linux-kernel - Re: [PATCH] x86/boot: This program cannot be run in DOS mode.$

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Date:   Mon, 8 Apr 2019 20:04:22 +0200
From:   Pavel Machek <pavel@....cz>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/boot: This program cannot be run in DOS mode.$

On Mon 2019-04-01 12:24:34, Pali Rohár wrote:
> Every EFI binary is in PE format. And we know that PE format needs to have
> MZ MS-DOS header as there is written offset to PE header.
> 
> Therefore generated bzImage binary with CONFIG_EFI_STUB option is MS-DOS
> executable binary.
> 
> We already know the "requirement" that Windows PE executable started in
> MS-DOS must print legendary and famous message to computer screen:
> "This program cannot be run in DOS mode."
> 
> But trying to run that bzImage of Linux kernel with MZ header just cause
> freezing whole MS-DOS instead of writing "the correct message" to user.
> This is not the compliant behavior of PE executables!
> 
> This patch fixes this problem. When Linux kernel compiled with
> CONFIG_EFI_STUB is started in MS-DOS then it prints message:
> 
>   This program cannot be run in DOS mode.
>   To load Linux kernel from DOS mode use LOADLIN.EXE.
> 
> So it also helps MS-DOS users how to "correctly" start this bzImage binary
> by mentioning LOADLIN.EXE. Note that MS-DOS strings are not null-terminated
> but rather dollar-terminated.
> 
> To have error message unified bugger_off_msg for BIOS boot block code was
> changed to:
> 
>   This program cannot be run in BIOS mode.
> 
> So if you copy generated bzImage directly to boot sector and try to boot it
> by BIOS you get this new updated message.
> 
> Due to fixed offset of setup header (0x1f1), PE header was moved after
> entrytext section. bstext and bsdata sections where is full MZ header and
> MS-DOS code is now bigger.

Hehe, I guess the patch may have been dropped due to april's fools?

You may want to retransmit?
								Pavel

> Signed-off-by: Pali Rohár <pali.rohar@...il.com>
> ---
>  arch/x86/boot/header.S | 55 +++++++++++++++++++++++++++++++++++++++++++-------
>  arch/x86/boot/setup.ld |  1 +
>  2 files changed, 49 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 850b8762e889..58fcab7d00c8 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -45,6 +45,46 @@ bootsect_start:
>  	# "MZ", MS-DOS header
>  	.byte 0x4d
>  	.byte 0x5a
> +
> +	# Explicitly enter this as bytes, or the assembler
> +	# tries to generate a 3-byte jump here, which causes
> +	# everything else to push off to the wrong offset.
> +	.byte	0xeb    # short (2-byte) jump
> +	.byte	bios_start-1f
> +1:
> +
> +	.org	0x04
> +	.short	3       # e_cp - Pages in file
> +	.short	0       # e_crlc - Relocations
> +	.short	4       # e_cparhdr - Size of header in paragraphs
> +	.short	0       # e_minalloc - Minimum extra paragraphs needed
> +	.short	0xffff  # e_maxalloc - Maximum extra paragraphs needed
> +	.short	0       # e_ss - Initial (relative) SS value
> +	.short	0       # e_sp - Initial SP value
> +	.short	0       # e_csum - Checksum
> +	.short	0       # e_ip - Initial IP value
> +	.short	-4      # e_cs - Initial (relative) CS value
> +	.short	0x40    # e_lfarlc - File address of relocation table
> +
> +	.org	0x3c
> +	.long	pe_header  # Offset to the PE header
> +
> +	#
> +	# MS-DOS start code
> +	#
> +	push	%cs
> +	pop	%ds
> +
> +	# Write msdos_msg string
> +	mov	$msdos_msg, %dx
> +	mov	$0x09, %ah
> +	int	$0x21
> +
> +	# Terminate program
> +	mov	$0x4c01, %ax
> +	int	$0x21
> +
> +bios_start:
>  #endif
>  
>  	# Normalize the start address
> @@ -80,22 +120,23 @@ bs_die:
>  	# invoke the BIOS reset code...
>  	ljmp	$0xf000,$0xfff0
>  
> +	.section ".bsdata", "a"
>  #ifdef CONFIG_EFI_STUB
> -	.org	0x3c
> -	#
> -	# Offset to the PE header.
> -	#
> -	.long	pe_header
> +msdos_msg:
> +	.ascii	"This program cannot be run in DOS mode.\r\n"
> +	.ascii	"To load Linux kernel from DOS mode use LOADLIN.EXE.\r\r\n"
> +	.ascii	"$"
>  #endif /* CONFIG_EFI_STUB */
>  
> -	.section ".bsdata", "a"
>  bugger_off_msg:
> -	.ascii	"Use a boot loader.\r\n"
> +	.ascii	"This program cannot be run in BIOS mode.\r\n"
> +	.ascii	"To load Linux kernel from BIOS mode use a boot loader.\r\n"
>  	.ascii	"\n"
>  	.ascii	"Remove disk and press any key to reboot...\r\n"
>  	.byte	0
>  
>  #ifdef CONFIG_EFI_STUB
> +	.section ".pedata", "a"
>  pe_header:
>  	.ascii	"PE"
>  	.word 	0
> diff --git a/arch/x86/boot/setup.ld b/arch/x86/boot/setup.ld
> index 0149e41d42c2..c0ec2fe09d2d 100644
> --- a/arch/x86/boot/setup.ld
> +++ b/arch/x86/boot/setup.ld
> @@ -16,6 +16,7 @@ SECTIONS
>  	. = 495;
>  	.header		: { *(.header) }
>  	.entrytext	: { *(.entrytext) }
> +	.pedata		: { *(.pedata) }
>  	.inittext	: { *(.inittext) }
>  	.initdata	: { *(.initdata) }
>  	__end_init = .;

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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