynrela->src + dynrela->addend;
> +			size = 4;
> +			break;
> +		case R_X86_64_64:
> +			loc = dynrela->dest;
> +			val = dynrela->src;
> +			size = 8;
> +			break;

This is x86-specific, so it definitely needs to go to arch/x86. The only 
hard precondition for arch to support live patching is ftrace with regs 
saving (we are currently in parallel working on extending the set of 
architectures that support this), so we shouldn't introduce any x86-isms 
into the generic code.

It seems to me that what basically needs to be done is to teach 
apply_relocate_add() about this kind of relocations and apply them as 
needed.

Thanks,

-- 
Jiri Kosina
SUSE Labs