Re: [PATCH] arm64: lds: move .got section out of .text
From: Fangrui Song
Date: Fri Apr 28 2023 - 17:06:19 EST
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. .got is initially writable, and
becomes read-only after relocation resolving.
I think this property is significant enough to make it outside of any rodata
segment.
(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?
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
>>