Re: [-mm patch] ptrace: make {put,get}reg work again for gs and fs

On Thu, Dec 21, 2006 at 11:22:05AM -0800, Jeremy Fitzhardinge wrote:
Frederik Deweerdt wrote:
Following the i386 pda patches, it's not possible to set gs or fs value
from gdb anymore. The following patch restores the old behaviour of
getting and setting thread.gs of thread.fs respectively.
Here's a gdb session *before* the patch:
(gdb) info reg
[...]
fs 0x33 51
gs 0x33 51
(gdb) set $fs=0xffff
(gdb) info reg
[...]
fs 0x33 51
gs 0x33 51
(gdb) set $gs=0xffffffff
(gdb) info reg
[...]
fs 0xffff 65535
gs 0x33 51

Another one *after* the patch:
(gdb) info reg
[...]
fs 0xd8 216


This doesn't look right. This is the kernel's %fs, not usermode's
(which should be 0).

gs 0x33 51
(gdb) set $fs=0xffff
(gdb) info reg
[...]
fs 0xffff 65535
gs 0x33 51
(gdb) set $gs=0xffff
(gdb) info reg
[...]
fs 0xffff 65535
gs 0xffff 65535

Hm. This shouldn't be possible since this is a bad selector, but I
guess ptrace/gdb doesn't really know that. If you run the target (even
single step it), these should revert to 0.

Here's a third session that looks better:

(gdb) info reg
[...]
fs 0x0 0
gs 0x33 51
(gdb) set $fs=0xffff
(gdb) info reg
[...]
fs 0xffff 65535
gs 0x33 51
(gdb) set $gs=0xffff
(gdb) info reg
[...]
fs 0xffff 65535
gs 0xffff 65535
(gdb) n
Single stepping until exit from function main,
which has no line number information.
Cannot find user-level thread for LWP 10751: generic error
(gdb) set $gs=0x33
(gdb) set $fs=0
(gdb) n
Single stepping until exit from function main,
which has no line number information.
0x08048c05 in __i686.get_pc_thunk.bx ()
(gdb) info reg
[...]
fs 0x0 0
gs 0x33 51

This is a -mm1 kernel + your efl_offset fix + the attached patch.
So the problem came from putreg still saving %gs to the stack where
there's no slot for it, whereas getreg got things right.

Regards,
Frederik

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xxxxxxxxx>


diff --git a/arch/i386/kernel/ptrace.c b/arch/i386/kernel/ptrace.c
index a803a49..d8f44db 100644
--- a/arch/i386/kernel/ptrace.c
+++ b/arch/i386/kernel/ptrace.c
@@ -89,14 +89,14 @@ static int putreg(struct task_struct *child,
unsigned long regno, unsigned long value)
{
switch (regno >> 2) {
- case FS:
+ case GS:
if (value && (value & 3) != 3)
return -EIO;
- child->thread.fs = value;
+ child->thread.gs = value;
return 0;
case DS:
case ES:
- case GS:
+ case FS:
if (value && (value & 3) != 3)
return -EIO;
value &= 0xffff;
-
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