[binutils-gdb] MIPS/GAS: Warn about unmatched REL HI16 relocations

[Maciej W. Rozycki]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=3492c9a216580892d7f601a0310f517c2be03149

commit 3492c9a216580892d7f601a0310f517c2be03149
Author: Maciej W. Rozycki <macro@orcam.me.uk>
Date:   Wed Jan 14 22:28:44 2026 +0000

    MIPS/GAS: Warn about unmatched REL HI16 relocations
    
    Ever since commit 749b8d9d455c ("PATCH: ld/4208: `final link failed: Bad
    value' when building Linux MIPS kernels."), which went in back in 2007,
    <https://inbox.sourceware.org/binutils/20070320043504.GA7735@lucon.org/>,
    we've been warning about unmatched HI16 REL relocations in the linker,
    as it has been concluded in the discussion on PR ld/4208 that we have no
    need to be forgiving about compiler bugs that lead to object files to be
    made that are not compliant with the psABI.
    
    However no corresponding update has ever been made to GAS, despite that
    it's always helpful to issue a warning as early as possible in the build
    process.  In this particular case GAS also has more information to hand
    as it always has the full original addend available that has been used
    for both the high-part and the low-part relocation, while the linker has
    to rely on the in-place value stored in the relocatable field by earlier
    processing, which has necessarily been truncated.  Therefore the linker
    may miss cases where unrelated high-part and low-part relocations have
    been put next to each other that had significantly different addends at
    assembly time (small differences are deliberately permitted where they
    are known not to affect any borrow from the high part).
    
    Additionally GAS is able to point at the exact place in sources where
    any offending orphan relocations have been used.
    
    Update GAS to issue the warning then where applicable and remove an old
    comment referring to GCC producing orphan HI16 relocations.
    
    Test cases will be added with a separate change.

Diff:
---
 gas/config/tc-mips.c | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
index 5a5e6b8cdaa..4440fb7cafb 100644
--- a/gas/config/tc-mips.c
+++ b/gas/config/tc-mips.c
@@ -7677,12 +7677,11 @@ micromips_add_label (void)
   S_SET_OTHER (s, ELF_ST_SET_MICROMIPS (S_GET_OTHER (s)));
 }
 
-/* If assembling microMIPS code, then return the microMIPS reloc
-   corresponding to the requested one if any.  Otherwise return
-   the reloc unchanged.  */
+/* Return the microMIPS relocation corresponding to RELOC if there is any
+   and MICROMIPS_P is true.  Otherwise return RELOC unchanged.  */
 
 static bfd_reloc_code_real_type
-micromips_map_reloc (bfd_reloc_code_real_type reloc)
+micromips_map_reloc (bfd_reloc_code_real_type reloc, bool micromips_p)
 {
   static const bfd_reloc_code_real_type relocs[][2] =
     {
@@ -7718,7 +7717,7 @@ micromips_map_reloc (bfd_reloc_code_real_type reloc)
   bfd_reloc_code_real_type r;
   size_t i;
 
-  if (!mips_opts.micromips)
+  if (!micromips_p)
     return reloc;
   for (i = 0; i < ARRAY_SIZE (relocs); i++)
     {
@@ -8199,7 +8198,8 @@ append_insn (struct mips_cl_insn *ip, expressionS *address_expr,
       /* Perform any necessary conversion to microMIPS relocations
 	 and find out how many relocations there actually are.  */
       for (i = 0; i < 3 && reloc_type[i] != BFD_RELOC_UNUSED; i++)
-	final_type[i] = micromips_map_reloc (reloc_type[i]);
+	final_type[i] = micromips_map_reloc (reloc_type[i],
+					     mips_opts.micromips);
 
       /* In a compound relocation, it is the final (outermost)
 	 operator that determines the relocated field.  */
@@ -14955,6 +14955,24 @@ parse_relocation (char **str, bfd_reloc_code_real_type *reloc)
   return false;
 }
 
+/* Return a relocation operator corresponding to RELOC.  */
+
+static const char *
+mips_op_for_relocation (bfd_reloc_code_real_type reloc)
+
+{
+  size_t i;
+
+  for (i = 0; i < ARRAY_SIZE (mips_percent_op); i++)
+    if (reloc == micromips_map_reloc (mips_percent_op[i].reloc,
+				      micromips_reloc_p (reloc)))
+      return mips_percent_op[i].str;
+  for (i = 0; i < ARRAY_SIZE (mips16_percent_op); i++)
+    if (reloc == mips16_percent_op[i].reloc)
+      return mips16_percent_op[i].str;
+
+  abort ();
+}
 
 /* Parse string STR as a 16-bit relocatable operand.  Store the
    expression in *EP and the relocations in the array starting
@@ -15816,11 +15834,7 @@ mips_frob_file (void)
 	 fixup_has_matching_lo_p() will return true.  For PC-relative
 	 relocations the distance between the offsets is retained
 	 according to expectations in `fixup_has_matching_lo_p',
-	 `_bfd_mips_elf_lo16_reloc' and `mips_elf_add_lo16_rel_addend'.
-
-	 We don't warn about unmatched high-part relocations since some
-	 versions of gcc have been known to emit dead "lui ...%hi(...)"
-	 instructions.  */
+	 `_bfd_mips_elf_lo16_reloc' and `mips_elf_add_lo16_rel_addend'.  */
       if (lo_pos != NULL)
 	{
 	  if (l->fixp->fx_r_type != BFD_RELOC_HI16_S_PCREL)
@@ -15832,6 +15846,11 @@ mips_frob_file (void)
 	      *lo_pos = l->fixp;
 	    }
 	}
+      else
+	as_warn_where
+	  (l->fixp->fx_file, l->fixp->fx_line,
+	   _("can't find matching low-part relocation for %s operator"),
+	   mips_op_for_relocation (l->fixp->fx_r_type));
     }
 }