anbox-modules/ashmem/deps.c

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#include <linux/mm.h>
#include <linux/kallsyms.h>
#include <linux/kprobes.h>
#include <linux/version.h>
#if (LINUX_VERSION_CODE >= KERNEL_VERSION(5,7,0))
#ifndef CONFIG_KPROBES
# error "Your kernel does not support KProbes, but this is required to compile ashmem as a kernel module on kernel 5.7 and later"
#endif
typedef unsigned long (*kallsyms_lookup_name_t)(const char *name);
static int dummy_kprobe_handler(struct kprobe *p, struct pt_regs *regs)
{
return 0;
}
static kallsyms_lookup_name_t get_kallsyms_lookup_name_ptr(void)
{
struct kprobe probe;
int ret;
kallsyms_lookup_name_t addr;
memset(&probe, 0, sizeof(probe));
probe.pre_handler = dummy_kprobe_handler;
probe.symbol_name = "kallsyms_lookup_name";
ret = register_kprobe(&probe);
if (ret)
return NULL;
addr = (kallsyms_lookup_name_t) probe.addr;
unregister_kprobe(&probe);
return addr;
}
#endif
/*
* On kernel 5.7 and later, kallsyms_lookup_name() can no longer be called from a kernel
* module for reasons described here: https://lwn.net/Articles/813350/
* As ashmem really needs to use kallsysms_lookup_name() to access some kernel
* functions that otherwise wouldn't be accessible, KProbes are used on later
* kernels to get the address of kallsysms_lookup_name(). The function is
* afterwards used just as before. This is a very dirty hack though and the much
* better solution would be if all the functions that are currently resolved
* with kallsysms_lookup_name() would get an EXPORT_SYMBOL() annotation to
* make them directly accessible to kernel modules.
*/
static unsigned long kallsyms_lookup_name_wrapper(const char *name)
{
#if (LINUX_VERSION_CODE >= KERNEL_VERSION(5,7,0))
static kallsyms_lookup_name_t func_ptr = NULL;
if (!func_ptr)
func_ptr = get_kallsyms_lookup_name_ptr();
return func_ptr(name);
#else
return kallsyms_lookup_name(name);
#endif
}
static int (*shmem_zero_setup_ptr)(struct vm_area_struct *) = NULL;
int shmem_zero_setup(struct vm_area_struct *vma)
{
if (!shmem_zero_setup_ptr)
shmem_zero_setup_ptr = kallsyms_lookup_name_wrapper("shmem_zero_setup");
return shmem_zero_setup_ptr(vma);
}