[PATCH] gas: elf: Relax rules for SHF_STRING sections

[Alan Modra]

On Fri, Jan 17, 2025 at 03:25:19PM +0000, Richard Earnshaw wrote:
> Commit af3394d97a8c5187085c0eec5fb03e8da88db5fb allowed sections
> declared with "S" (SHF_STRING) to specify the entity size, but then
> would warn if the entity size was omitted, as with the old syntax.
> 
> Unfortunately, since specifying the entity size is incompatible with
> binutils 2.43 or earlier, this makes it impossible to specify a
> strings section in source code without generating an assembly warning
> (the new syntax isn't supported in older assemblers and the old syntax
> generates warnings).
> 
> Nevertheless, the old code was wrong in that it did not set the entity
> size at all, in contravention of the ELF specification (though to date
> there are no known cases where this mattered outside of mergeable
> sections).
> 
> Fix this by permitting the original syntax without a warning again,
> but by defaulting the entity size to 1.  This is compatible with the
> most common case of strings being byte-based.

I think backward compatibility matters, and this change doesn't negate
Jan's fix for bad entsize, but rather allows a reasonable default.

> --- /dev/null
> +++ b/gas/testsuite/gas/elf/string.d
> @@ -0,0 +1,9 @@
> +#readelf: -S

You'll need -W or --wide here.

I also think we should do something about backwards compatibilty for
the ARM compiler configure bug, so that people can build released
versions of gcc.  This is what I have in mind, simpler than the patch
Thiago submitted.

diff --git a/gas/config/obj-elf.c b/gas/config/obj-elf.c
index 6670b9add0f..b3b2025938b 100644
--- a/gas/config/obj-elf.c
+++ b/gas/config/obj-elf.c
@@ -1324,13 +1324,27 @@ obj_elf_section (int push)
 		  && (bfd_section_flags (now_seg)
 		      & (SEC_MERGE | SEC_STRINGS)) != 0)
 		goto fetch_entsize;
-	      entsize = get_absolute_expression ();
-	      SKIP_WHITESPACE ();
-	      if (entsize <= 0)
+	      if (is_end_of_line[(unsigned char) *input_line_pointer])
+		{
+		  /* ??? This is here for older versions of gcc that
+		     test for gas string merge support with
+		     '.section .rodata.str, "aMS", @progbits, 1'
+		     Unfortunately '@' begins a comment on arm.
+		     This isn't as_warn because gcc tests with
+		     --fatal-warnings. */
+		  as_tsktsk (_("missing merge / string entity size, 1 assumed"));
+		  entsize = 1;
+		}
+	      else
 		{
-		  as_warn (_("invalid merge / string entity size"));
-		  attr &= ~(SHF_MERGE | SHF_STRINGS);
-		  entsize = 0;
+		  entsize = get_absolute_expression ();
+		  SKIP_WHITESPACE ();
+		  if (entsize <= 0)
+		    {
+		      as_warn (_("invalid merge / string entity size"));
+		      attr &= ~(SHF_MERGE | SHF_STRINGS);
+		      entsize = 0;
+		    }
 		}
 	    }
 	  else if ((attr & (SHF_MERGE | SHF_STRINGS)) != 0 && inherit

-- 
Alan Modra