From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vk0-x22b.google.com (mail-vk0-x22b.google.com [IPv6:2607:f8b0:400c:c05::22b]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 6700A81F7A for ; Thu, 8 Dec 2016 05:30:15 -0800 (PST) Received: by mail-vk0-x22b.google.com with SMTP id x186so229031671vkd.1 for ; Thu, 08 Dec 2016 05:30:15 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:from:date:message-id:subject:to; bh=LyDndjScZo+VdcvJbNABN8XS8Y6U0vX8WDB56KVOANQ=; b=uCjAhf9Bsqme9d0jLletBtikB/HeYQnPqB5vZxjeJ3qMapKsVHkyB7aMxpETJA8RLr HlUama8uFmkvCp+b4tTJS1rduMgOq1RZRfVQLJ+t2ljEyXuFiP8CEt4CAPJvngaG96a+ ho8LW30O8NS/m5tAbYJIEcblfFy2SpyFef1D4K3kmhFmgH22rOUob+/Jf4gtGSrY8dJA 77Y4SeWzXvZTESGvmHTJ7WN42SOtTq3M5JxpSrq7p/aY1r1hJpKC8l5vnE5wEmNdyARX U2ujKpziwdtw78KziRsiht5NEP/m/64qpggi6c1n0ItV0sx8zWSyt5TNmvAod4+ijCJN UIfQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:from:date:message-id:subject:to; bh=LyDndjScZo+VdcvJbNABN8XS8Y6U0vX8WDB56KVOANQ=; b=EjTOChPA1XcObLLlPVRVIEBQnDPoILX0KLbzYE1cBV3B86rrkEcp5sklDm4mHOcHgO YYaTr1uZxuR4VXfn1KCui3to4hxcXwiDIH050OVMrC3yidRq9syTmlUIZd/Csw7yVZls D1DBW5D/h2pPDCkSfwda+okT1P/2dviN+nkyjean6BDAMWwYfenryl5TyzDPr07+drhX v6H8NNVMuH5aGuJ5MN/485foXDXRL+7n+F/Lftu6qum/KATBXJCIN16T06Y/JdnwoPFN D4kFRSa/tdOHz7+8SiYs9J2eriwySL1luR/R110N/oVWq4XRlq4NfyR0H3l97nzY5Zmb 2gtQ== X-Gm-Message-State: AKaTC03DxGJMuhiTO17q/0RpcjFWCtIPjhuxiZlm7e6jXBtlMv0W7tSOYezuqKXNV2IM7QPe3AJmwXKBo+3YxA== X-Received: by 10.31.170.208 with SMTP id t199mr24073792vke.6.1481203814078; Thu, 08 Dec 2016 05:30:14 -0800 (PST) MIME-Version: 1.0 Received: by 10.31.34.2 with HTTP; Thu, 8 Dec 2016 05:30:13 -0800 (PST) From: Michael Zimmermann Date: Thu, 8 Dec 2016 14:30:13 +0100 Message-ID: To: "edk2-devel@lists.01.org" Subject: GCC needs __attribute__((returns_twice)) for SetJump X-BeenThere: edk2-devel@lists.01.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: EDK II Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 08 Dec 2016 13:30:15 -0000 Content-Type: text/plain; charset=UTF-8 When compiling with any ARM toolchain and Os, registers can get trashed when returning for the second time from SetJump because GCC only handles this correctly when using standard names like 'setjmp' or 'getcontext'. When different names are used you have to use the attribute 'returns_twice' to tell gcc to be extra careful. example: #define FN_NAME nonstandard_setjmp extern int FN_NAME(void*); void jmp_buf_set(void *jmpb, void (*f)(void)) { if (!FN_NAME(jmpb)) f(); } this code produces this wrong code with Os: 00000000 : 0: e92d4010 push {r4, lr} 4: e1a04001 mov r4, r1 8: ebfffffe bl 0 c: e3500000 cmp r0, #0 10: 01a03004 moveq r3, r4 14: 08bd4010 popeq {r4, lr} 18: 012fff13 bxeq r3 1c: e8bd4010 pop {r4, lr} 20: e12fff1e bx lr The generated code pushes backups of r4 and lr to the stack and then saves all registers using nonstandard_setjmp. Then it pops the stack and jumps to the function in r3 which is the main problem because now the function can overwrite our register backups on the stack. When we return a second time from the call to nonstandard_setjmp, the stack pointer has it's original(pushed) position and when the code pops r4 and lr from the stack the values are not guaranteed to be the same. When using a standard name like setjmp or getcontext or adding '__attribute__((returns_twice))' to nonstandard_setjmp's declaration the code looks different: 00000000 : 0: e92d4007 push {r0, r1, r2, lr} 4: e58d1004 str r1, [sp, #4] 8: ebfffffe bl 0 c: e3500000 cmp r0, #0 10: 059d3004 ldreq r3, [sp, #4] 14: 01a0e00f moveq lr, pc 18: 012fff13 bxeq r3 1c: e28dd00c add sp, sp, #12 20: e49de004 pop {lr} ; (ldr lr, [sp], #4) 24: e12fff1e bx lr Here the problem is being solved by restoring r3 from the stack without popping it. I would have sent a patch but since there's no define for 'returns_twice' yet and I don't know how other compilers handle this I want to discuss this first. Thanks Michael