linux-kernel - [tip: x86/kaslr] x86/kaslr: Drop unnecessary alignment in find_random_virt

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Message-ID: <159675713376.3192.5137291775003011083.tip-bot2@tip-bot2>
Date:   Thu, 06 Aug 2020 23:38:53 -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>,
        Ingo Molnar <mingo@...nel.org>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/kaslr] x86/kaslr: Drop unnecessary alignment in
 find_random_virt_addr()

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

Commit-ID:     eb38be6db516fb72ccf7282628b545a185b3bc7a
Gitweb:        https://git.kernel.org/tip/eb38be6db516fb72ccf7282628b545a185b3bc7a
Author:        Arvind Sankar <nivedita@...m.mit.edu>
AuthorDate:    Tue, 28 Jul 2020 18:57:17 -04:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 31 Jul 2020 11:08:17 +02:00

x86/kaslr: Drop unnecessary alignment in find_random_virt_addr()

Drop unnecessary alignment of image_size to CONFIG_PHYSICAL_ALIGN in
find_random_virt_addr, it cannot change the result: the largest valid
slot is the largest n that satisfies

  minimum + n * CONFIG_PHYSICAL_ALIGN + image_size <= KERNEL_IMAGE_SIZE

(since minimum is already aligned) and so n is equal to

  (KERNEL_IMAGE_SIZE - minimum - image_size) / CONFIG_PHYSICAL_ALIGN

even if image_size is not aligned to CONFIG_PHYSICAL_ALIGN.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-17-nivedita@alum.mit.edu
---
 arch/x86/boot/compressed/kaslr.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 0c64026..ce34a05 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -825,16 +825,12 @@ static unsigned long find_random_virt_addr(unsigned long minimum,
 {
 	unsigned long slots, random_addr;
 
-	/* Align image_size for easy slot calculations. */
-	image_size = ALIGN(image_size, CONFIG_PHYSICAL_ALIGN);
-
 	/*
 	 * There are how many CONFIG_PHYSICAL_ALIGN-sized slots
 	 * that can hold image_size within the range of minimum to
 	 * KERNEL_IMAGE_SIZE?
 	 */
-	slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
-		 CONFIG_PHYSICAL_ALIGN + 1;
+	slots = 1 + (KERNEL_IMAGE_SIZE - minimum - image_size) / CONFIG_PHYSICAL_ALIGN;
 
 	random_addr = kaslr_get_random_long("Virtual") % slots;