linux-kernel - Re: [tip: x86/boot] x86/boot: Fix incorrect startup_gdt

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Message-ID: <CBF7285B-C6F9-4539-ADA2-A626EAE1A73F@zytor.com>
Date:   Tue, 03 Oct 2023 14:56:09 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     linux-kernel@...r.kernel.org,
        tip-bot2 for Yuntao Wang <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org
CC:     Yuntao Wang <ytcoode@...il.com>, Ingo Molnar <mingo@...nel.org>,
        x86@...nel.org
Subject: Re: [tip: x86/boot] x86/boot: Fix incorrect startup_gdt_descr.size

On October 3, 2023 1:39:57 AM PDT, tip-bot2 for Yuntao Wang <tip-bot2@...utronix.de> wrote:
>The following commit has been merged into the x86/boot branch of tip:
>
>Commit-ID:     001470fed5959d01faecbd57fcf2f60294da0de1
>Gitweb:        https://git.kernel.org/tip/001470fed5959d01faecbd57fcf2f60294da0de1
>Author:        Yuntao Wang <ytcoode@...il.com>
>AuthorDate:    Mon, 07 Aug 2023 16:45:47 +08:00
>Committer:     Ingo Molnar <mingo@...nel.org>
>CommitterDate: Tue, 03 Oct 2023 10:28:29 +02:00
>
>x86/boot: Fix incorrect startup_gdt_descr.size
>
>Since the size value is added to the base address to yield the last valid
>byte address of the GDT, the current size value of startup_gdt_descr is
>incorrect (too large by one), fix it.
>
>[ mingo: This probably never mattered, because startup_gdt[] is only used
>         in a very controlled fashion - but make it consistent nevertheless. ]
>
>Fixes: 866b556efa12 ("x86/head/64: Install startup GDT")
>Signed-off-by: Yuntao Wang <ytcoode@...il.com>
>Signed-off-by: Ingo Molnar <mingo@...nel.org>
>Cc: "H. Peter Anvin" <hpa@...or.com>
>Link: https://lore.kernel.org/r/20230807084547.217390-1-ytcoode@gmail.com
>---
> arch/x86/kernel/head64.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
>index 49f7629..bbc2179 100644
>--- a/arch/x86/kernel/head64.c
>+++ b/arch/x86/kernel/head64.c
>@@ -80,7 +80,7 @@ static struct desc_struct startup_gdt[GDT_ENTRIES] = {
>  * while the kernel still uses a direct mapping.
>  */
> static struct desc_ptr startup_gdt_descr = {
>-	.size = sizeof(startup_gdt),
>+	.size = sizeof(startup_gdt)-1,
> 	.address = 0,
> };
> 

Well, it never mattered because there wasn't enough space at the end for an extra entry, but it is still Technically Correct™.