From: Arvind Sankar <email@example.com> To: Borislav Petkov <firstname.lastname@example.org> Cc: Arvind Sankar <email@example.com>, Thomas Gleixner <firstname.lastname@example.org>, Ingo Molnar <email@example.com>, "H. Peter Anvin" <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, Michael Matz <email@example.com> Subject: Re: [PATCH v2] x86/boot: Use 32-bit (zero-extended) move for z_output_len Date: Tue, 11 Feb 2020 14:27:39 -0500 [thread overview] Message-ID: <20200211192739.GA1761914@rani.riverdale.lan> (raw) In-Reply-To: <20200211181559.GI32279@zn.tnic> On Tue, Feb 11, 2020 at 07:15:59PM +0100, Borislav Petkov wrote: > On Tue, Feb 11, 2020 at 12:33:33PM -0500, Arvind Sankar wrote: > > z_output_len is the size of the decompressed payload (i.e. vmlinux + > > vmlinux.relocs) and is generated as an unsigned 32-bit quantity by > > mkpiggy.c. > > > > The current movq $z_output_len, %r9 instruction generates a > > sign-extended move to %r9. Using movl $z_output_len, %r9d will instead > > zero-extend into %r9, which is appropriate for an unsigned 32-bit > > quantity. This is also what we already do for z_input_len, the size of > > the compressed payload. > > Yes, thanks. > > What I'll also add to this is the fact that > > init_size: .long INIT_SIZE # kernel initialization size > > where z_output_len participates in through INIT_SIZE is a 32-bit > quantity determined by the ".long" so even if something made > z_output_len bigger than 32-bit by explicitly using MOVABS so that it > builds fine, you'd still get: > > arch/x86/boot/header.S: Assembler messages: > arch/x86/boot/header.S:568: Warning: value 0x10103b000 truncated to 0x103b000 > > as a warning. Yes, this is just a "neatening" patch to use a more appropriate instruction. There would have to be a lot of work to allow kernels to be bigger than 2Gb, they're currently limited to at most 1Gb (or even 0.5Gb if KASLR is disabled) by KERNEL_IMAGE_SIZE definition in asm/page_64_types.h and there are checks in the linker script and a bunch of other places, so the decompressed length can't be much bigger than that. > > Btw, while poking at this, we found out that the MOV really remains > MOV and not MOVABS if gas doesn't know what the quantity behind the > z_output_len symbol is, as it is a symbol assignment. Which, AFAIU, with > ELF64 objects, it should be using 8-byte quantities in the symbol table > to accommodate such assignments. But for some reason it doesn't. > > Anyway, Michael can correct me if I'm still imprecise here. For absolute symbols that it sees in the same file it does use 64-bit immediate move for movq if necessary, but otherwise seems to need the explicit opcode. > > Thx. > > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2020-02-11 19:27 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-01-07 19:44 [PATCH 1/3] x86/boot/compressed/32: Simplify calculation of output address Arvind Sankar 2020-01-07 19:44 ` [PATCH 2/3] x86/boot/compressed/64: Use leal to initialize boot stack pointer Arvind Sankar 2020-02-12 12:04 ` [tip: x86/boot] x86/boot/compressed/64: Use LEA " tip-bot2 for Arvind Sankar 2020-01-07 19:44 ` [PATCH 3/3] x86/boot/compressed/64: Use 32-bit move for z_output_len Arvind Sankar 2020-02-11 16:17 ` Borislav Petkov 2020-02-11 17:33 ` [PATCH v2] x86/boot: Use 32-bit (zero-extended) " Arvind Sankar 2020-02-11 18:15 ` Borislav Petkov 2020-02-11 19:27 ` Arvind Sankar [this message] 2020-02-12 9:47 ` Borislav Petkov 2020-02-12 12:04 ` [tip: x86/boot] x86/boot/compressed/64: Use 32-bit (zero-extended) MOV " tip-bot2 for Arvind Sankar 2020-01-23 11:04 ` [tip: x86/asm] x86/boot: Simplify calculation of output address tip-bot2 for Arvind Sankar
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