Re: [PATCH] BFD: Prevent writing the MIPS _gp_disp symbol into symbol tables

[Simon Atanasyan <simon at atanasyan dot com>]

The _gp_disp is a magic symbol, always implicitly defined by the linker.
It does not make a sense to write it into symbol tables for output files.
Moreover, now if the linker gets a version script, the _gp_disp symbol
gets zero version definition index.  The zero index means[1]:

"The symbol is local, not available outside the object."

But the _gp_disp symbol has GLOBAL binding.  That confuses some tools like
for example the LLD linker when they get such files as inputs.

This patch fixes the problem - it prevents writing the _gp_disp symbol
in regular and dynamic symbol tables.

This was tested by running ls test suite on mipsel-linux board.

References:

[1] "Linux Standard Base Specification", Section "10.7.2 Symbol
    Version Table", p. 32

2018-02-28  Simon Atanasyan  <simon@atanasyan.com>

bfd/

	* elf32-mips.c: (elf32_mips_fixup_symbol): New function.
	(elf_backend_fixup_symbol): New macro.
	* elfxx-mips.c: (mips_elf_output_extsym): Discard _gp_disp
	handling.
	(_bfd_mips_elf_finish_dynamic_symbol): Likewise.

ld/

	* testsuite/ld-mips-elf/mips-elf.exp: Add new gp-disp-sym test
	case to check that _gp_disp symbol is not included into symbol
	tables.
	* testsuite/ld-mips-elf/gp-disp-sym.s: Source code for
	the new test.
	* testsuite/ld-mips-elf/gp-disp-sym.d: Expectations for
	the new test.
	* testsuite/ld-mips-elf/mips16-pic-2.ad: Update for _gp_disp
	symbol removal.
	* testsuite/ld-mips-elf/mips16-pic-2.nd: Likewise.
	* testsuite/ld-mips-elf/pic-and-nonpic-3a.dd: Likewise.
	* testsuite/ld-mips-elf/tlslib-o32-hidden.got: Likewise.
	* testsuite/ld-mips-elf/tlslib-o32-ver.got: Likewise.
	* testsuite/ld-mips-elf/tlslib-o32.got: Likewise.

diff --git a/bfd/elf32-mips.c b/bfd/elf32-mips.c
index fa0cc15aba..e6fb9871f5 100644
--- a/bfd/elf32-mips.c
+++ b/bfd/elf32-mips.c
@@ -2414,6 +2414,22 @@ elf32_mips_write_core_note (bfd *abfd, char *buf, int *bufsiz, int note_type,
       }
     }
 }
+
+/* Remove the magic _gp_disp symbol from the symbol tables.  */
+
+static bfd_boolean
+elf32_mips_fixup_symbol (struct bfd_link_info *info,
+			 struct elf_link_hash_entry *h)
+{
+  const char *name = h->root.root.string;
+
+  if (h->dynindx != -1 && strcmp (name, "_gp_disp") == 0)
+    {
+      h->dynindx = -1;
+      _bfd_elf_strtab_delref (elf_hash_table (info)->dynstr, h->dynstr_index);
+    }
+  return TRUE;
+}
 
 /* Depending on the target vector we generate some version of Irix
    executables or "normal" MIPS ELF ABI executables.  */
@@ -2520,6 +2536,7 @@ static const struct ecoff_debug_swap mips_elf32_ecoff_debug_swap = {
 #define elf_backend_grok_prstatus	elf32_mips_grok_prstatus
 #define elf_backend_grok_psinfo		elf32_mips_grok_psinfo
 #define elf_backend_ecoff_debug_swap	&mips_elf32_ecoff_debug_swap
+#define elf_backend_fixup_symbol	elf32_mips_fixup_symbol
 
 #define elf_backend_got_header_size	(4 * MIPS_RESERVED_GOTNO)
 #define elf_backend_want_dynrelro	1
diff --git a/bfd/elfxx-mips.c b/bfd/elfxx-mips.c
index 285401367d..03f997b288 100644
--- a/bfd/elfxx-mips.c
+++ b/bfd/elfxx-mips.c
@@ -2903,12 +2903,6 @@ mips_elf_output_extsym (struct mips_elf_link_hash_entry *h, void *data)
 	      h->esym.asym.value =
 		mips_elf_hash_table (einfo->info)->procedure_count;
 	    }
-	  else if (strcmp (name, "_gp_disp") == 0 && ! NEWABI_P (einfo->abfd))
-	    {
-	      h->esym.asym.sc = scAbs;
-	      h->esym.asym.st = stLabel;
-	      h->esym.asym.value = elf_gp (einfo->abfd);
-	    }
 	  else
 	    h->esym.asym.sc = scUndefined;
 	}
@@ -10953,12 +10947,6 @@ _bfd_mips_elf_finish_dynamic_symbol (bfd *output_bfd,
       sym->st_info = ELF_ST_INFO (STB_GLOBAL, STT_SECTION);
       sym->st_value = 1;
     }
-  else if (strcmp (name, "_gp_disp") == 0 && ! NEWABI_P (output_bfd))
-    {
-      sym->st_shndx = SHN_ABS;
-      sym->st_info = ELF_ST_INFO (STB_GLOBAL, STT_SECTION);
-      sym->st_value = elf_gp (output_bfd);
-    }
   else if (SGI_COMPAT (output_bfd))
     {
       if (strcmp (name, mips_elf_dynsym_rtproc_names[0]) == 0
diff --git a/ld/testsuite/ld-mips-elf/gp-disp-sym.d b/ld/testsuite/ld-mips-elf/gp-disp-sym.d
new file mode 100644
index 0000000000..9450ce1d72
--- /dev/null
+++ b/ld/testsuite/ld-mips-elf/gp-disp-sym.d
@@ -0,0 +1,9 @@
+#source: gp-disp-sym.s
+#as: -EB -mips32 -32
+#ld: -melf32btsmip -shared
+#objdump: -tT
+
+#failif
+#...
+.*_gp_disp
+#...
diff --git a/ld/testsuite/ld-mips-elf/gp-disp-sym.s b/ld/testsuite/ld-mips-elf/gp-disp-sym.s
new file mode 100644
index 0000000000..c6380ba1fb
--- /dev/null
+++ b/ld/testsuite/ld-mips-elf/gp-disp-sym.s
@@ -0,0 +1,5 @@
+  .global foo
+  .text
+foo:
+  lui    $t0, %hi(_gp_disp)
+  addi   $t0, $t0, %lo(_gp_disp)
diff --git a/ld/testsuite/ld-mips-elf/mips-elf.exp b/ld/testsuite/ld-mips-elf/mips-elf.exp
index 13dbbc64f9..dcf114d79d 100644
--- a/ld/testsuite/ld-mips-elf/mips-elf.exp
+++ b/ld/testsuite/ld-mips-elf/mips-elf.exp
@@ -1004,6 +1004,9 @@ if { $linux_gnu } {
 # MIPS16 and microMIPS interlinking test.
 run_dump_test "mips16-and-micromips"
 
+# Test that _gp_disp symbol is not present in EXE or DSO
+run_dump_test "gp-disp-sym"
+
 # Export class call relocation tests.
 set abis [concat o32 [expr {$has_newabi ? "n32 n64" : ""}]]
 foreach { abi } $abis {
diff --git a/ld/testsuite/ld-mips-elf/mips16-pic-2.ad b/ld/testsuite/ld-mips-elf/mips16-pic-2.ad
index 52d3ea4c3b..689f0c2557 100644
--- a/ld/testsuite/ld-mips-elf/mips16-pic-2.ad
+++ b/ld/testsuite/ld-mips-elf/mips16-pic-2.ad
@@ -1,6 +1,6 @@
 # [MIPS_GOTSYM, MIPS_SYMTABNO) covers used4...used7.
 #...
- .* \(MIPS_SYMTABNO\) * 8
+ .* \(MIPS_SYMTABNO\) * 7
 #...
- .* \(MIPS_GOTSYM\) * 0x4
+ .* \(MIPS_GOTSYM\) * 0x3
 #pass
diff --git a/ld/testsuite/ld-mips-elf/mips16-pic-2.nd b/ld/testsuite/ld-mips-elf/mips16-pic-2.nd
index bc2cd38ee9..a2a579412f 100644
--- a/ld/testsuite/ld-mips-elf/mips16-pic-2.nd
+++ b/ld/testsuite/ld-mips-elf/mips16-pic-2.nd
@@ -1,9 +1,9 @@
 # used8 should come before MIPS_GOTSYM.
 #...
- +3: 000405bc +36 +FUNC +GLOBAL +DEFAULT .* used8
- +4: 00040574 +36 +FUNC +GLOBAL +DEFAULT .* used6
- +5: 00040598 +36 +FUNC +GLOBAL +DEFAULT .* used7
- +6: 00040550 +36 +FUNC +GLOBAL +DEFAULT .* used5
- +7: 0004052c +36 +FUNC +GLOBAL +DEFAULT .* used4
+ +2: 000405bc +36 +FUNC +GLOBAL +DEFAULT .* used8
+ +3: 00040574 +36 +FUNC +GLOBAL +DEFAULT .* used6
+ +4: 00040598 +36 +FUNC +GLOBAL +DEFAULT .* used7
+ +5: 00040550 +36 +FUNC +GLOBAL +DEFAULT .* used5
+ +6: 0004052c +36 +FUNC +GLOBAL +DEFAULT .* used4
 
 #pass
diff --git a/ld/testsuite/ld-mips-elf/pic-and-nonpic-3a.dd b/ld/testsuite/ld-mips-elf/pic-and-nonpic-3a.dd
index 3dcfe12cfc..b286f131f4 100644
--- a/ld/testsuite/ld-mips-elf/pic-and-nonpic-3a.dd
+++ b/ld/testsuite/ld-mips-elf/pic-and-nonpic-3a.dd
@@ -35,5 +35,5 @@ Disassembly of section \.MIPS\.stubs:
  c00:	8f998010 	lw	t9,-32752\(gp\)
  c04:	03e07825 	move	t7,ra
  c08:	0320f809 	jalr	t9
- c0c:	24180005 	li	t8,5
+ c0c:	24180004 	li	t8,4
 	\.\.\.
diff --git a/ld/testsuite/ld-mips-elf/tlslib-o32-hidden.got b/ld/testsuite/ld-mips-elf/tlslib-o32-hidden.got
index 563d8bb082..a746031f7e 100644
--- a/ld/testsuite/ld-mips-elf/tlslib-o32-hidden.got
+++ b/ld/testsuite/ld-mips-elf/tlslib-o32-hidden.got
@@ -4,11 +4,11 @@
 DYNAMIC RELOCATION RECORDS
 OFFSET   TYPE              VALUE 
 00000000 R_MIPS_NONE       \*ABS\*
-000403bc R_MIPS_TLS_TPREL32  \*ABS\*
-000403c0 R_MIPS_TLS_DTPMOD32  \*ABS\*
-000403c8 R_MIPS_TLS_DTPMOD32  \*ABS\*
+0004039c R_MIPS_TLS_TPREL32  \*ABS\*
+000403a0 R_MIPS_TLS_DTPMOD32  \*ABS\*
+000403a8 R_MIPS_TLS_DTPMOD32  \*ABS\*
 
 
 Contents of section .got:
- 403b0 00000000 80000000 00000380 00000008  ................
- 403c0 00000000 ffff8004 00000000 00000000  ................
+ 40390 00000000 80000000 00000360 00000008  ................
+ 403a0 00000000 ffff8004 00000000 00000000  ................
diff --git a/ld/testsuite/ld-mips-elf/tlslib-o32-ver.got b/ld/testsuite/ld-mips-elf/tlslib-o32-ver.got
index e675f9f64a..17a6385e8e 100644
--- a/ld/testsuite/ld-mips-elf/tlslib-o32-ver.got
+++ b/ld/testsuite/ld-mips-elf/tlslib-o32-ver.got
@@ -4,12 +4,12 @@
 DYNAMIC RELOCATION RECORDS
 OFFSET   TYPE              VALUE 
 00000000 R_MIPS_NONE       \*ABS\*
-000404d8 R_MIPS_TLS_DTPMOD32  \*ABS\*
-000404d0 R_MIPS_TLS_DTPMOD32  tlsvar_gd@@VER_1
-000404d4 R_MIPS_TLS_DTPREL32  tlsvar_gd@@VER_1
-000404cc R_MIPS_TLS_TPREL32  tlsvar_ie@@VER_1
+000404b8 R_MIPS_TLS_DTPMOD32  \*ABS\*
+000404b0 R_MIPS_TLS_DTPMOD32  tlsvar_gd@@VER_1
+000404b4 R_MIPS_TLS_DTPREL32  tlsvar_gd@@VER_1
+000404ac R_MIPS_TLS_TPREL32  tlsvar_ie@@VER_1
 
 
 Contents of section .got:
- 404c0 00000000 80000000 00000490 00000000  ................
- 404d0 00000000 00000000 00000000 00000000  ................
+ 404a0 00000000 80000000 00000470 00000000  ................
+ 404b0 00000000 00000000 00000000 00000000  ................
diff --git a/ld/testsuite/ld-mips-elf/tlslib-o32.got b/ld/testsuite/ld-mips-elf/tlslib-o32.got
index ad90fb019e..a389c30146 100644
--- a/ld/testsuite/ld-mips-elf/tlslib-o32.got
+++ b/ld/testsuite/ld-mips-elf/tlslib-o32.got
@@ -4,12 +4,12 @@ tmpdir/tlslib-o32.so:     file format elf32-tradbigmips
 DYNAMIC RELOCATION RECORDS
 OFFSET   TYPE              VALUE 
 00000000 R_MIPS_NONE       \*ABS\*
-00040448 R_MIPS_TLS_DTPMOD32  \*ABS\*
-00040440 R_MIPS_TLS_DTPMOD32  tlsvar_gd
-00040444 R_MIPS_TLS_DTPREL32  tlsvar_gd
-0004043c R_MIPS_TLS_TPREL32  tlsvar_ie
+00040428 R_MIPS_TLS_DTPMOD32  \*ABS\*
+00040420 R_MIPS_TLS_DTPMOD32  tlsvar_gd
+00040424 R_MIPS_TLS_DTPREL32  tlsvar_gd
+0004041c R_MIPS_TLS_TPREL32  tlsvar_ie
 
 
 Contents of section .got:
- 40430 00000000 80000000 00000400 00000000  ................
- 40440 00000000 00000000 00000000 00000000  ................
+ 40410 00000000 80000000 000003e0 00000000  ................
+ 40420 00000000 00000000 00000000 00000000  ................