Re: [PATCHv4 4/4] arm64: fix PAGE_OFFSET calc for flipped mm
On Fri, Jan 21, 2022 at 2:09 AM Philipp Rudo wrote:
>
> Hi Pingfan,
>
> On Tue, 18 Jan 2022 15:48:12 +0800
> Pingfan Liu wrote:
>
> > From: Kairui Song
> >
> > 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
> > (rebased and stripped by Pingfan )
> > Signed-off-by: Pingfan Liu
> > Cc: Simon Horman
> > Cc: Philipp Rudo
> > To: kexec@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
>
Thanks for your reviewing.
Regards,
Pingfan
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Re: [PATCHv4 4/4] arm64: fix PAGE_OFFSET calc for flipped mm
Hi Pingfan,
On Tue, 18 Jan 2022 15:48:12 +0800
Pingfan Liu wrote:
> From: Kairui Song
>
> 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
> (rebased and stripped by Pingfan )
> Signed-off-by: Pingfan Liu
> Cc: Simon Horman
> Cc: Philipp Rudo
> To: kexec@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.
Reviewed-by: Philipp Rudo
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[PATCHv4 4/4] arm64: fix PAGE_OFFSET calc for flipped mm
From: Kairui Song
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
(rebased and stripped by Pingfan )
Signed-off-by: Pingfan Liu
Cc: Simon Horman
Cc: Philipp Rudo
To: kexec@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;
--
2.31.1
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