From mboxrd@z Thu Jan 1 00:00:00 1970 Delivery-date: Thu, 16 Sep 2021 11:37:12 +0200 Received: from metis.ext.pengutronix.de ([2001:67c:670:201:290:27ff:fe1d:cc33]) by lore.white.stw.pengutronix.de with esmtp (Exim 4.92) (envelope-from ) id 1mQnpQ-0006du-CY for lore@lore.pengutronix.de; Thu, 16 Sep 2021 11:37:12 +0200 Received: from bombadil.infradead.org ([2607:7c80:54:e::133]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1mQnpP-0007RB-Af for lore@pengutronix.de; Thu, 16 Sep 2021 11:37:12 +0200 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:Message-Id:Date:Subject:Cc :To:From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References: List-Owner; bh=lrpyWvAZPUNsjGzVyUzduj9Ikh4vO0OP8cu2MrbTfNI=; b=CwYTNgP0ktkcck 8rXZRGojXKghIvrDbWHMtuzzxRTXZRSYTlHI/AHKSUsURlbnChSgGmlohoNqdUvZN3PUEVsLgkCvs 72lGqg5NEmj+pSzfSCnzwmRpa6BXLpUUiGsxrflchXiF3qX3cBBPuwX+KRQN5IwpHxdCel63I+12S pVhXmRI/LFCfxmmwoPUSV2oUxv0Itrhy2b0FiGnOVlYyB8sbg7mkQojS5Iinw0gBDMuwXNuQqA9RD s6WYV5KDrTt4ygiqIPA5deIrRT2YrizaQ+zKaolKiKIoV/HR8t8ZCYcD9GwbhDApYKYcBgilho7dH zkNjtCH89fKpAeIu+hOg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mQno9-00Aipb-W0; Thu, 16 Sep 2021 09:35:54 +0000 Received: from metis.ext.pengutronix.de ([2001:67c:670:201:290:27ff:fe1d:cc33]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1mQno4-00Aio3-3v for barebox@lists.infradead.org; Thu, 16 Sep 2021 09:35:49 +0000 Received: from dude.hi.pengutronix.de ([2001:67c:670:100:1d::7]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1mQno2-0006wq-TY; Thu, 16 Sep 2021 11:35:46 +0200 Received: from afa by dude.hi.pengutronix.de with local (Exim 4.92) (envelope-from ) id 1mQno2-0005mb-2s; Thu, 16 Sep 2021 11:35:46 +0200 From: Ahmad Fatoum To: barebox@lists.infradead.org Cc: Ahmad Fatoum , Jules Maselbas Date: Thu, 16 Sep 2021 11:35:43 +0200 Message-Id: <20210916093543.22138-1-a.fatoum@pengutronix.de> X-Mailer: git-send-email 2.30.2 MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210916_023548_217898_4A1DABD2 X-CRM114-Status: GOOD ( 19.58 ) X-BeenThere: barebox@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "barebox" X-SA-Exim-Connect-IP: 2607:7c80:54:e::133 X-SA-Exim-Mail-From: barebox-bounces+lore=pengutronix.de@lists.infradead.org X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on metis.ext.pengutronix.de X-Spam-Level: X-Spam-Status: No, score=-4.7 required=4.0 tests=AWL,BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,SPF_HELO_NONE,SPF_NONE autolearn=unavailable autolearn_force=no version=3.4.2 Subject: [PATCH] ARM: document arm_stack_setup() pitfalls X-SA-Exim-Version: 4.2.1 (built Wed, 08 May 2019 21:11:16 +0000) X-SA-Exim-Scanned: Yes (on metis.ext.pengutronix.de) Many arm32 board entry points use arm_setup_stack() to set up the stack from C code. This necessitates using __naked, which probably has been our most frequent cause of misscompiled C code. GCC is quite clear that: Only basic asm statements can safely be included in naked functions While using extended asm or a mixture of basic asm and C code may appear to work, they cannot be depended upon to work reliably and are not supported. But some boards use it anyway, because it's nice to avoid writing assembly. Reading generated assembly to spot compiler miscompilation isn't that nice though, so add some documentation, comments and compiler diagnostics to hopefully reduce future porting effort. Signed-off-by: Ahmad Fatoum --- Cc: Jules Maselbas --- Documentation/devel/porting.rst | 21 +++++++++++++++++++++ arch/arm/include/asm/common.h | 22 ++++++++++++++++++++++ 2 files changed, 43 insertions(+) diff --git a/Documentation/devel/porting.rst b/Documentation/devel/porting.rst index 97b787327c9a..699badbec672 100644 --- a/Documentation/devel/porting.rst +++ b/Documentation/devel/porting.rst @@ -169,6 +169,27 @@ Looking at other boards you might see some different patterns: needs to be done at start. If a board similar to yours does this, you probably want to do likewise. + - ``__naked``: All functions called before stack is correctly initialized must be + marked with this attribute. Otherwise, function prologue and epilogue may access + the uninitialized stack. If the compiler for the target architecture doesn't + support the attribute, stack must be set up in non-inline assembly: + either a barebox assembly entry point or in earlier firmware. + The compiler may still spill excess local C variables used in a naked function + to the stack before it was initialized. + A naked function should thus preferably only contain inline assembly, set up a + stack and jump directly after to a ``noinline`` non naked function where the + stack is then normally usable. + + - ``noinline``: Compiler code inlining is oblivious to stack modification. + If you want to ensure a new function has its own stack frame (e.g. after setting + up the stack in a ``__naked`` function), you must jump to a ``__noreturn noinline`` + function. + + - ``arm_setup_stack``: For 32-bit ARM, ``arm_setup_stack`` initialized the stack + top when called from a naked C function, which allows to write the entry point + directly in C. The stack pointer will be decremented before pushing values. + Avoid interleaving with C-code. See ``__naked`` above for more details. + - ``__dtb_z_my_board_start[];``: Because the PBL normally doesn't parse anything out of the device tree blob, boards can benefit from keeping the device tree blob compressed and only unpack it in barebox proper. Such LZO-compressed device trees diff --git a/arch/arm/include/asm/common.h b/arch/arm/include/asm/common.h index d03ee6273fe5..88e2991c9324 100644 --- a/arch/arm/include/asm/common.h +++ b/arch/arm/include/asm/common.h @@ -1,6 +1,8 @@ #ifndef __ASM_ARM_COMMON_H #define __ASM_ARM_COMMON_H +#include + static inline unsigned long get_pc(void) { unsigned long pc; @@ -46,8 +48,28 @@ static inline unsigned long get_sp(void) return sp; } +extern void __compiletime_error( + "arm_setup_stack() called outside of naked function. On ARM64, " + "stack should be setup in non-inline assembly before branching to C entry point." +) __unsafe_stack_setup(void); + +/* + * Sets up new stack growing down from top within a naked C function. + * The first stack word will be top - sizeof(word). + * + * Avoid interleaving with C code as much as possible and and jump + * ASAP to a noinline function. + */ static inline void arm_setup_stack(unsigned long top) { + if (IS_ENABLED(CONFIG_CPU_V8)) { + /* + * There are no naked functions on ARM64 and thus it's never + * safe to call arm_setup_stack(). + */ + __unsafe_stack_setup(); + } + __asm__ __volatile__("mov sp, %0" : : "r"(top)); -- 2.30.2 _______________________________________________ barebox mailing list barebox@lists.infradead.org http://lists.infradead.org/mailman/listinfo/barebox