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

["Pali Rohár" <pali.rohar@gmail.com>]

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On Monday 08 April 2019 20:04:22 Pavel Machek wrote:
> 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?

Yea, it was mean as first April patch :-) I tested it with MS-DOS 7.0
and DOSBox and it worked.

> You may want to retransmit?

Do you really think it can be useful? :-)

> 								Pavel
> 
> > Signed-off-by: Pali Rohár <pali.rohar@gmail.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 = .;
> 

-- 
Pali Rohár
pali.rohar@gmail.com

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