[PATCHv4 4/4] arm64: fix PAGE_OFFSET calc for flipped mm

[Pingfan Liu]

On Fri, Jan 21, 2022 at 2:09 AM Philipp Rudo <prudo at redhat.com> wrote:
>
> Hi Pingfan,
>
> On Tue, 18 Jan 2022 15:48:12 +0800
> Pingfan Liu <piliu at redhat.com> wrote:
>
> > From: Kairui Song <kasong at tencent.com>
> >
> > Since kernel commit 14c127c957c1 ('arm64: mm: Flip kernel VA space'),
> > the memory layout on arm64 have changed, and kexec-tools can no longer
> > get the the right PAGE_OFFSET based on _text symbol.
> >
> > Prior to that, the kimage (_text) lays above PAGE_END with this layout:
> > 0               -> VA_START                 : Usespace
> > VA_START        -> VA_START + 256M          : BPF JIT, Modules
> > VA_START + 256M -> PAGE_OFFSET - (~GB misc) : Vmalloc (KERNEL _text HERE)
> > PAGE_OFFSET     -> ...                      : * Linear map *
> >
> > And here we have:
> > VA_START    = -1UL << VA_BITS
> > PAGE_OFFSET = -1UL << (VA_BITS - 1)
> > _text < -1UL << (VA_BITS - 1)
> >
> > Kernel image lays somewhere between VA_START and PAGE_OFFSET, so we just
> > calc VA_BITS by getting the highest unset bit of _text symbol address,
> > and shift one less bit of VA_BITS to get page offset. This works as long
> > as KASLR don't put kernel in a too high location (which is commented inline).
> >
> > And after that commit, kernel layout have changed:
> > 0               -> PAGE_OFFSET              : Userspace
> > PAGE_OFFSET     -> PAGE_END                 : * Linear map *
> > PAGE_END        -> PAGE_END + 128M          : bpf jit region
> > PAGE_END + 128M -> PAGE_END + 256MB         : modules
> > PAGE_END + 256M -> ...                      : vmalloc (KERNEL _text HERE)
> >
> > Here we have:
> > PAGE_OFFSET = -1UL << VA_BITS
> > PAGE_END    = -1UL << (VA_BITS - 1)
> > _text > -1UL << (VA_BITS - 1)
> >
> > Kernel image now lays above PAGE_END, so we have to shift one more bit to
> > get the VA_BITS, and shift the exact VA_BITS for PAGE_OFFSET.
> >
> > We can simply check if "_text > -1UL << (VA_BITS - 1)" is true to judge
> > which layout is being used and shift the page offset occordingly.
> >
> > Signed-off-by: Kairui Song <kasong at tencent.com>
> > (rebased and stripped by Pingfan )
> > Signed-off-by: Pingfan Liu <piliu at redhat.com>
> > Cc: Simon Horman <horms at verge.net.au>
> > Cc: Philipp Rudo <prudo at redhat.com>
> > To: kexec at lists.infradead.org
> > ---
> >  kexec/arch/arm64/kexec-arm64.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/kexec/arch/arm64/kexec-arm64.c b/kexec/arch/arm64/kexec-arm64.c
> > index 793799b..ce7a5bb 100644
> > --- a/kexec/arch/arm64/kexec-arm64.c
> > +++ b/kexec/arch/arm64/kexec-arm64.c
> > @@ -923,13 +923,25 @@ out:
> >
> >  int get_page_offset(unsigned long *page_offset)
> >  {
> > +     unsigned long long text_sym_addr, kernel_va_mid;
> >       int ret;
> >
> > +     text_sym_addr = get_kernel_sym("_text");
> > +     if (text_sym_addr == 0) {
> > +             fprintf(stderr, "Can't get the symbol of _text to calculate page_offset.\n");
> > +             return -1;
> > +     }
> > +
> >       ret = get_va_bits();
> >       if (ret < 0)
> >               return ret;
> >
> > -     if (va_bits < 52)
> > +     /* Since kernel 5.4, kernel image is put above
> > +      * UINT64_MAX << (va_bits - 1)
> > +      */
> > +     kernel_va_mid = UINT64_MAX << (va_bits - 1);
> > +     /* older kernel */
> > +     if (text_sym_addr < kernel_va_mid)
> >               *page_offset = UINT64_MAX << (va_bits - 1);
> >       else
> >               *page_offset = UINT64_MAX << va_bits;
>
> I would drop the kernel_va_mid and simply use
>
>         *page_offset = UINT64_MAX << (va_bits - 1)
>         if (*page_offset > text_sym_addr > *page_offset)
>                 *page_offset = UINT64_MAX << va_bits
>
> but that's more a matter of taste.
>
Ah, I kept kernel_va_mid dedicatedly to illustrate the purpose.

> Reviewed-by: Philipp Rudo <prudo at redhat.com>
>
Thanks for your reviewing.

Regards,

Pingfan