[PATCH] x86_64, i386: Add command line length to boot protocol

From: Bernhard Walle
Date: Sun Feb 04 2007 - 13:06:14 EST

Because the command line is increased to 2048 characters after 2.6.20-rc6-mm1,
it's not possible for boot loaders and userspace tools to determine the length
of the command line the kernel can understand. The benefit of knowing that
length is that users can be warned if the command line size is too long which
prevents surprise if things don't work after bootup.

This patch updates the boot protocol to contain a field called "cmdline_size"
that contain the length of the command line. It also updates documentation to
set the length to 256 characters before boot protocol 2.06, simply because
there was no kernel before that has another limit than 256 characters.

The patch also adds missing fields (of protocol version 2.05) to the x86_64
setup code.


Signed-off-by: Bernhard Walle <bwalle@xxxxxxx>

---
Documentation/i386/boot.txt | 23 +++++++++++++++++------
arch/i386/boot/setup.S | 7 ++++++-
arch/x86_64/boot/setup.S | 15 ++++++++++++++-
3 files changed, 37 insertions(+), 8 deletions(-)

Index: linux-2.6.20-rc6-mm3/Documentation/i386/boot.txt
===================================================================
--- linux-2.6.20-rc6-mm3.orig/Documentation/i386/boot.txt
+++ linux-2.6.20-rc6-mm3/Documentation/i386/boot.txt
@@ -2,7 +2,7 @@
----------------------------

H. Peter Anvin <hpa@xxxxxxxxx>
- Last update 2007-01-26
+ Last update 2007-02-03

On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
@@ -35,9 +35,13 @@ Protocol 2.03: (Kernel 2.4.18-pre1) Expl
initrd address available to the bootloader.

Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
+
Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
Introduce relocatable_kernel and kernel_alignment fields.

+Protocol 2.06: (Kernel 2.6.21) Added a field that contains the size of
+ the boot command line
+

**** MEMORY LAYOUT

@@ -133,6 +137,8 @@ Offset Proto Name Meaning
022C/4 2.03+ initrd_addr_max Highest legal initrd address
0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
+0235/3 N/A pad2 Unused
+0238/4 2.06+ cmdline_size Maximum size of the kernel command line

(1) For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.
@@ -233,6 +239,12 @@ filled out, however:
if your ramdisk is exactly 131072 bytes long and this field is
0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)

+ cmdline_size:
+ The maximum size of the command line including the terminating
+ zero. This means that the command line can contain at most
+ (cmdline_size - 1) characters. With protocol version 2.05 and
+ earlier, the maximum size was 256.
+

**** THE KERNEL COMMAND LINE

@@ -241,11 +253,10 @@ loader to communicate with the kernel.
relevant to the boot loader itself, see "special command line options"
below.

-The kernel command line is a null-terminated string currently up to
-255 characters long, plus the final null. A string that is too long
-will be automatically truncated by the kernel, a boot loader may allow
-a longer command line to be passed to permit future kernels to extend
-this limit.
+The kernel command line is a null-terminated string. The maximum
+length can be retrieved from the field cmdline_size. Before protocol
+version 2.06, the maximum was 255 characters. A string that is too
+long will be automatically truncated by the kernel.

If the boot protocol version is 2.02 or later, the address of the
kernel command line is given by the header field cmd_line_ptr (see
Index: linux-2.6.20-rc6-mm3/arch/i386/boot/setup.S
===================================================================
--- linux-2.6.20-rc6-mm3.orig/arch/i386/boot/setup.S
+++ linux-2.6.20-rc6-mm3/arch/i386/boot/setup.S
@@ -52,6 +52,7 @@
#include <asm/boot.h>
#include <asm/e820.h>
#include <asm/page.h>
+#include <asm/setup.h>

/* Signature words to ensure LILO loaded us right */
#define SIG1 0xAA55
@@ -81,7 +82,7 @@ start:
# This is the setup header, and it must start at %cs:2 (old 0x9020:2)

.ascii "HdrS" # header signature
- .word 0x0205 # header version number (>= 0x0105)
+ .word 0x0206 # header version number (>= 0x0105)
# or else old loadlin-1.5 will fail)
realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
start_sys_seg: .word SYSSEG
@@ -171,6 +172,10 @@ relocatable_kernel: .byte 0
pad2: .byte 0
pad3: .word 0

+cmdline_size: .long COMMAND_LINE_SIZE #length of the command line,
+ #added with boot protocol
+ #version 2.06
+
trampoline: call start_of_setup
.align 16
# The offset at this point is 0x240
Index: linux-2.6.20-rc6-mm3/arch/x86_64/boot/setup.S
===================================================================
--- linux-2.6.20-rc6-mm3.orig/arch/x86_64/boot/setup.S
+++ linux-2.6.20-rc6-mm3/arch/x86_64/boot/setup.S
@@ -51,6 +51,7 @@
#include <asm/boot.h>
#include <asm/e820.h>
#include <asm/page.h>
+#include <asm/setup.h>

/* Signature words to ensure LILO loaded us right */
#define SIG1 0xAA55
@@ -80,7 +81,7 @@ start:
# This is the setup header, and it must start at %cs:2 (old 0x9020:2)

.ascii "HdrS" # header signature
- .word 0x0204 # header version number (>= 0x0105)
+ .word 0x0206 # header version number (>= 0x0105)
# or else old loadlin-1.5 will fail)
realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
start_sys_seg: .word SYSSEG
@@ -155,6 +156,18 @@ cmd_line_ptr: .long 0 # (Header versio
# low memory 0x10000 or higher.

ramdisk_max: .long 0xffffffff
+
+kernel_alignment: .long CONFIG_PHYSICAL_START #physical addr alignment
+ #(not relocatable =>
+ #fixed start == alignment)
+
+relocatable_kernel: .byte 0 #x86_64 is currently not
+pad2: .byte 0 #relocatable
+pad3: .word 0
+
+cmdline_size: .long COMMAND_LINE_SIZE #length of the command line,
+ #added with boot protocol
+ #version 2.06

trampoline: call start_of_setup
.align 16

-
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