[BUG?] x86_64 : Can not read /dev/kmem ?

[Eric Dumazet]

Hi Andi

I tried to read /dev/kmem on x86_64 (linux-2.6.11) and got no success.

read() or pread() returns EINVAL

I tried mmap() too : mmap() calls succeed, but as soon the user process 
dereference memory, we get :

tinfo: Corrupted page table at address 2aaaaaabf800
PGD 8a983067 PUD c7e5a067 PMD 91588067 PTE ffffffff8048a025
Bad pagetable: 000d [1] SMP
CPU 0
Modules linked in: ipt_REJECT
Pid: 10892, comm: tinfo Not tainted 2.6.11
RIP: 0033:[<0000000000100562>] [<0000000000100562>]
RSP: 002b:00007ffffffff790  EFLAGS: 00010217
RAX: 00002aaaaaabf000 RBX: 00002aaaaabbe000 RCX: 00002aaaaac8fc0c
RDX: 0000000000000001 RSI: 0000000000001000 RDI: 0000000000000000
RBP: 00007ffffffff7f8 R08: 0000000000000003 R09: ffffffff8048a000
R10: 0000000000000001 R11: 0000000000000206 R12: 00000000001005b0
R13: 0000000000000001 R14: 00002aaaaadfdfe8 R15: 0000000000100530
FS:  00002aaaaabcb970(0000) GS:ffffffff804866c0(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00002aaaaaabf800 CR3: 0000000090368000 CR4: 00000000000006e0
Process tinfo (pid: 10892, threadinfo ffff8100901b0000, task 
ffff8100c7d976c0)

RIP [<0000000000100562>] RSP <00007ffffffff790>



Thank you

Eric Dumazet
----------------------------------------------------------------
# cat tinfo.c

#define _XOPEN_SOURCE 500
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>

struct tcp_hashinfo {
        struct tcp_ehash_bucket *__tcp_ehash;
        struct tcp_bind_hashbucket *__tcp_bhash;

        int __tcp_bhash_size;
        int __tcp_ehash_size;

} tcp_hashinfo;

#define TCPINFO_ADDR 0xffffffff8048a000 /* tcp_hashinfo */

int main()
{
int fd = open("/dev/kmem", O_RDONLY) ;


if (pread(fd, &tcp_hashinfo, sizeof(tcp_hashinfo), TCPINFO_ADDR) == -1) {
        lseek(fd, TCPINFO_ADDR, 0) ;
        if (read(fd, &tcp_hashinfo, sizeof(tcp_hashinfo)) == -1) {
                perror("Can not read /dev/kmem ?") ;
                return 1 ;
                }
        }
printf("ehash=%p esize=%d bhash=%p bsize=%d\n",
        tcp_hashinfo.__tcp_ehash,
        tcp_hashinfo.__tcp_ehash_size,
        tcp_hashinfo.__tcp_bhash,
        tcp_hashinfo.__tcp_bhash_size) ;
return 0 ;
}
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