linux-kernel - [tip: x86/boot] x86/boot/compressed/64: Use 32-bit (zero-extended) MOV for z_output

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Date:   Wed, 12 Feb 2020 12:04:37 -0000
From:   "tip-bot2 for Arvind Sankar" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Borislav Petkov <bp@...e.de>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/boot] x86/boot/compressed/64: Use 32-bit (zero-extended)
 MOV for z_output_len

The following commit has been merged into the x86/boot branch of tip:

Commit-ID:     a86255fe5258714e1f7c1bdfe95f08e4d098d450
Gitweb:        https://git.kernel.org/tip/a86255fe5258714e1f7c1bdfe95f08e4d098d450
Author:        Arvind Sankar <nivedita@...m.mit.edu>
AuthorDate:    Tue, 11 Feb 2020 12:33:33 -05:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Wed, 12 Feb 2020 11:15:31 +01:00

x86/boot/compressed/64: Use 32-bit (zero-extended) MOV for z_output_len

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 is already done for z_input_len, the
size of the compressed payload.

[ bp:

  Also, z_output_len cannot be a 64-bit quantity because it participates
  in:

  init_size:              .long INIT_SIZE         # kernel initialization size

  through INIT_SIZE which is a 32-bit quantity determined by the .long
  directive (vs .quad for 64-bit). Furthermore, if it really must be a
  64-bit quantity, then the insn must be MOVABS which can accommodate a
  64-bit immediate and which the toolchain does not generate automatically.
]

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lkml.kernel.org/r/20200211173333.1722739-1-nivedita@alum.mit.edu
---
 arch/x86/boot/compressed/head_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index d1220de..68f31c4 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -482,7 +482,7 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
 	leaq	input_data(%rip), %rdx  /* input_data */
 	movl	$z_input_len, %ecx	/* input_len */
 	movq	%rbp, %r8		/* output target address */
-	movq	$z_output_len, %r9	/* decompressed length, end of relocs */
+	movl	$z_output_len, %r9d	/* decompressed length, end of relocs */
 	call	extract_kernel		/* returns kernel location in %rax */
 	popq	%rsi