linux-kernel - Re: [PATCH 1/2] x86/vdso: Remove obsolete "fake section table" reservation

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Date:   Tue, 4 Dec 2018 10:22:39 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 1/2] x86/vdso: Remove obsolete "fake section table"
 reservation

On Tue, Dec 04, 2018 at 08:17:40AM -0800, Sean Christopherson wrote:
> At one point the vDSO image was manually stripped down by vdso2c in an
> attempt to minimize the size of the image mapped into userspace.  Part
> of that stripping process involved building a fake section table so as
> not to break userspace processes that parse the section table.  Memory
> for the fake section table was reserved in the .rodata section so that
> vdso2c could simply copy the entire PT_LOAD segment into the userspace
> image after building the fake table.
> 
> Eventually, the entire fake section table approach was dropped in favor
> of stripping the vdso "the old fashioned way", i.e. via objdump -S.
> But, the reservation in .rodata for the fake table was left behind.
> Remove the reserveration along with a few other related defines and
> section entries.
> 
> Removing the fake section table placeholder zaps a whopping 0x340 bytes
> from the 64-bit vDSO image, which drops the current image's size to
> under 4k, i.e. reduces the effective size of the userspace vDSO mapping
> by a full page.
> 
> Fixes: da861e18eccc ("x86, vdso: Get rid of the fake section mechanism")
> Cc: Andy Lutomirski <luto@...capital.net>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
> diff --git a/arch/x86/entry/vdso/vdso2c.c b/arch/x86/entry/vdso/vdso2c.c
> index 4674f58581a1..2479a454b15c 100644
> --- a/arch/x86/entry/vdso/vdso2c.c
> +++ b/arch/x86/entry/vdso/vdso2c.c
> @@ -98,12 +98,6 @@ struct vdso_sym required_syms[] = {
>  	[sym_hpet_page] = {"hpet_page", true},
>  	[sym_pvclock_page] = {"pvclock_page", true},
>  	[sym_hvclock_page] = {"hvclock_page", true},
> -	[sym_VDSO_FAKE_SECTION_TABLE_START] = {
> -		"VDSO_FAKE_SECTION_TABLE_START", false
> -	},
> -	[sym_VDSO_FAKE_SECTION_TABLE_END] = {
> -		"VDSO_FAKE_SECTION_TABLE_END", false
> -	},

Doh, I missed removing the definitions for sym_VDSO_FAKE_SECTION_TABLE_*.

>  	{"VDSO32_NOTE_MASK", true},
>  	{"__kernel_vsyscall", true},
>  	{"__kernel_sigreturn", true},
> -- 
> 2.19.2
>