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

[John Baldwin <jhb@FreeBSD.org>]

On 1/18/24 10:51 AM, Tom Tromey wrote:
> 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.

Ouch, good find.

-- 
John Baldwin