On Wed, 18 Nov, at 09:14:23AM, Ingo Molnar wrote:
>
> * Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
>
> > > > + npages = (_end - _text) >> PAGE_SHIFT;
> > >
> > > You really need to PFN_ALIGN _end and _text. Has been wrong in the
> > > existing code as well.
> >
> > Hmm... very good point.
>
> So I think we should instead guarantee that _end and _text are page aligned.
>
> _text is already page aligned:
>
> SECTIONS
> {
> #ifdef CONFIG_X86_32
> . = LOAD_OFFSET + LOAD_PHYSICAL_ADDR;
> phys_startup_32 = startup_32 - LOAD_OFFSET;
> #else
> . = __START_KERNEL;
> phys_startup_64 = startup_64 - LOAD_OFFSET;
> #endif
>
> /* Text and read-only data */
> .text : AT(ADDR(.text) - LOAD_OFFSET) {
> _text = .;
>
> The reason for aligning _end as well is that we already page-align the BSS and BRK
> sections of the kernel and its various section boundary symbols:
>
> /* BSS */
> . = ALIGN(PAGE_SIZE);
> .bss : AT(ADDR(.bss) - LOAD_OFFSET) {
> __bss_start = .;
> *(.bss..page_aligned)
> *(.bss)
> . = ALIGN(PAGE_SIZE);
> __bss_stop = .;
> }
>
> . = ALIGN(PAGE_SIZE);
> .brk : AT(ADDR(.brk) - LOAD_OFFSET) {
> __brk_base = .;
> . += 64 * 1024; /* 64k alignment slop space */
> *(.brk_reservation) /* areas brk users have reserved */
> __brk_limit = .;
> }
>
> _end = .;
>
> STABS_DEBUG
> DWARF_DEBUG
>
> _end is the only odd one out, so we should align it as well - because it's easy to
> make such pfn conversion bugs.
FWIW, I saw no changes in either 32-bit or 64-bit vmlinux size when
building with the following patch, so it seems like a pretty easy win,
---
>From 25ad518fa52e589f110376ae06e42fb20b3e4188 Mon Sep 17 00:00:00 2001
From: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Date: Fri, 20 Nov 2015 11:46:11 +0000
Subject: [PATCH] x86: Page align _end to avoid pfn conversion bugs
Ingo noted that if we can guarantee _end is aligned to PAGE_SIZE we
can automatically avoid bugs along the lines of,
size = _end - _text >> PAGE_SHIFT
which is missing a call to PFN_ALIGN(). The EFI mixed mode contains
this bug, for example.
_text is already aligned to PAGE_SIZE through the use of
LOAD_PHYSICAL_ADDR, and the BSS and BRK sections are explicitly
aligned in the linker script, so it makes sense to align _end to
match.
Reported-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "H . Peter Anvin" <hpa@xxxxxxxxx>
Cc: Toshi Kani <toshi.kani@xxxxxx>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/vmlinux.lds.S | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 74e4bf11f562..4f1994257a18 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -325,6 +325,7 @@ SECTIONS
__brk_limit = .;
}
+ . = ALIGN(PAGE_SIZE);
_end = .;
STABS_DEBUG
--
2.6.2
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