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.

4 files changed, 36 insertions, 0 deletions

diff --git a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
index 947a266..1d793da 100644
--- a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
+++ b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
@@ -173,6 +173,8 @@ if [istarget "riscv*-*-*"] {
     run_dump_test "attr-phdr"
     run_dump_test "relax-max-align-gp"
     run_dump_test "uleb128"
+    run_dump_test "pr31179"
+    run_dump_test "pr31179-r"
     run_ld_link_tests [list \
 	[list "Weak reference 32" "-T weakref.ld -m[riscv_choose_ilp32_emul]" "" \
 	    "-march=rv32i -mabi=ilp32" {weakref32.s} \
diff --git a/ld/testsuite/ld-riscv-elf/pr31179-r.d b/ld/testsuite/ld-riscv-elf/pr31179-r.d
new file mode 100644
index 0000000..cd5c98e
--- /dev/null
+++ b/ld/testsuite/ld-riscv-elf/pr31179-r.d
@@ -0,0 +1,10 @@
+#source: pr31179.s
+#as:
+#readelf: -Wr
+
+Relocation section '.rela.text' at .*
+[ 	]+Offset[ 	]+Info[ 	]+Type[ 	]+.*
+[0-9a-f]+[ 	]+[0-9a-f]+[ 	]+R_RISCV_SET_ULEB128[ 	]+[0-9a-f]+[ 	]+bar \+ 1
+[0-9a-f]+[ 	]+[0-9a-f]+[ 	]+R_RISCV_SUB_ULEB128[ 	]+[0-9a-f]+[ 	]+foo \+ 0
+[0-9a-f]+[ 	]+[0-9a-f]+[ 	]+R_RISCV_SET_ULEB128[ 	]+[0-9a-f]+[ 	]+bar \+ 1
+[0-9a-f]+[ 	]+[0-9a-f]+[ 	]+R_RISCV_SUB_ULEB128[ 	]+[0-9a-f]+[ 	]+foo \+ 1
diff --git a/ld/testsuite/ld-riscv-elf/pr31179.d b/ld/testsuite/ld-riscv-elf/pr31179.d
new file mode 100644
index 0000000..a3228db
--- /dev/null
+++ b/ld/testsuite/ld-riscv-elf/pr31179.d
@@ -0,0 +1,11 @@
+#source: pr31179.s
+#as:
+#ld: --check-uleb128
+#objdump: -sj .text
+#warning: .*R_RISCV_SUB_ULEB128 with non-zero addend, please rebuild by binutils 2.42 or up
+
+.*:[ 	]+file format .*
+
+Contents of section .text:
+
+[ 	]+[0-9a-f]+[ 	]+00000303[ 	]+.*
diff --git a/ld/testsuite/ld-riscv-elf/pr31179.s b/ld/testsuite/ld-riscv-elf/pr31179.s
new file mode 100644
index 0000000..5c4b4b5
--- /dev/null
+++ b/ld/testsuite/ld-riscv-elf/pr31179.s
@@ -0,0 +1,13 @@
+.globl _start
+_start:
+
+foo:
+.2byte 0
+bar:
+
+.uleb128 bar - foo + 1
+
+reloc:
+.reloc reloc, R_RISCV_SET_ULEB128, bar + 1
+.reloc reloc, R_RISCV_SUB_ULEB128, foo + 1
+.byte 0x0