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.
> Andrew, this goes on top of
> ptrace-fix-efl_offset-value-according-to-i386-pda-changes.patch
> sent by Jeremy yesterday.
>
Don't think this is quite right yet. Assuming the %gs->%fs patch has
been applied, then the target %fs should be on its stack, and target %gs
will be in thread.gs. I'm not sure that thread.fs has any use, but I'd
want to double check vm86 to be sure.
J
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