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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Sat, 29 Apr 2023 07:13:48 +0100
Message-ID: <CAMj1kXGw9zz-PraPRx12uWkorhV0t8vRwG=qH7-3C75eBH2nAg@mail.gmail.com>
Subject: Re: [PATCH] arm64: lds: move .got section out of .text
To:     Fangrui Song <maskray@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Bill Wendling <morbo@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>
Content-Type: text/plain; charset="UTF-8"
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On Fri, 28 Apr 2023 at 23:29, Fangrui Song <maskray@google.com> wrote:
>
> On 2023-04-28, Ard Biesheuvel wrote:
> >On Fri, 28 Apr 2023 at 22:06, Fangrui Song <maskray@google.com> wrote:
> >>
> >> On 2023-04-28, Ard Biesheuvel wrote:
> >> >On Fri, 28 Apr 2023 at 19:58, Fangrui Song <maskray@google.com> 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@google.com> 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@google.com>
> >> >> >> ---
> >> >> >>  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.
>

Yes, but we don't use .data.rel.ro in the kernel because we don't
compile with -fpic/-fpie

> >> 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.
>

Why? The PT_LOAD headers are never consumed by anything. On arm64, we
never consume the ELF - it is converted to a flat binary during the
build.

> 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.
>

But we are not in user space. There is no ELF loader at runtime that
consumes any of this, The entire layout is self-described by the
linker script using symbols that the code links to directly.

So frankly, if we could just avoid PT_LOAD headers entirely, or add a
PHDRS {} section that defines a single region, I'd be ok with that.
But tweaking the layout to reduce the number of PT_LOAD headers seems
unnecessary to me.

And moving the GOT into a writable area just to make the ELF metadata
look tidier is out of the question - that could turn into a security
bug.


> ---
>
> 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.
>

The kernel is remapped a number of times during the boot - in the
initial mapping everything is read-only-exec except bss, and the
physical address is used as the virtual address. When it is remapped
again using the actual virtual addresses, we have two pseudo-segments,
where text/rodata/inittext are RX and initdata and data are RW. Then,
another mapping is created where text and inittext are RX, and rodata,
initdata and data are RW, until finally inittext and initdata are
unmapped and freed, and rodata is remapped read-only.

In the context of the above, why does it matter what the PT_LOAD headers say?