Re: [PATCH 4/5] RISC-V: Protect .init.text & .init.data

[Anup Patel]

On Thu, Oct 22, 2020 at 7:01 AM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
>
> On Fri, Oct 16, 2020 at 11:24 AM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Oct 13, 2020 at 10:24 PM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Oct 13, 2020 at 6:21 PM Jim Wilson <jimw@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Oct 13, 2020 at 3:25 PM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
> > > > > This happens only when copy_from_user is called from function that is
> > > > > annotated with __init.
> > > > > Adding Kito & Jim for their input
> > > > >
> > > > > @kito, @Jim: Please let me know if I should create a issue in
> > > > > riscv-gnu-toolchain repo or somewhere else.
> > > >
> > > > I can't do anything useful without a testcase that I can use to
> > > > reproduce the problem.  The interactions here are complex, so pointing
> > > > at lines of code or kernel config options doesn't give me any useful
> > > > info.
> > > >
> > > > Relaxation can convert calls to a jal.  I don't know of any open bugs
> > > > in this area that can generate relocation errors.  if it is a
> > > > relaxation error then turning off relaxation should work around the
> > > > problem as you suggested.
> > > >
> > > > A kernel build problem is serious.  I think this is worth a bug
> > > > report.  FSF binutils or riscv-gnu-toolchain is fine.
> > > >
> > >
> > > I have created an issue with detailed descriptions and reproduction steps.
> > > Please let me know if you need anything else.
> > >
> >
> > It may be a toolchain issue. Here is the ongoing discussion in case
> > anybody else is interested.
> >
> > https://github.com/riscv/riscv-gnu-toolchain/issues/738
> >
> > > > Jim
> > >
> > >
> > >
> > > --
> > > Regards,
> > > Atish
> >
> >
> >
> > --
> > Regards,
> > Atish
>
> Thanks to Jim, we know the cause now. Jim has provided an excellent
> analysis of the issue in the github issue report.
> https://github.com/riscv/riscv-gnu-toolchain/issues/738
>
> To summarize, the linker relaxation code is not aware of the
> alignments between sections.
> That's why it relaxes the calls from .text to .init.text and  converts
> a auipc+jalr pair to jal even if the address can't be fit +/- 1MB.
>
> There are few ways we can solve this problem.
>
> 1. As per Jim's suggestion, linker relaxation code is aware of the
> section alignments. We can mark .init.text as a 2MB aligned section.
>    For calls within a section, section's alignment will be used in the
> calculation. For calls across sections, e.g. from .init.text to .text,
> the maximum
>    section alignment of every section will be used. Thus, all
> relaxation within .init.text and between any sections will be
> impacted.
>    Thus, it avoids the error but results in the following increase in
> size of various sections.
>      section           change in size (in bytes)
>      .head.text      +4
>      .text               +40
>      .init.text.        +6530
>      .exit.text        +84
>
> The only significant increase is .init.text but it is freed after
> boot. Thus, I don't see any significant performance degradation due to
> that.
>
> Here is the diff
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -51,7 +51,13 @@ SECTIONS
>         . = ALIGN(SECTION_ALIGN);
>         __init_begin = .;
>         __init_text_begin = .;
> -       INIT_TEXT_SECTION(PAGE_SIZE)
> +       . = ALIGN(PAGE_SIZE);                                   \
> +       .init.text : AT(ADDR(.init.text) - LOAD_OFFSET)
> ALIGN(SECTION_ALIGN) {  \
> +               _sinittext = .;                                         \
> +               INIT_TEXT                                               \
> +               _einittext = .;                                         \
> +       }
> +
>         . = ALIGN(8);
>         __soc_early_init_table : {
>                 __soc_early_init_table_start = .;
>
> 2. We will continue to keep head.txt & .init.text together before
> .text. However, we will map the pages that contain head & init.text at
> page
>     granularity so that .head.text and init.text can have different
> permissions. I have not measured the performance impact of this but it
> won't
>     too bad given that the combined size of sections .head.txt &
> .init.text is 200K. So we are talking about page level permission only
> for
>     ~50 pages during boot.
>
> 3. Keep head.text in a separate 2MB aligned section. .init.text will
> follow .head.text in its own section as well. This increases the
> kernel
>     size by 2MB for MMU kernels. For nommu case, it will only increase
> by 64 bytes due to smaller section alignment for nommu kernels.
>
> Both solutions 1 & 2 come at minimal performance on boot time while
> solution 3 comes at increased kernel size.
>
> Any preference ?

I prefer solution#3 because:
1. This will help us avoid special handling of static objects
2.  This will make RISC-V linker script more aligned with other
     major architectures

I don't think solution#3 will increase the size of the kernel by 2MB. We
can make head.text part of text section. There will be only one section
alignment between text section and init section but the existing section
alignment between init section and text section will be removed. In other
words, number of section alignments will remain same.

Regards,
Anup