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

[Fangrui Song]

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 .

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