I found this following problem while executing cat/proc/iomem. The 
command causes following BUG.

x236:/linux-2.6.17-rc1/fs # cat /proc/iomem
Segmentation fault
x236:/linux-2.6.17-rc1/fs #

Dmesg has this following stack trace.

Same problem with 2.6.17-rc2. But not with 2.6.16-rc6 [ some old 
precompiled 2.6.16 kernel i had on my machine ]. cat on /proc/ioports 
also causes similar BUG.

Attaching the dmesg log for reference. Arch is IA32.

BUG: unable to handle kernel NULL pointer dereference at virtual address 
00000010
  printing eip:
c011cdef
*pde = 00000000
Oops: 0000 [#1]
Modules linked in:
CPU:    0
EIP:    0060:[<c011cdef>]    Not tainted VLI
EFLAGS: 00010246   (2.6.17-rc1 #12)
EIP is at r_show+0x134/0x157
eax: 00000000   ebx: c05358e8   ecx: ffffffff   edx: 00000000
esi: f54a26c7   edi: c05358eb   ebp: f44f7f14   esp: f44f7ecc
ds: 007b   es: 007b   ss: 0068
Process cat (pid: 11983, threadinfo=f44f6000 task=f54a2540)
Stack: <0>c0157422 c70ff7c0 f4790920 00000000 c70fd6c8 f44f7eec c04ffb30 
c05bd9b8
        f44f7f14 c011cc45 f44f6000 f44f7f14 00000008 00000000 c05bd99c 
c721d280
        f54a26c5 c053961d f44f7f60 c017acf9 f5d94cc0 c721d280 f5f9e2c0 
f47afddc
Call Trace:
  <c01034c8> show_stack_log_lvl+0xcf/0xdf   <c01036b8> 
show_registers+0x192/0x208
  <c01038b1> die+0x100/0x1ee   <c0111858> do_page_fault+0x460/0x670
  <c01030bb> error_code+0x4f/0x54   <c017acf9> seq_read+0xc6/0x3f1
  <c015a3b4> vfs_read+0xa8/0x177   <c015a748> sys_read+0x47/0x6e
  <c0102e2b> sysenter_past_esp+0x54/0x75
Code: c0 83 c4 3c 5b 5e 5f 5d c3 8b 55 ec 8b 42 10 89 54 24 04 c7 04 24 
50 6a 53 c0 89 44 24 08 e8 af af ff ff e9 69 ff ff ff 8b 55 ec <8b> 42 
10 89 54 24 08 89 44 24 0c 8b 45 08 c7 04 24 74 6a 53 c0
  BUG: cat/11983, active lock [f5d94ce0(f5d94cc0-f5d94d40)] freed!
  <c01033f7> show_trace+0x20/0x22   <c0103524> dump_stack+0x1e/0x20
  <c012e199> mutex_debug_check_no_locks_freed+0x10e/0x17e   <c01578f2> 
kfree+0x40/0x73
  <c017b32e> seq_release+0x22/0x2a   <c015b4e9> __fput+0x163/0x192
  <c015b384> fput+0x18/0x1a   <c0159cb9> filp_close+0x45/0x6f
  <c0119428> close_files+0x7f/0x9e   <c0119497> put_files_struct+0x22/0x51
  <c0119f5c> do_exit+0x13b/0x404   <c010399f> do_trap+0x0/0xb4
  <c0111858> do_page_fault+0x460/0x670   <c01030bb> error_code+0x4f/0x54
  <c017acf9> seq_read+0xc6/0x3f1   <c015a3b4> vfs_read+0xa8/0x177
  <c015a748> sys_read+0x47/0x6e   <c0102e2b> sysenter_past_esp+0x54/0x75
  [f5d94ce0] {seq_open}
.. held by:               cat:11983 [f54a2540, 115]
... acquired at:               seq_read+0x27/0x3f1

Thanks
-Sachin