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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Fn5z5p0KWkT8F7kdqXLtUSD00/eNhULjEnaQyt7HO3c=; b=o++UZzvdR8rBObcm4ChcSQkddrBmtSsmL85ae9A0WEbdoq7HVUiWjO9EtZkMg4NmHE W90QCu6Vl2wvqnerRJZHsicn8xb1JAwIs1hIV6CZbgCCyxgKc0Q7NieT2OZ2YAOn3cls Ju7fcZde6JKHyo37iD51cFMeWEi+iepkB8efMOeu+9rYM+rZ5OUo2cbTWFayBvz7Hm4I 6K8nAYFIZCCGrRcU42QemIJp2hBfmfnyiSuy5cHAakaNs4bawJLufz8liR/V/WOyUGtC zQtjUU8Bf+Z7Mcd3msIeaO9M64nJhizv4BG2B9C5JIKj/iUph8b6gIuM1w6oHlGKwcQ9 vIMw== X-Gm-Message-State: AOAM531C9zuvdFdwcJj/WEOPZnqmHQiBQLV1KxjzrbyJDajXwuQiRhy4 /o8W1iSz2NGRZ/IM8BtdO9KJ0YCn+8K6aMODOE+/QA== X-Google-Smtp-Source: ABdhPJxEx7C1NCFiroHZ1htfv1qkh42nY8oGzcnOH06wOhFq2SsYUT8w4mdvW5J7C4BtXsEWH+72M+TnlFjXmM5sSRs= X-Received: by 2002:a17:90b:889:: with SMTP id bj9mr28426pjb.101.1600703630255; Mon, 21 Sep 2020 08:53:50 -0700 (PDT) MIME-Version: 1.0 References: <20200918201436.2932360-1-samitolvanen@google.com> <20200918201436.2932360-4-samitolvanen@google.com> In-Reply-To: <20200918201436.2932360-4-samitolvanen@google.com> From: Nick Desaulniers Date: Mon, 21 Sep 2020 08:53:39 -0700 Message-ID: Subject: Re: [PATCH v3 03/30] x86/boot/compressed: Disable relocation relaxation To: Sami Tolvanen X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200921_115352_687235_FEC25B9A X-CRM114-Status: GOOD ( 29.35 ) 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: linux-arch , "maintainer:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" , Kees Cook , "Paul E. McKenney" , Kernel Hardening , Peter Zijlstra , Greg Kroah-Hartman , Masahiro Yamada , Linux Kbuild mailing list , LKML , Steven Rostedt , clang-built-linux , Arvind Sankar , linux-pci@vger.kernel.org, Will Deacon , Linux ARM Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org It looks like this just hit mainline over the weekend. FWIW. EOM. On Fri, Sep 18, 2020 at 1:14 PM Sami Tolvanen wrote: > > From: Arvind Sankar > > The x86-64 psABI [0] specifies special relocation types > (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset > Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker > can take advantage of for optimization (relaxation) at link time. This > is supported by LLD and binutils versions 2.26 onwards. > > The compressed kernel is position-independent code, however, when using > LLD or binutils versions before 2.27, it must be linked without the -pie > option. In this case, the linker may optimize certain instructions into > a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo. > > This potential issue has been present with LLD and binutils-2.26 for a > long time, but it has never manifested itself before now: > - LLD and binutils-2.26 only relax > movq foo@GOTPCREL(%rip), %reg > to > leaq foo(%rip), %reg > which is still position-independent, rather than > mov $foo, %reg > which is permitted by the psABI when -pie is not enabled. > - gcc happens to only generate GOTPCREL relocations on mov instructions. > - clang does generate GOTPCREL relocations on non-mov instructions, but > when building the compressed kernel, it uses its integrated assembler > (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as), > which has so far defaulted to not generating the GOTPCRELX > relocations. > > Nick Desaulniers reports [1,2]: > A recent change [3] to a default value of configuration variable > (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's > integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX > relocations. LLD will relax instructions with these relocations based > on whether the image is being linked as position independent or not. > When not, then LLD will relax these instructions to use absolute > addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with > Clang and linked with LLD to fail to boot. > > Patch series [4] is a solution to allow the compressed kernel to be > linked with -pie unconditionally, but even if merged is unlikely to be > backported. As a simple solution that can be applied to stable as well, > prevent the assembler from generating the relaxed relocation types using > the -mrelax-relocations=no option. For ease of backporting, do this > unconditionally. > > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65 > [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/ > [2] https://github.com/ClangBuiltLinux/linux/issues/1121 > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0 > [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/ > > Reported-by: Nick Desaulniers > Signed-off-by: Arvind Sankar > Tested-by: Nick Desaulniers > Tested-by: Sedat Dilek > Reviewed-by: Nick Desaulniers > Cc: stable@vger.kernel.org > --- > arch/x86/boot/compressed/Makefile | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile > index 3962f592633d..ff7894f39e0e 100644 > --- a/arch/x86/boot/compressed/Makefile > +++ b/arch/x86/boot/compressed/Makefile > @@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign > KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=) > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables > KBUILD_CFLAGS += -D__DISABLE_EXPORTS > +# Disable relocation relaxation in case the link is not PIE. > +KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no) > > KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__ > GCOV_PROFILE := n > -- > 2.28.0.681.g6f77f65b4e-goog > -- Thanks, ~Nick Desaulniers _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel