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 <[email protected]>
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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