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Wed, 14 Feb 2024 23:56:38 -0800 (PST) X-Forwarded-Encrypted: i=1; AJvYcCVNMh/+roFBG3ssnL0lMR3HnbL2takR3P6GjdEhYak/WpqZ0ptrwpWiXOBAjOsS+lCnw3kWG7pvGh1qZwHCnt1jDUdv4coQvcZdFvOvkBxhTp8QqTuKXoKGIFbGM7UUBjkPD+7Yl0aV8uD7aukW3opdBYAQEyD0qmX+4AHnZAmxu1rgtZsRooYUTGgyULTYZh/tZJjmbxeyRXCXZQEwY7e+9LEo X-Gm-Message-State: AOJu0YwB1caySH3ncamAwBq8rnKzbhZMeromuLBbdi7Z0L05kGccana9 GYVaj7Dhx4W3R3EWQwNsuiwpupnA/3c+tPwR/kh+klE/zklw1p4Y1Y4y82gcCNVIFX5dZgwwbvg Llsw+QN+fKf8MYkW+t8MuMmyM0JA= X-Received: by 2002:ac2:43cd:0:b0:512:87a2:adf6 with SMTP id u13-20020ac243cd000000b0051287a2adf6mr528637lfl.22.1707983796842; Wed, 14 Feb 2024 23:56:36 -0800 (PST) Precedence: bulk X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20240214221847.2066632-1-ross.philipson@oracle.com> <20240214221847.2066632-2-ross.philipson@oracle.com> In-Reply-To: <20240214221847.2066632-2-ross.philipson@oracle.com> From: Ard Biesheuvel Date: Thu, 15 Feb 2024 08:56:25 +0100 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v8 01/15] x86/boot: Place kernel_info at a fixed offset To: Ross Philipson Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-integrity@vger.kernel.org, linux-doc@vger.kernel.org, linux-crypto@vger.kernel.org, kexec@lists.infradead.org, linux-efi@vger.kernel.org, dpsmith@apertussolutions.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, dave.hansen@linux.intel.com, mjg59@srcf.ucam.org, James.Bottomley@hansenpartnership.com, peterhuewe@gmx.de, jarkko@kernel.org, jgg@ziepe.ca, luto@amacapital.net, nivedita@alum.mit.edu, herbert@gondor.apana.org.au, davem@davemloft.net, kanth.ghatraju@oracle.com, trenchboot-devel@googlegroups.com Content-Type: text/plain; charset="UTF-8" On Wed, 14 Feb 2024 at 23:31, Ross Philipson wrote: > > From: Arvind Sankar > > There are use cases for storing the offset of a symbol in kernel_info. > For example, the trenchboot series [0] needs to store the offset of the > Measured Launch Environment header in kernel_info. > Why? Is this information consumed by the bootloader? I'd like to get away from x86 specific hacks for boot code and boot images, so I would like to explore if we can avoid kernel_info, or at least expose it in a generic way. We might just add a 32-bit offset somewhere in the first 64 bytes of the bootable image: this could co-exist with EFI bootable images, and can be implemented on arm64, RISC-V and LoongArch as well. > Since commit (note: commit ID from tip/master) > > commit 527afc212231 ("x86/boot: Check that there are no run-time relocations") > > run-time relocations are not allowed in the compressed kernel, so simply > using the symbol in kernel_info, as > > .long symbol > > will cause a linker error because this is not position-independent. > > With kernel_info being a separate object file and in a different section > from startup_32, there is no way to calculate the offset of a symbol > from the start of the image in a position-independent way. > > To enable such use cases, put kernel_info into its own section which is > placed at a predetermined offset (KERNEL_INFO_OFFSET) via the linker > script. This will allow calculating the symbol offset in a > position-independent way, by adding the offset from the start of > kernel_info to KERNEL_INFO_OFFSET. > > Ensure that kernel_info is aligned, and use the SYM_DATA.* macros > instead of bare labels. This stores the size of the kernel_info > structure in the ELF symbol table. > > Signed-off-by: Arvind Sankar > Cc: Ross Philipson > Signed-off-by: Ross Philipson > --- > arch/x86/boot/compressed/kernel_info.S | 19 +++++++++++++++---- > arch/x86/boot/compressed/kernel_info.h | 12 ++++++++++++ > arch/x86/boot/compressed/vmlinux.lds.S | 6 ++++++ > 3 files changed, 33 insertions(+), 4 deletions(-) > create mode 100644 arch/x86/boot/compressed/kernel_info.h > > diff --git a/arch/x86/boot/compressed/kernel_info.S b/arch/x86/boot/compressed/kernel_info.S > index f818ee8fba38..c18f07181dd5 100644 > --- a/arch/x86/boot/compressed/kernel_info.S > +++ b/arch/x86/boot/compressed/kernel_info.S > @@ -1,12 +1,23 @@ > /* SPDX-License-Identifier: GPL-2.0 */ > > +#include > #include > +#include "kernel_info.h" > > - .section ".rodata.kernel_info", "a" > +/* > + * If a field needs to hold the offset of a symbol from the start > + * of the image, use the macro below, eg > + * .long rva(symbol) > + * This will avoid creating run-time relocations, which are not > + * allowed in the compressed kernel. > + */ > + > +#define rva(X) (((X) - kernel_info) + KERNEL_INFO_OFFSET) > > - .global kernel_info > + .section ".rodata.kernel_info", "a" > > -kernel_info: > + .balign 16 > +SYM_DATA_START(kernel_info) > /* Header, Linux top (structure). */ > .ascii "LToP" > /* Size. */ > @@ -19,4 +30,4 @@ kernel_info: > > kernel_info_var_len_data: > /* Empty for time being... */ > -kernel_info_end: > +SYM_DATA_END_LABEL(kernel_info, SYM_L_LOCAL, kernel_info_end) > diff --git a/arch/x86/boot/compressed/kernel_info.h b/arch/x86/boot/compressed/kernel_info.h > new file mode 100644 > index 000000000000..c127f84aec63 > --- /dev/null > +++ b/arch/x86/boot/compressed/kernel_info.h > @@ -0,0 +1,12 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > + > +#ifndef BOOT_COMPRESSED_KERNEL_INFO_H > +#define BOOT_COMPRESSED_KERNEL_INFO_H > + > +#ifdef CONFIG_X86_64 > +#define KERNEL_INFO_OFFSET 0x500 > +#else /* 32-bit */ > +#define KERNEL_INFO_OFFSET 0x100 > +#endif > + > +#endif /* BOOT_COMPRESSED_KERNEL_INFO_H */ > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S > index 083ec6d7722a..718c52f3f1e6 100644 > --- a/arch/x86/boot/compressed/vmlinux.lds.S > +++ b/arch/x86/boot/compressed/vmlinux.lds.S > @@ -7,6 +7,7 @@ OUTPUT_FORMAT(CONFIG_OUTPUT_FORMAT) > > #include > #include > +#include "kernel_info.h" > > #ifdef CONFIG_X86_64 > OUTPUT_ARCH(i386:x86-64) > @@ -27,6 +28,11 @@ SECTIONS > HEAD_TEXT > _ehead = . ; > } > + .rodata.kernel_info KERNEL_INFO_OFFSET : { > + *(.rodata.kernel_info) > + } > + ASSERT(ABSOLUTE(kernel_info) == KERNEL_INFO_OFFSET, "kernel_info at bad address!") > + > .rodata..compressed : { > *(.rodata..compressed) > } > -- > 2.39.3 >