RISC-V: PR31179, The SET/ADD/SUB fix breaks ABI compatibility with 2.41 objects

* Problematic fix commit,
2029e13917d53d2289d3ebb390c4f40bd2112d21
RISC-V: Clarify the behaviors of SET/ADD/SUB relocations

* Bugzilla,
https://sourceware.org/bugzilla/show_bug.cgi?id=31179#c5

The addend of SUB_ULEB128 should be zero if using .uleb128, but we make it
non-zero by accident in assembler before.  This causes troubles by applying
the above commit, since the calculation is changed to support .reloc *SUB*
relocations with non-zero addend.

We encourage people to rebuild their stuff to get the non-zero addend of
SUB_ULEB128, but that might need some times, so report warnings to inform
people need to rebuild their stuff if --check-uleb128 is enabled.

Since the failed .reloc cases for ADD/SET/SUB/ULEB128 are rarely to use,
it may acceptable that stop supproting them until people rebuld their stuff,
maybe half-year or a year later.  Or maybe we should teach people that don't
write the .reloc R_RISCV_SUB* with non-zero constant, and then report
warnings/errors in assembler.

bfd/
	* elfnn-riscv.c (perform_relocation): Ignore the non-zero addend of
	R_RISCV_SUB_ULEB128.
	(riscv_elf_relocate_section): Report warnings to inform people need
	to rebuild their stuff if --check-uleb128 is enabled.  So that can
	get the right non-zero addend of R_RISCV_SUB_ULEB128.
	* elfxx-riscv.h (struct riscv_elf_params): Added bool check_uleb128.
ld/
	* NEWS: Updated.
	* emultempl/riscvelf.em: Added linker risc-v target options,
	--[no-]check-uleb128, to enable/disable checking if the addend of
	uleb128 is non-zero or not.  So that people will know they need to
	rebuild the objects with binutils 2.42 and up, to get the right zero
	addend of SUB_ULEB128 relocation, or they may get troubles if using
	.reloc.
	* ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.
	* ld/testsuite/ld-riscv-elf/pr31179*: New test cases.

2 files changed, 32 insertions, 14 deletions

diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
index 042266e..509d61e 100644
--- a/bfd/elfnn-riscv.c
+++ b/bfd/elfnn-riscv.c
@@ -1735,19 +1735,9 @@ perform_relocation (const reloc_howto_type *howto,
   if (howto->pc_relative)
     value -= sec_addr (input_section) + rel->r_offset;
 
-  switch (ELFNN_R_TYPE (rel->r_info))
-    {
-    case R_RISCV_SUB6:
-    case R_RISCV_SUB8:
-    case R_RISCV_SUB16:
-    case R_RISCV_SUB32:
-    case R_RISCV_SUB64:
-    case R_RISCV_SUB_ULEB128:
-      value -= rel->r_addend;
-      break;
-    default:
-      value += rel->r_addend;
-    }
+  /* PR31179, ignore the non-zero addend of R_RISCV_SUB_ULEB128.  */
+  if (ELFNN_R_TYPE (rel->r_info) != R_RISCV_SUB_ULEB128)
+    value += rel->r_addend;
 
   switch (ELFNN_R_TYPE (rel->r_info))
     {
@@ -2530,9 +2520,35 @@ riscv_elf_relocate_section (bfd *output_bfd,
 	  if (uleb128_set_rel != NULL
 	      && uleb128_set_rel->r_offset == rel->r_offset)
 	    {
-	      relocation = uleb128_set_vma - relocation + uleb128_set_rel->r_addend;
+	      relocation = uleb128_set_vma - relocation
+			   + uleb128_set_rel->r_addend;
 	      uleb128_set_vma = 0;
 	      uleb128_set_rel = NULL;
+
+	      /* PR31179, the addend of SUB_ULEB128 should be zero if using
+		 .uleb128, but we make it non-zero by accident in assembler,
+		 so just ignore it in perform_relocation, and make assembler
+		 continue doing the right thing.  Don't reset the addend of
+		 SUB_ULEB128 to zero here since it will break the --emit-reloc,
+		 even though the non-zero addend is unexpected.
+
+		 We encourage people to rebuild their stuff to get the
+		 non-zero addend of SUB_ULEB128, but that might need some
+		 times, so report warnings to inform people need to rebuild
+		 if --check-uleb128 is enabled.  However, since the failed
+		 .reloc cases for ADD/SET/SUB/ULEB128 are rarely to use, it
+		 may acceptable that stop supproting them until people rebuld
+		 their stuff, maybe half-year or one year later.  I believe
+		 this might be the least harmful option that we should go.
+
+		 Or maybe we should teach people that don't write the
+		 .reloc R_RISCV_SUB* with non-zero constant, and report
+		 warnings/errors in assembler.  */
+	      if (htab->params->check_uleb128
+		  && rel->r_addend != 0)
+		_bfd_error_handler (_("%pB: warning: R_RISCV_SUB_ULEB128 with"
+				      " non-zero addend, please rebuild by"
+				      " binutils 2.42 or up"), input_bfd);
 	    }
 	  else
 	    {
diff --git a/bfd/elfxx-riscv.h b/bfd/elfxx-riscv.h
index abcb409..6df2471 100644
--- a/bfd/elfxx-riscv.h
+++ b/bfd/elfxx-riscv.h
@@ -31,6 +31,8 @@ struct riscv_elf_params
 {
   /* Whether to relax code sequences to GP-relative addressing.  */
   bool relax_gp;
+  /* Whether to check if SUB_ULEB128 relocation has non-zero addend.  */
+  bool check_uleb128;
 };
 
 extern void riscv_elf32_set_options (struct bfd_link_info *,