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Date:   Fri, 28 Apr 2023 18:58:14 +0000
From:   Fangrui Song <maskray@google.com>
To:     Ard Biesheuvel <ardb@kernel.org>
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>
Subject: Re: [PATCH] arm64: lds: move .got section out of .text
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References: <20230428050442.180913-1-maskray@google.com>
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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
>>