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Mon, 22 Jan 2024 16:57:07 +0000 (UTC) (envelope-from jhb@FreeBSD.org) Message-ID: <94184f20-5e5e-44a3-839f-4959dbbed61a@FreeBSD.org> Date: Mon, 22 Jan 2024 08:57:06 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] Mask x86 segment registers in Windows gdbserver Content-Language: en-US To: Tom Tromey Cc: gdb-patches@sourceware.org References: <20231219144823.1070597-1-tromey@adacore.com> <3c3d8d0c-8e61-46c0-88d5-fecce4112b59@FreeBSD.org> <87wmt72tah.fsf@tromey.com> <87edeemq5f.fsf@tromey.com> From: John Baldwin In-Reply-To: <87edeemq5f.fsf@tromey.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-4.9 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_NONE,SPF_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: 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