[PATCH v2 0/2] aarch64: fix DT_RELR with discarded sections

[PATCH v2 0/2] aarch64: fix DT_RELR with discarded sections

[Szabolcs Nagy]

v2:
- update commit msg
- simplify the testcase

Szabolcs Nagy (2):
  aarch64: Fix DT_RELR support with discarded sections
  aarch64: Test DT_RELR with discarded sections

 bfd/elfnn-aarch64.c                           |  4 ++
 ld/testsuite/ld-aarch64/aarch64-elf.exp       |  2 +
 ld/testsuite/ld-aarch64/relr-discard-pie.d    | 15 +++++
 ld/testsuite/ld-aarch64/relr-discard-shared.d | 15 +++++
 ld/testsuite/ld-aarch64/relr-discard.ld       | 13 ++++
 ld/testsuite/ld-aarch64/relr-discard.s        | 63 +++++++++++++++++++
 6 files changed, 112 insertions(+)
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard-pie.d
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard-shared.d
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard.ld
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard.s

-- 
2.25.1

[PATCH v2 1/2] aarch64: Fix DT_RELR support with discarded sections

[Szabolcs Nagy]

In case of discarded sections, via /DISCARD/ or .gnu.linkonce,
relr relocation accounting was wrong.  This broke building linux.

The issue was that the *_relocate_section logic was copied to
*_late_size_sections to record relative relocations that can
be packed, however *_relocate_section is not called on sections
that are discarded, while record_relr_non_got_relocs is called
for all input sections. The fix is to filter out the discarded
sections with the same logic that is used to count non-GOT
relocs for local symbols earlier.

GOT relocations are affected too if all sections are discarded
that reference the GOT entry of a particular symbol, however
this can cause unused GOT entries independently of DT_RELR, and
the only difference with DT_RELR is that a relative reloc may be
emitted instead of a R_AARCH64_NONE for the unused GOT entry
which is acceptable. A proper fix would require redoing the GOT
refcounting after we know the discarded sections.
---
v2:
- only commit msg update: i realized that the GOT case is not
  handled right when DT_RELR is disabled so there is no point
  trying to fix that here and unnecessary relative relocation
  in such corner-case is acceptable. i updated the test case
  patch to cover the GOT cases that currently work.
---
 bfd/elfnn-aarch64.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/bfd/elfnn-aarch64.c b/bfd/elfnn-aarch64.c
index 81f1a646125..02873f2ceb2 100644
--- a/bfd/elfnn-aarch64.c
+++ b/bfd/elfnn-aarch64.c
@@ -9327,6 +9327,10 @@ record_relr_non_got_relocs (bfd *input_bfd, struct bfd_link_info *info,
     return true;
   if (sec->alignment_power == 0)
     return true;
+  if (!bfd_is_abs_section (sec)
+      && bfd_is_abs_section (sec->output_section))
+    /* Input section has been discarded.  */
+    return true;
   sreloc = elf_section_data (sec)->sreloc;
   if (sreloc == NULL)
     return true;
-- 
2.25.1

[PATCH v2 2/2] aarch64: Test DT_RELR with discarded sections

[Szabolcs Nagy]

---
 ld/testsuite/ld-aarch64/aarch64-elf.exp       |  2 +
 ld/testsuite/ld-aarch64/relr-discard-pie.d    | 15 +++++
 ld/testsuite/ld-aarch64/relr-discard-shared.d | 15 +++++
 ld/testsuite/ld-aarch64/relr-discard.ld       | 13 ++++
 ld/testsuite/ld-aarch64/relr-discard.s        | 63 +++++++++++++++++++
 5 files changed, 108 insertions(+)
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard-pie.d
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard-shared.d
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard.ld
 create mode 100644 ld/testsuite/ld-aarch64/relr-discard.s

diff --git a/ld/testsuite/ld-aarch64/aarch64-elf.exp b/ld/testsuite/ld-aarch64/aarch64-elf.exp
index 38929ffc3c6..0e70ca3e028 100644
--- a/ld/testsuite/ld-aarch64/aarch64-elf.exp
+++ b/ld/testsuite/ld-aarch64/aarch64-elf.exp
@@ -475,6 +475,8 @@ run_dump_test_lp64 "relr-got-pie"
 run_dump_test_lp64 "relr-got-shared"
 run_dump_test_lp64 "relr-text-pie"
 run_dump_test_lp64 "relr-text-shared"
+run_dump_test_lp64 "relr-discard-pie"
+run_dump_test_lp64 "relr-discard-shared"
 
 if { ![skip_sframe_tests] } {
   run_dump_test "sframe-simple-1"
diff --git a/ld/testsuite/ld-aarch64/relr-discard-pie.d b/ld/testsuite/ld-aarch64/relr-discard-pie.d
new file mode 100644
index 00000000000..b1d16e30bc9
--- /dev/null
+++ b/ld/testsuite/ld-aarch64/relr-discard-pie.d
@@ -0,0 +1,15 @@
+#source: relr-discard.s
+#ld: -pie --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
+#readelf: -rW
+
+Relocation section '\.rela\.dyn' at offset 0x1.* contains 4 entries:
+    Offset             Info             Type               Symbol's Value  Symbol's Name \+ Addend
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+
+Relocation section '\.relr\.dyn' at offset 0x1.* contains 2 entries which relocate 2 locations:
+Index: Entry            Address           Symbolic Address
+0000:  0000000000020008 0000000000020008  _GLOBAL_OFFSET_TABLE_ \+ 0x8
+0001:  0000000000000003 0000000000020010  _GLOBAL_OFFSET_TABLE_ \+ 0x10
diff --git a/ld/testsuite/ld-aarch64/relr-discard-shared.d b/ld/testsuite/ld-aarch64/relr-discard-shared.d
new file mode 100644
index 00000000000..b5f3ad87666
--- /dev/null
+++ b/ld/testsuite/ld-aarch64/relr-discard-shared.d
@@ -0,0 +1,15 @@
+#source: relr-discard.s
+#ld: -shared --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
+#readelf: -rW
+
+Relocation section '\.rela\.dyn' at offset 0x1.* contains 5 entries:
+    Offset             Info             Type               Symbol's Value  Symbol's Name \+ Addend
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000000000  0000000000000000 R_AARCH64_NONE                            0
+0000000000020010  .*00000401 R_AARCH64_GLOB_DAT     000000000001000c sym_global \+ 0
+
+Relocation section '\.relr\.dyn' at offset 0x1.* contains 1 entry which relocates 1 location:
+Index: Entry            Address           Symbolic Address
+0000:  0000000000020008 0000000000020008  _GLOBAL_OFFSET_TABLE_ \+ 0x8
diff --git a/ld/testsuite/ld-aarch64/relr-discard.ld b/ld/testsuite/ld-aarch64/relr-discard.ld
new file mode 100644
index 00000000000..288ba23e11c
--- /dev/null
+++ b/ld/testsuite/ld-aarch64/relr-discard.ld
@@ -0,0 +1,13 @@
+OUTPUT_ARCH(aarch64)
+ENTRY(_start)
+SECTIONS
+{
+  /DISCARD/ : { *(.discard.*) }
+
+  . = 0x10000;
+  .text : { *(.text) }
+  . = 0x20000;
+  .got  : { *(.got) *(.got.plt)}
+  . = 0x30000;
+  .data : { *(.data) *(.data.*) }
+}
diff --git a/ld/testsuite/ld-aarch64/relr-discard.s b/ld/testsuite/ld-aarch64/relr-discard.s
new file mode 100644
index 00000000000..0ed9b4a9098
--- /dev/null
+++ b/ld/testsuite/ld-aarch64/relr-discard.s
@@ -0,0 +1,63 @@
+// Test DT_RELR with references in discarded sections.
+
+.text
+.p2align 3
+.global _start
+_start:
+	nop
+
+sym_local:
+	nop
+
+.global sym_hidden
+.hidden sym_hidden
+sym_hidden:
+	nop
+
+.global sym_global
+sym_global:
+	nop
+
+.global sym_global_abs
+.set sym_global_abs, 42
+
+.global sym_weak_undef
+.weak sym_weak_undef
+
+.section .discard.got_local,"ax"
+	adrp	x0, :got:sym_local
+	ldr	x0, [x0, :got_lo12:sym_local]
+
+.section .discard.got_global,"ax"
+	adrp	x0, :got:sym_global
+	ldr	x0, [x0, :got_lo12:sym_global]
+
+.section .discard.local,"a"
+.p2align 1
+discard_local:
+.xword sym_local
+
+.section .discard.hidden,"a"
+.p2align 1
+discard_hidden:
+.xword sym_hidden
+
+.section .discard.global,"a"
+.p2align 1
+discard_global:
+.xword sym_global
+
+.section .discard.global_abs,"a"
+.p2align 1
+discard_global_abs:
+.xword sym_global_abs
+
+.section .discard.weak_undef,"a"
+.p2align 1
+discard_weak_undef:
+.xword sym_weak_undef
+
+.section .discard._DYNAMIC,"a"
+.p2align 1
+discard_DYNAMIC:
+.xword _DYNAMIC
-- 
2.25.1

Re: [PATCH v2 1/2] aarch64: Fix DT_RELR support with discarded sections

[Alan Modra]

On Tue, Jun 04, 2024 at 06:41:34PM +0100, Szabolcs Nagy wrote:
> --- a/bfd/elfnn-aarch64.c
> +++ b/bfd/elfnn-aarch64.c
> @@ -9327,6 +9327,10 @@ record_relr_non_got_relocs (bfd *input_bfd, struct bfd_link_info *info,
>      return true;
>    if (sec->alignment_power == 0)
>      return true;
> +  if (!bfd_is_abs_section (sec)
> +      && bfd_is_abs_section (sec->output_section))
> +    /* Input section has been discarded.  */
> +    return true;

I think you should be using discarded_section here and in
elfNN_aarch64_late_size_sections.

>    sreloc = elf_section_data (sec)->sreloc;
>    if (sreloc == NULL)
>      return true;
> -- 
> 2.25.1

-- 
Alan Modra

Re: [PATCH v2 2/2] aarch64: Test DT_RELR with discarded sections

[Alan Modra]

On Tue, Jun 04, 2024 at 06:41:45PM +0100, Szabolcs Nagy wrote:
> ---
>  ld/testsuite/ld-aarch64/aarch64-elf.exp       |  2 +
>  ld/testsuite/ld-aarch64/relr-discard-pie.d    | 15 +++++
>  ld/testsuite/ld-aarch64/relr-discard-shared.d | 15 +++++
>  ld/testsuite/ld-aarch64/relr-discard.ld       | 13 ++++
>  ld/testsuite/ld-aarch64/relr-discard.s        | 63 +++++++++++++++++++
>  5 files changed, 108 insertions(+)
>  create mode 100644 ld/testsuite/ld-aarch64/relr-discard-pie.d
>  create mode 100644 ld/testsuite/ld-aarch64/relr-discard-shared.d
>  create mode 100644 ld/testsuite/ld-aarch64/relr-discard.ld
>  create mode 100644 ld/testsuite/ld-aarch64/relr-discard.s
> 
> diff --git a/ld/testsuite/ld-aarch64/aarch64-elf.exp b/ld/testsuite/ld-aarch64/aarch64-elf.exp
> index 38929ffc3c6..0e70ca3e028 100644
> --- a/ld/testsuite/ld-aarch64/aarch64-elf.exp
> +++ b/ld/testsuite/ld-aarch64/aarch64-elf.exp
> @@ -475,6 +475,8 @@ run_dump_test_lp64 "relr-got-pie"
>  run_dump_test_lp64 "relr-got-shared"
>  run_dump_test_lp64 "relr-text-pie"
>  run_dump_test_lp64 "relr-text-shared"
> +run_dump_test_lp64 "relr-discard-pie"
> +run_dump_test_lp64 "relr-discard-shared"
>  
>  if { ![skip_sframe_tests] } {
>    run_dump_test "sframe-simple-1"
> diff --git a/ld/testsuite/ld-aarch64/relr-discard-pie.d b/ld/testsuite/ld-aarch64/relr-discard-pie.d
> new file mode 100644
> index 00000000000..b1d16e30bc9
> --- /dev/null
> +++ b/ld/testsuite/ld-aarch64/relr-discard-pie.d
> @@ -0,0 +1,15 @@
> +#source: relr-discard.s
> +#ld: -pie --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
> +#readelf: -rW
> +
> +Relocation section '\.rela\.dyn' at offset 0x1.* contains 4 entries:
> +    Offset             Info             Type               Symbol's Value  Symbol's Name \+ Addend
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +

I must say I don't really like matching bad behaviour in a test, but I
guess this ensures you don't have both .rela.dyn and .relr.dyn relocs
against G_O_T, so OK.

> +Relocation section '\.relr\.dyn' at offset 0x1.* contains 2 entries which relocate 2 locations:
> +Index: Entry            Address           Symbolic Address
> +0000:  0000000000020008 0000000000020008  _GLOBAL_OFFSET_TABLE_ \+ 0x8
> +0001:  0000000000000003 0000000000020010  _GLOBAL_OFFSET_TABLE_ \+ 0x10
> diff --git a/ld/testsuite/ld-aarch64/relr-discard-shared.d b/ld/testsuite/ld-aarch64/relr-discard-shared.d
> new file mode 100644
> index 00000000000..b5f3ad87666
> --- /dev/null
> +++ b/ld/testsuite/ld-aarch64/relr-discard-shared.d
> @@ -0,0 +1,15 @@
> +#source: relr-discard.s
> +#ld: -shared --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
> +#readelf: -rW
> +
> +Relocation section '\.rela\.dyn' at offset 0x1.* contains 5 entries:
> +    Offset             Info             Type               Symbol's Value  Symbol's Name \+ Addend
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> +0000000000020010  .*00000401 R_AARCH64_GLOB_DAT     000000000001000c sym_global \+ 0
> +
> +Relocation section '\.relr\.dyn' at offset 0x1.* contains 1 entry which relocates 1 location:
> +Index: Entry            Address           Symbolic Address
> +0000:  0000000000020008 0000000000020008  _GLOBAL_OFFSET_TABLE_ \+ 0x8
> diff --git a/ld/testsuite/ld-aarch64/relr-discard.ld b/ld/testsuite/ld-aarch64/relr-discard.ld
> new file mode 100644
> index 00000000000..288ba23e11c
> --- /dev/null
> +++ b/ld/testsuite/ld-aarch64/relr-discard.ld
> @@ -0,0 +1,13 @@
> +OUTPUT_ARCH(aarch64)
> +ENTRY(_start)
> +SECTIONS
> +{
> +  /DISCARD/ : { *(.discard.*) }
> +
> +  . = 0x10000;
> +  .text : { *(.text) }
> +  . = 0x20000;
> +  .got  : { *(.got) *(.got.plt)}
> +  . = 0x30000;
> +  .data : { *(.data) *(.data.*) }
> +}
> diff --git a/ld/testsuite/ld-aarch64/relr-discard.s b/ld/testsuite/ld-aarch64/relr-discard.s
> new file mode 100644
> index 00000000000..0ed9b4a9098
> --- /dev/null
> +++ b/ld/testsuite/ld-aarch64/relr-discard.s
> @@ -0,0 +1,63 @@
> +// Test DT_RELR with references in discarded sections.
> +
> +.text
> +.p2align 3
> +.global _start
> +_start:
> +	nop
> +
> +sym_local:
> +	nop
> +
> +.global sym_hidden
> +.hidden sym_hidden
> +sym_hidden:
> +	nop
> +
> +.global sym_global
> +sym_global:
> +	nop
> +
> +.global sym_global_abs
> +.set sym_global_abs, 42
> +
> +.global sym_weak_undef
> +.weak sym_weak_undef
> +
> +.section .discard.got_local,"ax"
> +	adrp	x0, :got:sym_local
> +	ldr	x0, [x0, :got_lo12:sym_local]
> +
> +.section .discard.got_global,"ax"
> +	adrp	x0, :got:sym_global
> +	ldr	x0, [x0, :got_lo12:sym_global]
> +
> +.section .discard.local,"a"
> +.p2align 1
> +discard_local:
> +.xword sym_local
> +
> +.section .discard.hidden,"a"
> +.p2align 1
> +discard_hidden:
> +.xword sym_hidden
> +
> +.section .discard.global,"a"
> +.p2align 1
> +discard_global:
> +.xword sym_global
> +
> +.section .discard.global_abs,"a"
> +.p2align 1
> +discard_global_abs:
> +.xword sym_global_abs
> +
> +.section .discard.weak_undef,"a"
> +.p2align 1
> +discard_weak_undef:
> +.xword sym_weak_undef
> +
> +.section .discard._DYNAMIC,"a"
> +.p2align 1
> +discard_DYNAMIC:
> +.xword _DYNAMIC
> -- 
> 2.25.1

-- 
Alan Modra

Re: [PATCH v2 1/2] aarch64: Fix DT_RELR support with discarded sections

[Szabolcs Nagy]

The 06/06/2024 10:47, Alan Modra wrote:
> On Tue, Jun 04, 2024 at 06:41:34PM +0100, Szabolcs Nagy wrote:
> > --- a/bfd/elfnn-aarch64.c
> > +++ b/bfd/elfnn-aarch64.c
> > @@ -9327,6 +9327,10 @@ record_relr_non_got_relocs (bfd *input_bfd, struct bfd_link_info *info,
> >      return true;
> >    if (sec->alignment_power == 0)
> >      return true;
> > +  if (!bfd_is_abs_section (sec)
> > +      && bfd_is_abs_section (sec->output_section))
> > +    /* Input section has been discarded.  */
> > +    return true;
> 
> I think you should be using discarded_section here and in
> elfNN_aarch64_late_size_sections.

i was not sure why late_size_sections did it this way.

i will fix it.

Re: [PATCH v2 2/2] aarch64: Test DT_RELR with discarded sections

[Szabolcs Nagy]

The 06/06/2024 10:53, Alan Modra wrote:
> On Tue, Jun 04, 2024 at 06:41:45PM +0100, Szabolcs Nagy wrote:
> > --- /dev/null
> > +++ b/ld/testsuite/ld-aarch64/relr-discard-pie.d
> > @@ -0,0 +1,15 @@
> > +#source: relr-discard.s
> > +#ld: -pie --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
> > +#readelf: -rW
> > +
> > +Relocation section '\.rela\.dyn' at offset 0x1.* contains 4 entries:
> > +    Offset             Info             Type               Symbol's Value  Symbol's Name \+ Addend
> > +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> > +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> > +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> > +0000000000000000  0000000000000000 R_AARCH64_NONE                            0
> > +
> 
> I must say I don't really like matching bad behaviour in a test, but I
> guess this ensures you don't have both .rela.dyn and .relr.dyn relocs
> against G_O_T, so OK.

i think we can fix relr behaviour and the test once
https://sourceware.org/bugzilla/show_bug.cgi?id=31847
is fixed, i will add a comment.