linux-kernel - Re: [PATCH v2 3/8] x86-64: Allow alternative patching in the vDSO

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Message-ID: <CADZ9YHiSmjtXx1Ue4tfUzN9zpj2qjABY_fhMP+SOm2_vYrrFfw@mail.gmail.com>
Date:	Mon, 11 Jul 2011 16:41:51 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Andy Lutomirski <luto@....edu>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, John Stultz <johnstul@...ibm.com>,
	Borislav Petkov <bp@...64.org>
Subject: Re: [PATCH v2 3/8] x86-64: Allow alternative patching in the vDSO

On Sun, Jul 10, 2011 at 9:22 AM, Andy Lutomirski <luto@....edu> wrote:
> This code is short enough and different enough from the module
> loader that it's not worth trying to share anything.
>
> Signed-off-by: Andy Lutomirski <luto@....edu>
> ---
>  arch/x86/vdso/vma.c |   30 ++++++++++++++++++++++++++++++
>  1 files changed, 30 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/vdso/vma.c b/arch/x86/vdso/vma.c
> index 7abd2be..ba92244 100644
> --- a/arch/x86/vdso/vma.c
> +++ b/arch/x86/vdso/vma.c
> @@ -23,11 +23,41 @@ extern unsigned short vdso_sync_cpuid;
>  static struct page **vdso_pages;
>  static unsigned vdso_size;
>
> +static void patch_vdso(void *vdso, size_t len)

I think, patch_vdso should mark with __init. Since it's been only
called from an __init function. We might hit a section mismatch, even
if we don't hit section mismatch, the will function get removed after
system boot - saves some .text size.

> +{
> +       Elf64_Ehdr *hdr = vdso;
> +       Elf64_Shdr *sechdrs, *alt_sec = 0;
> +       char *secstrings;
> +       void *alt_data;
> +       int i;
> +
> +       BUG_ON(len < sizeof(Elf64_Ehdr));
> +       BUG_ON(memcmp(hdr->e_ident, ELFMAG, SELFMAG) != 0);
> +
> +       sechdrs = (void *)hdr + hdr->e_shoff;
> +       secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
> +
> +       for (i = 1; i < hdr->e_shnum; i++) {
> +               Elf64_Shdr *shdr = &sechdrs[i];
> +               if (!strcmp(secstrings + shdr->sh_name, ".altinstructions")) {
> +                       alt_sec = shdr;
> +                       goto found;
> +               }
> +       }
> +       return;  /* nothing to patch */

If there's nothing to patch, I think (perhaps) - it would be nice to
let user know through printk.

Thanks,
Rakib.
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