Messages in this thread |  | | Date | Tue, 11 Feb 2020 19:15:59 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v2] x86/boot: Use 32-bit (zero-extended) move for z_output_len |
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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.
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.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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