Re: [PATCH] arm64: lds: move .got section out of .text

[Ard Biesheuvel]

On Fri, 28 Apr 2023 at 19:58, Fangrui Song <maskray@xxxxxxxxxx> wrote:
>
> On 2023-04-28, Ard Biesheuvel wrote:
> >Hello Fangrui,
>
> Hello Ard, thank you for the rapid response.
>
> >On Fri, 28 Apr 2023 at 06:05, Fangrui Song <maskray@xxxxxxxxxx> wrote:
> >>
> >> Currently, the .got section is placed within the output section .text.
> >> However, when .got is non-empty, the SHF_WRITE flag is set when linked
> >> by lld. GNU ld recognizes .text as a special section and ignores the
> >> SHF_WRITE flag. By renaming .text, we can also get the SHF_WRITE flag.
> >>
> >> Conventionally, the .got section is placed just before .got.plt (which
> >> should be empty and omitted in the kernel). Therefore, we move the .got
> >> section to a conventional location (between .text and .data) and remove
> >> the unneeded `. = ALIGN(16)`.
> >>
> >> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
> >> ---
> >>  arch/arm64/kernel/vmlinux.lds.S | 20 ++++++++++----------
> >>  1 file changed, 10 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> >> index b9202c2ee18e..2bcb3b30db41 100644
> >> --- a/arch/arm64/kernel/vmlinux.lds.S
> >> +++ b/arch/arm64/kernel/vmlinux.lds.S
> >> @@ -181,18 +181,8 @@ SECTIONS
> >>                         KPROBES_TEXT
> >>                         HYPERVISOR_TEXT
> >>                         *(.gnu.warning)
> >> -               . = ALIGN(16);
> >> -               *(.got)                 /* Global offset table          */
> >>         }
> >>
> >> -       /*
> >> -        * Make sure that the .got.plt is either completely empty or it
> >> -        * contains only the lazy dispatch entries.
> >> -        */
> >> -       .got.plt : { *(.got.plt) }
> >> -       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
> >> -              "Unexpected GOT/PLT entries detected!")
> >> -
> >>         . = ALIGN(SEGMENT_ALIGN);
> >>         _etext = .;                     /* End of text section */
> >>
> >> @@ -247,6 +237,16 @@ SECTIONS
> >>
> >>         . = ALIGN(SEGMENT_ALIGN);
> >>         __inittext_end = .;
> >> +
> >> +       .got : { *(.got) }
> >
> >This is the .init region, which gets freed and unmapped after boot. If
> >the GOT is non-empty, it needs to remain mapped, so we cannot place it
> >here.
>
> Thanks.  I did not know the constraint.
>
> >We have the same issue with the .rodata section, which incorporates
> >variables marked as __ro_after_init, which are not const qualified. So
> >given that .rodata is already emitted as WA, and we cannot do anything
> >about that, let's move the GOT in there.
>
> Yes, writable .data..ro_after_init and __jump_table sections in
> include/asm-generic/vmlinux.lds.h (#define RO_DATA(align)) makes the
> output section .rodata writable.  Perhaps this is very difficult to fix,
> and we will have writable .rodata for a long time.
>
> What do you think of moving .got/.got.plt immediately before .data?
> I want to place .got/.got.plt before the guaranteed-writable sections,
> not some sections which are "unfortunately" writable (.rodata, __modver,
> .hyp.rodata, .rodata.text, etc).
>
> For userspace programs, either linked with GNU ld or lld, .got/.got.plt
> are usually immediately before .data .
>

I don't think that would be the right choice.

We have five pseudo-segments in the kernel

text (RX)
rodata (R)
inittext (RX)
initdata (RW)
data (RW)

where the init ones disappear entirely when the boot completes.

The GOT should not be modifiable, so it should not be in .data. So the
only appropriate 'segment' for the GOT is rodata

Note that we don't use PIC codegen for the kernel, so all const
qualified data structures containing statically initialized global
pointer variables are emitted into .rodata as well, and relocated at
boot. So having the GOT in rodata too makes sense imho.




> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index b9202c2ee18e..48bd7c25b6ab 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -181,18 +181,8 @@ SECTIONS
>                         KPROBES_TEXT
>                         HYPERVISOR_TEXT
>                         *(.gnu.warning)
> -               . = ALIGN(16);
> -               *(.got)                 /* Global offset table          */
>         }
>
> -       /*
> -        * Make sure that the .got.plt is either completely empty or it
> -        * contains only the lazy dispatch entries.
> -        */
> -       .got.plt : { *(.got.plt) }
> -       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
> -              "Unexpected GOT/PLT entries detected!")
> -
>         . = ALIGN(SEGMENT_ALIGN);
>         _etext = .;                     /* End of text section */
>
> @@ -286,6 +276,15 @@ SECTIONS
>         __initdata_end = .;
>         __init_end = .;
>
> +       .got : { *(.got) }
> +       /*
> +        * Make sure that the .got.plt is either completely empty or it
> +        * contains only the lazy dispatch entries.
> +        */
> +       .got.plt : { *(.got.plt) }
> +       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
> +              "Unexpected GOT/PLT entries detected!")
> +
>         _data = .;
>         _sdata = .;
>         RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
> --
> 2.40.1.495.gc816e09b53d-goog
>
>
> >> +       /*
> >> +        * Make sure that the .got.plt is either completely empty or it
> >> +        * contains only the lazy dispatch entries.
> >> +        */
> >> +       .got.plt : { *(.got.plt) }
> >> +       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
> >> +              "Unexpected GOT/PLT entries detected!")
> >> +
> >>         __initdata_begin = .;
> >>
> >>         init_idmap_pg_dir = .;
> >> --
> >> 2.40.1.495.gc816e09b53d-goog
> >>