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142 lines
5.9 KiB
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From viro@ftp.linux.org.uk Fri Feb 27 20:21:34 2015
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Received: (at 629506) by bugs.debian.org; 27 Feb 2015 20:21:34 +0000
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for 629506@bugs.debian.org; Fri, 27 Feb 2015 20:21:34 +0000
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for 629506@bugs.debian.org; Fri, 27 Feb 2015 19:53:49 +0000
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Date: Fri, 27 Feb 2015 19:53:49 +0000
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From: Al Viro <viro@ZenIV.linux.org.uk>
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To: 629506@bugs.debian.org
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Subject: memtest86+.bin crashes if loader ends up putting it not at 9000:0000
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Message-ID: <20150227195349.GN29656@ZenIV.linux.org.uk>
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MIME-Version: 1.0
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Sender: Al Viro <viro@ftp.linux.org.uk>
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X-Greylist: delayed 1658 seconds by postgrey-1.34 at buxtehude; Fri, 27 Feb 2015 20:21:33 UTC
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FWIW, the effects described in this bug report are 100% reproducible
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on any version, as long as the loader (lilo, grub, whatever) ends up putting
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the bootsect+setup in any location below 9000:0000.
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zImage-type images consist of 3 parts - bootsect, setup and
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payload. Payload is loaded at 1000:0, bootsect and setup - n:0 and
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n+2:0 resp., near the top of lowmem. Payload is protected mode code,
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setup - real mode one. Bootsect isn't executed at all; when the
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same image is booted directly it would've been the only part loaded
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by BIOS and it would copy itself and read the rest of the image to
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expected locations and pass control to setup. Values in it can be
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used by setup, though, so it must be present even when the image had been
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brought in by a loader.
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The thing is, we can't be guaranteed n == 0x9000. E.g. ACPI and
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SMM have every right to use _anything_ in range 512K..1M for their
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state, declaring it reserved. That's what BIOS int 0x12 is for - it
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reports how high (in kilobytes) can you go in lowmem without running into
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reserved areas. And while having it report 512K is rare, something like
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20K reserved just below the VRAM (i.e. report 620K) is nothing unusual.
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I hadn't looked into details of GRUB behaviour, but LILO puts its
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secondary loader as high in lowmem as it can, then puts the bootsect and
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setup parts of image below that. It tries to load at 9000:0 if possible,
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but if there's no space, it'll go lower. With the sizes it uses 20K reserved
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below 640K is enough to push bootsect + setup combination (just) below 9000:0.
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And memtest86+ setup has that 0x9000 hardwired - this
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gdt_48:
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.word 0x800 # gdt limit=2048, 256 GDT entries
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.word 512+gdt - start,0x9 # gdt base = 0X9xxxx
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in setup.S really depends on 'start' (entry to setup) loaded at
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0x90200 physical. Have it loaded at any other address and you'll trigger
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an exception as soon as you try to assign any segment register after you switch
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to protected mode. Which will escalate to triple-fault and reboot the
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damn thing immediately.
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GRUB might be better or worse in triggering that "load not at
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9000:0" situation, but it can't possibly avoid it in all cases. If nothing
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else, 9000:0 might be within one of the reserved areas. It simply isn't
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guaranteed to be available, period.
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Another issue is that setup of memtest86+ expects to see in %dx
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the value left there by the bootsect of memtest86+. Fortunately, it's
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not critical - in
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movw $INITSEG, %ax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %fs
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movw %ax, %ss # reset the stack to INITSEG:0x4000-12.
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movw %dx, %sp
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push %cs
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pop %ds
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we really need only the last two instructions. The stack footprint
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of what follows is fairly low *and* these values are not used past the
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reload of segment registers immediately after protected mode switch.
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It's not the only problem in there - the bits after
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# start from grub-a20.patch
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are clearly cargo-culted from grub, badly. There it was a part of a function
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that expects an argument on stack (it can turn A20 both on and off) and that
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got blindly copied, nevermind that the value on stack is random or that
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we follow it with (unconditional) use of 8042-based method anyway. I'd rather
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see upstream opinion on that particular piece of code first, though.
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Anyway, the patch below fixes dependency on being loaded at 9000:0 and it
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seems to work here without regressions. Have fun...
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--- memtest86+-5.01/setup.S 2013-08-09 22:01:58.000000000 -0400
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+++ fixed/setup.S 2015-02-27 14:18:47.000000000 -0500
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@@ -26,14 +26,13 @@
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# APs also execute this code
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#ljmp $INITSEG, $(reload - start + 0x200)
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reload:
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- movw $INITSEG, %ax
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- movw %ax, %ds
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- movw %ax, %es
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- movw %ax, %fs
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- movw %ax, %ss # reset the stack to INITSEG:0x4000-12.
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- movw %dx, %sp
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+ xorl %eax, %eax
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push %cs
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- pop %ds
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+ pop %ax
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+ movw %ax, %ds
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+ shll $4, %eax
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+ addl %eax, (gdt_48 - start + 2)
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+
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lidt idt_48 - start # load idt with 0,0
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lgdt gdt_48 - start # load gdt with whatever appropriate
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@@ -88,6 +87,7 @@
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %ss
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+ xorl %esp, %esp # 32bit code will set it then
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movw %ax, %fs
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movw %ax, %gs
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@@ -144,7 +144,7 @@
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gdt_48:
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.word 0x800 # gdt limit=2048, 256 GDT entries
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- .word 512+gdt - start,0x9 # gdt base = 0X9xxxx
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+ .word gdt - start,0 # gdt base, needs to be adjusted
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msg1:
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.asciz "Setup.S\r\n"
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