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Date:   Tue, 12 Jan 2021 11:39:07 +0000
From:   "Metzger, Markus T" <>
To:     "Bae, Chang Seok" <>,
        "Lutomirski, Andy" <>
CC:     Borislav Petkov <>, Andy Lutomirski <>,
        "" <>, x86-ml <>,
        lkml <>
Subject: RE: gdbserver + fsgsbase kaputt

> The GDB behavior looks to be different between the two cases -- with vs
> without gdb server, when I checked the GS/GSBASE values on the ptrace front.

64-bit GDB doesn't support FSGSBASE for 32-bit inferiors and it looks like gdbserver
might not support FSGSBASE, at all.

I had added support for the former as part of the tests I wrote about a year ago [1]
but never submitted the patch.  Was the discussion ever concluded?

The general behavior should be that GDB reads a regset, overwrites the registers it
knows about, and writes it back again to preserve the original values of registers it
doesn't know about.

When I log the values that are read and written for FSGSBASE, however, it looks like
ptrace is returning a non-zero GS_BASE on a read and gdbserver is writing zero on
the next write.

Chang, is that also what you were seeing?



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