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Date: Thu, 21 May 2020 11:11:18 -0700
From: Kees Cook <keescook@...omium.org>
To: Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com, rick.p.edgecombe@...el.com,
Tony Luck <tony.luck@...el.com>, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 5/9] x86: Make sure _etext includes function sections
On Thu, May 21, 2020 at 09:56:36AM -0700, Kristen Carlson Accardi wrote:
> When using -ffunction-sections to place each function in
> it's own text section so it can be randomized at load time, the
> linker considers these .text.* sections "orphaned sections", and
> will place them after the first similar section (.text). In order
> to accurately represent the end of the text section and the
> orphaned sections, _etext must be moved so that it is after both
> .text and .text.* The text size must also be calculated to
> include .text AND .text.*
>
> Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Tested-by: Tony Luck <tony.luck@...el.com>
> ---
> arch/x86/kernel/vmlinux.lds.S | 18 +++++++++++++++++-
> include/asm-generic/vmlinux.lds.h | 2 +-
> 2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 1bf7e312361f..044f7528a2f0 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -147,8 +147,24 @@ SECTIONS
> #endif
> } :text =0xcccc
>
> - /* End of text section, which should occupy whole number of pages */
> +#ifdef CONFIG_FG_KASLR
> + /*
> + * -ffunction-sections creates .text.* sections, which are considered
> + * "orphan sections" and added after the first similar section (.text).
> + * Adding this ALIGN statement causes the address of _etext
> + * to be below that of all the .text.* orphaned sections
> + */
> + . = ALIGN(PAGE_SIZE);
> +#endif
> _etext = .;
> +
> + /*
> + * the size of the .text section is used to calculate the address
> + * range for orc lookups. If we just use SIZEOF(.text), we will
> + * miss all the .text.* sections. Calculate the size using _etext
> + * and _stext and save the value for later.
> + */
> + text_size = _etext - _stext;
> . = ALIGN(PAGE_SIZE);
I don't think there's any need for this #ifdef (nor the trailing ALIGN).
I think leave the comment to explain why the ALIGN is being moved before
the _etext.
A repeated ALIGN won't move the position:
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -147,9 +147,11 @@ SECTIONS
#endif
} :text =0xcccc
+ . = ALIGN(PAGE_SIZE);
/* End of text section, which should occupy whole number of * pages */
_etext = .;
. = ALIGN(PAGE_SIZE);
+ _ktext = .;
X86_ALIGN_RODATA_BEGIN
RO_DATA(PAGE_SIZE)
$ nm vmlinux | grep '_[ek]text'
ffffffff81e02000 R _etext
ffffffff81e02000 R _ktext
--
Kees Cook
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