Benjamin Herrenschmidt wrote:
On Mon, 2008-02-25 at 19:47 -0600, Maynard Johnson wrote:
Hi,
I'm developing a kernel module that needs to parse the in-memory ELF
objects for a shared library (libc, to be specific). When running my
test on a 32-bit library, it works fine, but for a 64-bit library, the
very first copy_from_user() fails:
Elf64_Ehdr ehdr;
copy_from_user(&ehdr, location_of_lib, sizeof(Elf64_Ehdr);
I talked this over a bit with Will Schmidt. He determined that
access_ok (being done as a result of copy_from_user) was failing, but we
don't know why. I have 32-bit and 64-bit testcases that start up and
then pause, waiting for input. We look at the entry for libc in
/proc/<pid>/maps, and the permissions are the same for both 32-bit and
64-bit.
I've run this test on both a stock SLES 10 SP1 kernel and on 2.6.24.
I'm sure this is a user error, but for the life of me, I don't know what
I'm doing wrong.
Can anyone out there help?
I would have to look at the code.
Ben,
I've pared down the code to a minimal testcase and attached the source
file. Here are the makefile rules to build it:
----------------------------------------------
obj-m := uaccess_test.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
clean:
rm -f *.mod.c *.ko *.o .*.cmd
rm -rf .tmp_versions
----------------------------------------------
Instructions:
1. Write a simple C program that will pause, waiting for input, so that
you can obtain the address of libc to pass into the uaccess_test kernel
module. For example:
#include <stdio.h>
int main(void)
{
printf("Press Enter to continue.\n");
getchar();
return 0;
}
--------------
2. Compile C program as 32-bit; then run it. While the program is
waiting for input, obtain its PID and do 'cat /proc/<pid>/maps' to get
the address of where libc is loaded.
3. From the dir where you build the uaccess_test kernel module:
'insmod ./uaccess_test.ko lib_addr=0x<mem_loc_libc>'
This should succeed. dmesg to verify.
4. Unload the module.
5. Recompile your C program with -m64; start it up and obtain the
address of libc again (now a 64-bit address).
6. Load the uaccess_test kernel module and pass
'lib_addr=0x<mem_loc_libc>'. Note that this time, the load fails.
dmesg to see debug printk's.
Thanks for any light you can shed on this!
-Maynard
Ben.
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <asm/uaccess.h>
static long lib_addr;
module_param(lib_addr, long, 0);
MODULE_PARM_DESC(lib_addr, "lib_addr");
static unsigned long parse_elf64(unsigned long start_loc)
{
Elf64_Ehdr * ehdr;
int ret = 0;
ehdr = kmalloc(sizeof(Elf64_Ehdr), GFP_KERNEL);
if (copy_from_user((void *)ehdr, (void *) start_loc,
sizeof(Elf64_Ehdr))) {
printk("cannot get Elf64_Ehdr from "
"start_loc %lx\n", start_loc);
goto out;
}
if (ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
printk("EI_CLASS of Elf64_Hdr is incorrect! %d\n",
ehdr->e_ident[EI_CLASS]);
goto out;
}
if (ehdr->e_type != ET_DYN) {
printk(KERN_INFO "LPA: "
"%s, line %d: Unexpected e_type %u parsing ELF\n",
__FUNCTION__, __LINE__, ehdr->e_type);
goto out;
}
ret = ehdr->e_ident[EI_CLASS];
printk(KERN_INFO "Elf class from Ehdr is %d\n", ret);
out:
return ret;
}
static unsigned long parse_elf32(unsigned long start_loc)
{
Elf32_Ehdr ehdr;
int ret = 0;
if (copy_from_user(&ehdr, (void *) start_loc, sizeof (ehdr)))
goto out;
if (ehdr.e_ident[EI_CLASS] != ELFCLASS32)
goto out;
if (ehdr.e_type != ET_DYN) {
printk(KERN_INFO
"%s, line %d: Unexpected e_type %u parsing ELF\n",
__FUNCTION__, __LINE__, ehdr.e_type);
goto out;
}
ret = ehdr.e_ident[EI_CLASS];
printk(KERN_INFO "Elf class from Ehdr is %d\n", ret);
out:
return ret;
}
int find_ehdr(unsigned long start_loc)
{
int ret = 0;
if (!(ret = parse_elf32(start_loc)))
ret = parse_elf64(start_loc);
return ret;
}
int __init init_module(void)
{
if (!(find_ehdr(lib_addr))) {
printk(KERN_INFO "uaccess test failed\n");
return -1;
}
printk(KERN_INFO "uaccess test succeeded\n");
return 0;
}
void __exit cleanup_module(void)
{
}
MODULE_LICENSE("GPL");
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