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

From: Fangrui Song
Date: Fri Apr 28 2023 - 18:29:21 EST

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On 2023-04-28, Ard Biesheuvel wrote:

On Fri, 28 Apr 2023 at 22:06, Fangrui Song <maskray@xxxxxxxxxx> wrote:

On 2023-04-28, Ard Biesheuvel wrote:
>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.

arm64's vmlinux is linked with -shared -Bsymbolic and .got has many
entries that need to be relocated.

Actually, it is mostly empty, given that we don't use -fpic/-fpie codegen.

Yes, .got is usually small, e.g. 0x68 in my `make ARCH=arm64 LLVM=1 -j60 defconfig vmlinux` build.

.got is initially writable, and
becomes read-only after relocation resolving.

The same applies to .rodata, given that we don't use -fpic/fpie
codegen. And note that I am not referring to .data..ro_after_init here
- I mean .rodata itself, which carries all const qualified global data
structures with statically initialized pointer fields. These are
conceptually the same as GOT entries - the only difference is that
they are not created by the linker and are referenced explicitly from
the C code.

I agree that they are very similar, but there is a distinction, that's
why we have both .rodata and .data.rel.ro (relro), and the latter is for sections
that need relocations.

I think this property is significant enough to make it outside of any rodata
segment.

I disagree.

(In the userspace .got is typically placed in PT_GNU_RELRO by using -z relro.)

If we remove inittext and initdata pseudo-segments which disappear after
the boot, we have

text (RX)
rodata (R)
/// where is .got (RELRO)
data (RW)

If we consider .got distinct from rodata, I think placing .got immediately
after rodata or immediately before data is both fine?

We can place it anywhere in the rodata pseudo-segment. What user space
does is not quite relevant here - what is relevant is that we already
have a pseudo-segment with the right properties. Adding a new segment
just for the GOT which is almost empty and is mapped with the same
attributes as rodata seems unnecessary to me.

Do you have any practical concerns? Or is it just tidiness?

Unfortunately, vmlinux has many PT_LOAD program headers. I am trying to
find the best position for .got that makes the most sense. When making
this decision, I aim to create a layout that looks more normal and is
closer to user-space programs. Additionally, I want to reduce the number
of PT_LOAD program headers.

You are arguing that .got should be part of rodata. However, I disagree
with this perspective from both practical and linker layout viewpoints.

If we place .got below RO_DATA(PAGE_SIZE), it will immediately follow
.note or .BTF (see NOTES in include/asm-generic/vmlinux.lds.h).
There isn't an option to drop SHF_WRITE from .got, and
we will need a separate PT_LOAD program header for .got.

Alternatively, if we place .got below .hyp.rodata, we can eliminate one
PT_LOAD because the transition from .hyp.rodata to .got does not require
a separate PT_LOAD. However, having a half-writable .got before
.rodata.text feels strange to me.

I feel that the region between .text/.rodata and .data is too messy.
Placing .got immediately before .data makes the layout look more normal
and closer to user space without causing any pessimization.

---

My linker layout viewpoint is that ideally a program's program headers
should looke like

https://maskray.me/blog/2020-11-15-explain-gnu-linker-options#layout-related

R PT_LOAD
RX PT_LOAD
RW PT_LOAD (overlaps with PT_GNU_RELRO) .got, .data.rel.ro (if present)
RW PT_LOAD

For historical reason, I can expect swapped R and RX.
Sections such as .got and .data.rel.ro are different from both .rodata
and .data.

>
>
>
>> 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
>> >>

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