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[209.85.167.52]) by smtp.gmail.com with ESMTPSA id s12sm1080901lfg.44.2021.06.18.19.14.14 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 18 Jun 2021 19:14:14 -0700 (PDT) Received: by mail-lf1-f52.google.com with SMTP id m21so19660208lfg.13 for ; Fri, 18 Jun 2021 19:14:14 -0700 (PDT) X-Received: by 2002:a05:6512:3d13:: with SMTP id d19mr5376578lfv.41.1624068854147; Fri, 18 Jun 2021 19:14:14 -0700 (PDT) MIME-Version: 1.0 References: <1623979642-14983-1-git-send-email-schmitzmic@gmail.com> <91865b90-c597-6119-5e14-dfe521a33489@gmail.com> <2b2ba866-104c-afea-9c29-145e9d80c2d5@gmail.com> In-Reply-To: From: Linus Torvalds Date: Fri, 18 Jun 2021 19:13:58 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] m68k: save extra registers on more syscall entry points To: Michael Schmitz Cc: Geert Uytterhoeven , linux-arch , linux-m68k , "Eric W. Biederman" , Andreas Schwab Content-Type: multipart/mixed; boundary="00000000000024960405c514ffcb" Precedence: bulk List-ID: X-Mailing-List: linux-m68k@vger.kernel.org --00000000000024960405c514ffcb Content-Type: text/plain; charset="UTF-8" On Fri, Jun 18, 2021 at 6:54 PM Linus Torvalds wrote: > > The registers being zero at that point is actually expected, so that's > not much of a hint. But yeah, clearly I got some stack initialization > offset or something wrong there, and I don't know modern m68k nearly > well enough to even guess where I screwed up. Oh. I think I might have an idea. In that ret_from_kernel_thread code, after it returns from thge kernel thread, I did addql #4,%sp RESTORE_SWITCH_STACK jra ret_from_exception where that "RESTORE_SWITCH_STACK" loads the user space registers from the user-space switch stack. BUT. The switch stack also has that "retpc", and that one should just be popped. So I think that code should read addql #4,%sp RESTORE_SWITCH_STACK addql #4,%sp jra ret_from_exception because we want to restore the switch stack registers (they'll all be 0) but we then want to get rid of retpc too before we jump to ret_from_exception, which will do the RESTORE_ALL. (The first 'addql' is to remove the argument we've pushed on the stack earlier in ret_from_kernel_thread, the second addql is to remove retpc). All the *normal* uses of RESTORE_SWITCH_STACK will just do "rts", but that's because they were called from the shallower stack. The ret_from_kernel_thread case is kind of special, because it had that stack frame layout built up manually, rather than have a call to it. In that sense, it's a bit more like the 'do_trace_entry/exit' code, which builds that switch stack using subql #4,%sp SAVE_SWITCH_STACK and then undoes it using that same RESTORE_SWITCH_STACK addql #4,%sp sequence that I _should_ have done in ret_from_kernel_thread. (Also, see ret_from_signal) I've attached an updated patch just in case my incoherent ramblings above didn't explain what I meant. It's the same as the previous patch, it just has that one extra stack update there. Does _this_ one perhaps work? 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