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Date: Fri, 9 Nov 2018 17:57:50 +0800
From: Li Zhijian <zhijianx.li@...el.com>
To: Ingo Molnar <mingo@...nel.org>,
Li Zhijian <lizhijian@...fujitsu.com>,
Juergen Gross <jgross@...e.com>
Cc: Peter Maydell <peter.maydell@...aro.org>, x86@...nel.org,
hpa@...or.com, bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
QEMU Developers <qemu-devel@...gnu.org>,
Philip Li <philip.li@...el.com>, linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Kees Cook <keescook@...omium.org>
Subject: Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1
to allow up to 4G initrd
On 11/9/2018 3:20 PM, Ingo Molnar wrote:
> * Li Zhijian <lizhijian@...fujitsu.com> wrote:
>
>>> If the kernel initrd creation process creates an initrd which
>>> is larger than 2GB and also claims that it can't be placed
>>> with any part of it above 2GB, then that sounds like a bug
>>> in the initrd creation process...
>> Exactly, it's a real problem.
>>
>> Add x86 maintainers and LKML:
>>
>> The background is that QEMU want to support up to 4G initrd. but linux header (
>> initrd_addr_max field) only allow 2G-1.
>> Is one of the below approaches reasonable:
>> 1) change initrd_addr_max to 4G-1 directly simply(arch/x86/boot/header.S)?
>> 2) lie QEMU bootloader the initrd_addr_max is 4G-1 even though header said 2G-1
>> 3) any else
> A 10 years old comment from hpa says:
>
> initrd_addr_max: .long 0x7fffffff
> # (Header version 0x0203 or later)
> # The highest safe address for
> # the contents of an initrd
> # The current kernel allows up to 4 GB,
> # but leave it at 2 GB to avoid
> # possible bootloader bugs.
>
> To avoid the potential of bugs lurking in dozens of major and hundreds of
> minor iterations of various Linux bootloaders I'd prefer a real solution
> and extend it - because if there's a 2GB initrd for some weird reason
> today there might be a 4GB one in two years.
thank a lots. that's amazing.
>
> The real solution would be to:
>
> - Extend the boot protocol with a 64-bit field, named initrd_addr64_max
> or such.
> - We don't change the old field - but if the new field is set by new
> kernels then new bootloaders can use that as a new initrd_addr64_max
> value. (or reject to load the kernel if the address is too high.)
>
> - The kernel build should also emit a warning when building larger than
> 2GB initrds, with a list of bootloaders that support the new protocol.
Actually i just knew QEMU(Seabios + optionrom(linuxboot_dma.bin)) can support ~4GB initrd so far.
i just drafted at patch to add this field. could you have a look.
another patch which is to document initrd_addr64_max is ongoing.
commit db463ac9c1975f115d1ce2acb82d530c2b63b888
Author: Li Zhijian <lizhijian@...fujitsu.com>
Date: Fri Nov 9 17:24:14 2018 +0800
x86: Add header field initrd_addr64_max
Years ago, kernel had support load ~4GB initrd. But for some weird reasons (
avoid possible bootloader bugs), it only allow leave initrd under 2GB address
space(see initrd_addr_max fild at arch/x86/boot/header.S).
So modern bootloaders have not chance to load >=2G initrd previously.
To avoid the potential of bugs lurking in dozens of major and hundreds of
minor iterations of various Linux bootloaders. Ingo suggests to add a new field
initrd_addr64_max. If bootloader believes that it can load initrd to >=2G
address space, it can use initrd_addr64_max as the maximum loading address in
stead of the old field initrd_addr_max.
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 4c881c8..5fc3ebe 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -300,7 +300,7 @@ _start:
# Part 2 of the header, from the old setup.S
.ascii "HdrS" # header signature
- .word 0x020e # header version number (>= 0x0105)
+ .word 0x020f # header version number (>= 0x0105)
# or else old loadlin-1.5 will fail)
.globl realmode_swtch
realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
@@ -562,6 +562,12 @@ acpi_rsdp_addr: .quad 0 # 64-bit physical pointer to the
# ACPI RSDP table, added with
# version 2.14
+#ifdef CONFIG_INITRD_SIZE_4GB
+initrd_addr64_max: .quad 0xffffffff # allow ~4G initrd since 2.15
+#else
+initrd_addr64_max: .quad 0
+#endif
+
# End of setup header #####################################################
.section ".entrytext", "ax"
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 22f89d0..b86013d 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -90,6 +90,7 @@ struct setup_header {
__u32 init_size;
__u32 handover_offset;
__u64 acpi_rsdp_addr;
+ __u64 initrd_addr64_max;
} __attribute__((packed));
struct sys_desc_table {
diff --git a/init/Kconfig b/init/Kconfig
index a4112e9..611d4af 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1080,6 +1080,14 @@ config BLK_DEV_INITRD
If unsure say Y.
+config INITRD_SIZE_4GB
+ bool "4G size initrd support"
+ depends on (X86 || X86_64)
+ help
+ This option enables support ~4GB initrd.
+
+ if unsure say N.
+
if BLK_DEV_INITRD
source "usr/Kconfig"
Thanks
Zhijian
> Or something along those lines.
>
> Thanks,
>
> Ingo
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