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d=efficios.com; s=default; t=1560009166; bh=1zYPEOa390T+4ygk6CtXUOuek0d/tel39qFmVv01juo=; h=Date:From:To:Message-ID:MIME-Version; b=nmY+u3lkKGhd4/aQFS9wb3Y9ZomWqTVuBO+UzEIHuF3VVQ9HR2N5MffthLUcypuV+ O4SVrRQd/kc70mezBkUXbFa9I3mSAb5PLmTO1H7GsFmohcsfB3XBPzgVCHK9VJp4Vy 4iOSurbuqnJOn0IH8VkH0nKd+bUdguycBTHWmt6zllSHXlnJPj/BBk5LBhCdwwGkqn KDBiczy+fKgFc585bxrqFDOSZADWJYtIA+fxGZbcCFo4xXIBh2Qn2kRMasmv6L0PuC IUDil25QL5GgHV2GKmiZffrGBIdbNvQWadJtePVf6Em4CFXxniKH03lOBVXtlcWU/R eNk74JO39IXtA== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10026) with ESMTP id v03xN4kWi2lL; Sat, 8 Jun 2019 11:52:46 -0400 (EDT) Received: from mail02.efficios.com (mail02.efficios.com [167.114.142.138]) by mail.efficios.com (Postfix) with ESMTP id 1D44F1D3986; Sat, 8 Jun 2019 11:52:46 -0400 (EDT) Date: Sat, 8 Jun 2019 11:52:45 -0400 (EDT) From: Mathieu Desnoyers To: Will Deacon , Russell King Message-ID: <716499178.40175.1560009165920.JavaMail.zimbra@efficios.com> In-Reply-To: <1975020343.35751.1559844149532.JavaMail.zimbra@efficios.com> References: <20190429152803.7719-9-mathieu.desnoyers@efficios.com> <20190503193858.9676-1-mathieu.desnoyers@efficios.com> <1975020343.35751.1559844149532.JavaMail.zimbra@efficios.com> Subject: Re: [PATCH v2 for 5.2 08/12] rseq/selftests: arm: use udf instruction for RSEQ_SIG MIME-Version: 1.0 X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.12_GA_3803 (ZimbraWebClient - FF67 (Linux)/8.8.12_GA_3794) Thread-Topic: rseq/selftests: arm: use udf instruction for RSEQ_SIG Thread-Index: YcGezvm7ivFdjjSZdlA+YYoxUnlhzUxULrHf X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190608_085258_118504_0B7B56D5 X-CRM114-Status: GOOD ( 22.70 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Joel Fernandes , Peter Zijlstra , Catalin Marinas , Dave Watson , Andi Kleen , linux-kselftest , "H. Peter Anvin" , Chris Lameter , shuah , Ingo Molnar , Michael Kerrisk , "Paul E . McKenney" , Paul Turner , Boqun Feng , Josh Triplett , rostedt , Ben Maurer , Thomas Gleixner , linux-arm-kernel , linux-api , linux-kernel , Andy Lutomirski , Andrew Morton , Linus Torvalds Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org ----- On Jun 6, 2019, at 8:02 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote: > ----- On May 3, 2019, at 3:38 PM, Mathieu Desnoyers > mathieu.desnoyers@efficios.com wrote: > >> Use udf as the guard instruction for the restartable sequence abort >> handler. >> >> Previously, the chosen signature was not a valid instruction, based >> on the assumption that it could always sit in a literal pool. However, >> there are compilation environments in which literal pools are not >> available, for instance execute-only code. Therefore, we need to >> choose a signature value that is also a valid instruction. >> >> Handle compiling with -mbig-endian on ARMv6+, which generates binaries >> with mixed code vs data endianness (little endian code, big endian >> data). >> >> Else mismatch between code endianness for the generated signatures and >> data endianness for the RSEQ_SIG parameter passed to the rseq >> registration will trigger application segmentation faults when the >> kernel try to abort rseq critical sections. >> >> Prior to ARMv6, -mbig-endian generates big-endian code and data, so >> endianness should not be reversed in that case. > > And of course it cannot be that easy. This breaks when building in > thumb mode (-mthumb). Output from librseq arm32 build [1] (code similar > to what is found in the rseq selftests): > > CC rseq.lo > /tmp/ccu6Jw1b.s: Assembler messages: > /tmp/ccu6Jw1b.s:297: Error: cannot determine Thumb instruction size. Use > .inst.n/.inst.w instead > /tmp/ccu6Jw1b.s:490: Error: cannot determine Thumb instruction size. Use > .inst.n/.inst.w instead > Makefile:460: recipe for target 'rseq.lo' failed > > This appears to be caused by a missing .arm directive in RSEQ_SIG_DATA. > Fixing with: > > - asm volatile ("b 2f\n\t" \ > + asm volatile (".arm\n\t" \ > + "b 2f\n\t" \ > > gets the build to go further, but breaks at: > > CC basic_percpu_ops_test.o > /tmp/ccpHOMHZ.s: Assembler messages: > /tmp/ccpHOMHZ.s:148: Error: misaligned branch destination > /tmp/ccpHOMHZ.s:956: Error: misaligned branch destination > Makefile:378: recipe for target 'basic_percpu_ops_test.o' failed > > I suspect it's caused by the change from: > > - ".word " __rseq_str(RSEQ_SIG) "\n\t" \ > > to > > + ".arm\n\t" \ > + ".inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \ > > which changes the mode from thumb to arm for the rest of the > inline asm within __RSEQ_ASM_DEFINE_ABORT. Better yet, there appears > to be no way to save the arm/thumb state and restore it afterwards. > > I'm really starting to wonder if we should go our of our way to try > to get this signature to be a valid instruction on arm32. Perhaps > we should consider going back to use ".word" on arm32 so it ensures > it uses data endianness (which matches the parameter received by the > sys_rseq system call), let objdump and friends print it as a literal > pool (which it is), and just choose an instruction which has little > chances to appear for the cases we care about between ARM32 BE, LE > and THUMB. Perhaps a 32-bit palindrome ? Bonus points if this is a > trap instruction in common configurations for odd-cases-debugging > purposes. So I'm not particularly proud of the result, but I found a rather ugly way to figure out if we are currently in thumb mode within an inline asm, and restore that mode: test the length of a nop instruction with a ".if" asm statement. Do we want to go for this kind of approach, or should we revert back to a ".word" and accept that the rseq signature before the abort handler will be seen as data rather than an instruction on arm32 ? Is there a better way to do this ? Thanks, Mathieu diff --git a/include/rseq/rseq-arm.h b/include/rseq/rseq-arm.h index 1ce9231..b6c36dd 100644 --- a/include/rseq/rseq-arm.h +++ b/include/rseq/rseq-arm.h @@ -43,7 +43,14 @@ ({ \ int sig; \ asm volatile ("b 2f\n\t" \ + "3:\n\t" \ + "nop\n\t" \ + "4:\n\t" \ + ".arm\n\t" \ "1: .inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \ + ".if ((4b - 3b) == 2)\n\t" \ + ".thumb\n\t" \ + ".endif\n\t" \ "2:\n\t" \ "ldr %[sig], 1b\n\t" \ : [sig] "=r" (sig)); \ @@ -125,8 +132,14 @@ do { \ __rseq_str(table_label) ":\n\t" \ ".word " __rseq_str(version) ", " __rseq_str(flags) "\n\t" \ ".word " __rseq_str(start_ip) ", 0x0, " __rseq_str(post_commit_offset) ", 0x0, " __rseq_str(abort_ip) ", 0x0\n\t" \ + "333:\n\t" \ + "nop\n\t" \ + "444:\n\t" \ ".arm\n\t" \ ".inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \ + ".if ((444b - 333b) == 2)\n\t" \ + ".thumb\n\t" \ + ".endif\n\t" \ __rseq_str(label) ":\n\t" \ teardown \ "b %l[" __rseq_str(abort_label) "]\n\t" -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel