[PATCH v3 0/2] aarch64: fix DT_RELR with discarded sections

[PATCH v3 0/2] aarch64: fix DT_RELR with discarded sections

[Szabolcs Nagy]

v3:
- use discarded_section()
- adjust commit msg
- comment about bug in testcase
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                           |  5 +-
 ld/testsuite/ld-aarch64/aarch64-elf.exp       |  2 +
 ld/testsuite/ld-aarch64/relr-discard-pie.d    | 18 ++++++
 ld/testsuite/ld-aarch64/relr-discard-shared.d | 18 ++++++
 ld/testsuite/ld-aarch64/relr-discard.ld       | 13 ++++
 ld/testsuite/ld-aarch64/relr-discard.s        | 63 +++++++++++++++++++
 6 files changed, 117 insertions(+), 2 deletions(-)
 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 v3 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
record_relr_non_got_relocs to find the relative relocs 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 in *_late_size_sections for local symbols earlier.
Use the discarded_section helper in both cases to clarify the
intent and handle all corner-cases consistently.

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, see bug 31850.
---
v3:
- use discarded_section()
- adjust commit msg
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 | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/bfd/elfnn-aarch64.c b/bfd/elfnn-aarch64.c
index 81f1a646125..2221de0a480 100644
--- a/bfd/elfnn-aarch64.c
+++ b/bfd/elfnn-aarch64.c
@@ -9327,6 +9327,8 @@ record_relr_non_got_relocs (bfd *input_bfd, struct bfd_link_info *info,
     return true;
   if (sec->alignment_power == 0)
     return true;
+  if (discarded_section (sec))
+    return true;
   sreloc = elf_section_data (sec)->sreloc;
   if (sreloc == NULL)
     return true;
@@ -9602,8 +9604,7 @@ elfNN_aarch64_late_size_sections (bfd *output_bfd ATTRIBUTE_UNUSED,
 	  for (p = (struct elf_dyn_relocs *)
 	       (elf_section_data (s)->local_dynrel); p != NULL; p = p->next)
 	    {
-	      if (!bfd_is_abs_section (p->sec)
-		  && bfd_is_abs_section (p->sec->output_section))
+	      if (discarded_section (p->sec))
 		{
 		  /* Input section has been discarded, either because
 		     it is a copy of a linkonce section or due to
-- 
2.25.1

[PATCH v3 2/2] aarch64: Test DT_RELR with discarded sections

[Szabolcs Nagy]

---
v3:
- add comment about bug.
---
 ld/testsuite/ld-aarch64/aarch64-elf.exp       |  2 +
 ld/testsuite/ld-aarch64/relr-discard-pie.d    | 18 ++++++
 ld/testsuite/ld-aarch64/relr-discard-shared.d | 18 ++++++
 ld/testsuite/ld-aarch64/relr-discard.ld       | 13 ++++
 ld/testsuite/ld-aarch64/relr-discard.s        | 63 +++++++++++++++++++
 5 files changed, 114 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..7e7f341bc36
--- /dev/null
+++ b/ld/testsuite/ld-aarch64/relr-discard-pie.d
@@ -0,0 +1,18 @@
+#source: relr-discard.s
+#ld: -pie --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
+#readelf: -rW
+
+# Note: There are unnecessary GOT entries and *_NONE relocations
+# for those GOT entries and discarded locations, this is bug 31850.
+
+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..9260a6d69c4
--- /dev/null
+++ b/ld/testsuite/ld-aarch64/relr-discard-shared.d
@@ -0,0 +1,18 @@
+#source: relr-discard.s
+#ld: -shared --no-apply-dynamic-relocs -z pack-relative-relocs -T relr-discard.ld
+#readelf: -rW
+
+# Note: There are unnecessary GOT entries and *_NONE relocations
+# for those GOT entries and discarded locations, this is bug 31850.
+
+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  .* 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 v3 0/2] aarch64: fix DT_RELR with discarded sections

[Alan Modra]

On Thu, Jun 06, 2024 at 11:52:58AM +0100, Szabolcs Nagy wrote:
> v3:
> - use discarded_section()
> - adjust commit msg
> - comment about bug in testcase
> v2:
> - update commit msg
> - simplify the testcase

These are OK.

-- 
Alan Modra