Re: [PATCH] Mask x86 segment registers in Windows gdbserver

[Tom Tromey <tromey@adacore.com>]

John> LGTM.  Treating the x86 seg regs as 32-bits has required workarounds in
John> various places, but not easily fixable unfortunately.

Tom> Well, to my surprise, this patch caused regressions in our internal
Tom> tester.

Tom> It works again if I explicitly exclude the 'ss' register, like:

Tom>   if (!windows_process.wow64_process)
Tom>     return r >= AMD64_CS_REGNUM && r <= AMD64_GS_REGNUM && r != 19;

Tom> Now, this seems "ok" to do in some sense.  However, I can't explain why
Tom> native gdb works fine, as it has the same logic in
Tom> amd64_windows_segment_register_p.

Tom> I'd welcome any thoughts you (or anybody) might have.

I finally figured it out.  The Windows CONTEXT structure uses 16-bit
types for these registers' fields.  So, gdbserver is actually
reading/writing past the end of the field here.

My patch correctly truncates the value, but the truncated value is
preserved and written back -- overwriting other fields in the thread
context.

I have a new patch that fixes this problem and some other related things
as well.  I'll send it once it's been through a bit more testing.

Tom