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    SubjectRe: [PATCH 2/5] x86-64: Allow alternative patching in the vDSO
    On Tue, Jun 07, 2011 at 03:32:39PM -0400, Andy Lutomirski 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@mit.edu>
    > ---
    > arch/x86/vdso/vma.c | 32 ++++++++++++++++++++++++++++++++
    > 1 files changed, 32 insertions(+), 0 deletions(-)
    >
    > diff --git a/arch/x86/vdso/vma.c b/arch/x86/vdso/vma.c
    > index 7abd2be..b8b074c1 100644
    > --- a/arch/x86/vdso/vma.c
    > +++ b/arch/x86/vdso/vma.c
    > @@ -23,11 +23,43 @@ extern unsigned short vdso_sync_cpuid;
    > static struct page **vdso_pages;
    > static unsigned vdso_size;
    >
    > +static void patch_vdso(void *vdso, size_t len)
    > +{
    > + 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;
    > + break;
    > + }
    > + }
    > +
    > + if (!alt_sec)
    > + return; /* nothing to patch */

    Just a minor nitpick:

    Maybe do

    for (.. ) {

    if (!strcmp(...)) {
    alt_sec = shdr;
    goto apply;
    }
    }
    return;

    apply:

    to save yourself the if (!alt_sec) test and the alt_sec = 0 aka NULL
    assignment above.

    > +
    > + alt_data = (void *)hdr + alt_sec->sh_offset;
    > +
    > + apply_alternatives(alt_data, alt_data + alt_sec->sh_size);
    > +}
    > +
    > static int __init init_vdso_vars(void)
    > {
    > int npages = (vdso_end - vdso_start + PAGE_SIZE - 1) / PAGE_SIZE;
    > int i;
    >
    > + patch_vdso(vdso_start, vdso_end - vdso_start);
    > +
    > vdso_size = npages << PAGE_SHIFT;
    > vdso_pages = kmalloc(sizeof(struct page *) * npages, GFP_KERNEL);
    > if (!vdso_pages)
    > --
    > 1.7.5.2

    Thanks.

    --
    Regards/Gruss,
    Boris.

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