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

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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