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SubjectRe: [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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