linux-kernel - Re: [PATCH 2/2] livepatch: Fix the bug if the function name is larger than KSYM_NAME

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Date:	Tue, 14 Apr 2015 08:26:29 +0800
From:	Minfei Huang <minfei.huang@...mail.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
CC:	sjenning@...hat.com, jkosina@...e.cz, vojtech@...e.cz,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] livepatch: Fix the bug if the function name is
 larger than KSYM_NAME_LEN-1

On 04/13/15 at 06:13P, Josh Poimboeuf wrote:
> On Sun, Apr 12, 2015 at 09:15:54PM +0800, Minfei Huang wrote:
> > For now, the kallsyms will only store the first (KSYM_NAME_LEN-1). The
> > kallsyms name is same for the function which first (KSYM_NAME_LEN-1) is
> > same, but the rest is not.
> > 
> > Then function will never be patched, although function name and address
> > are provided both. The reason caused this bug is livepatch cannt
> > recognize the function name.
> > 
> > Now, livepatch will verify the function name with first (KSYM_NAME_LEN-1)
> > and address, if provided. Once they are matched, we can confirm that the
> > patched function is found.
> 
> From scripts/kallsyms.c:
> 
> 	if (strlen(str) > KSYM_NAME_LEN) {
> 		fprintf(stderr, "Symbol %s too long for kallsyms (%zu vs %d).\n"
> 				"Please increase KSYM_NAME_LEN both in kernel and kallsyms.c\n",
> 			str, strlen(str), KSYM_NAME_LEN);
> 		return -1;
> 	}
> 
> So I think such a long symbol name wouldn't be added to the kallsyms
> database in the first place.
> 

Actually, kernel allows overlength function name to be used. Following
is my testing module.

We can got the address in /proc/kallsyms.
$ cat /proc/kallsyms | grep sysfs_print
ffffffffa0000000 t sys_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_pri  [sysfs_print]
ffffffffa0000010 t kobj_release [sysfs_print]
ffffffffa0000020 t sys_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_pri  [sysfs_print]
ffffffffa00004e0 b root_kobj    [sysfs_print]
ffffffffa0000200 d print_ktype  [sysfs_print]
ffffffffa00004a0 b print_kobj   [sysfs_print]
ffffffffa000004c t sys_print_exit   [sysfs_print]
ffffffffa0000144 r __func__.14514   [sysfs_print]
ffffffffa0000230 d kobj_attrs   [sysfs_print]
ffffffffa0000240 d sys_print_kobj_attr  [sysfs_print]
ffffffffa0000260 d __this_module    [sysfs_print]
ffffffffa000004c t cleanup_module   [sysfs_print]

Code:

static ssize_t sys_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_store(struct kobject *kobj, s
        const char *buf, size_t count)
{
    return count;
}

static ssize_t sys_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf)
{
    return snprintf(buf, PAGE_SIZE-1, "%s\n", "This is printed by module");
}

static struct kobj_attribute sys_print_kobj_attr = __ATTR_RW(sys_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_print_p
static struct attribute *kobj_attrs[] = {
    &sys_print_kobj_attr.attr,
    NULL
};

Thanks
Minfei

> -- 
> Josh
> --
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