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[222.152.189.37]) by smtp.gmail.com with ESMTPSA id j20sm11824131pfc.203.2021.07.16.22.38.04 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Fri, 16 Jul 2021 22:38:09 -0700 (PDT) Subject: Re: [PATCH v4 0/3] m68k: Improved switch stack handling To: "Eric W. Biederman" References: <1624407696-20180-1-git-send-email-schmitzmic@gmail.com> <87zgunzovm.fsf@disp2133> <3b4f287b-7be2-0e7b-ae5a-6c11972601fb@gmail.com> Cc: geert@linux-m68k.org, linux-arch@vger.kernel.org, linux-m68k@lists.linux-m68k.org, torvalds@linux-foundation.org, schwab@linux-m68k.org From: Michael Schmitz Message-ID: <1b656c02-925c-c4ba-03d3-f56075cdfac5@gmail.com> Date: Sat, 17 Jul 2021 17:38:01 +1200 User-Agent: Mozilla/5.0 (X11; Linux ppc; rv:45.0) Gecko/20100101 Icedove/45.4.0 MIME-Version: 1.0 In-Reply-To: <3b4f287b-7be2-0e7b-ae5a-6c11972601fb@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org Am 16.07.2021 um 11:10 schrieb Michael Schmitz: > Eric, > > On 16/07/21 1:29 am, Eric W. Biederman wrote: >> >> I have been digging into this some more and I have found one place >> that I am having a challenge dealing with. >> >> In arch/m68k/fpsp040/skeleton.S there is an assembly version of >> copy_from_user that calls fpsp040_die when the bytes can not be read. >> >> Now fpsp040_die is just: >> >> /* >> * This function is called if an error occur while accessing >> * user-space from the fpsp040 code. >> */ >> asmlinkage void fpsp040_die(void) >> { >> do_exit(SIGSEGV); >> } >> The problem here is the instruction emulation performed in the fpsp040 >> code performs a very minimal saving of registers. I don't think even >> the normal system call entry point registers that are saved are present >> at that point. >> >> Is there any chance you can help me figure out how to get a stack frame >> with all of the registers present before fpsp040_die is called? > > I suppose adding the following code (untested) to entry.S: > > ENTRY(fpsp040_die) > SAVE_ALL_INT > jbsr fpsp040_die_c > jra ret_from_exception > > along with renaming above C entry point to fpsp040_die_c would add the > basic saved registers, but these would not necessarily reflect the state > of the processor when the fpsp040 trap was called. Is that what you're > after? I should have looked more closely at skeleton.S - most FPU exceptions handled there call trap_c the same way as is done for generic traps, i.e. SAVE_ALL_INT before, ret_from_exception after. Instead of adding code to entry.S, much better to add it in skeleton.S. I'll try to come up with a way to test this code path (calling fpsp040_die from the dz exception hander seems much the easiest way) to make sure this doesn't have side effects. Does do_exit() ever return? Cheers, Michael