Re: [PATCH] readelf: allow build with LLVM/clang

[Mark Wielaard <mark@klomp.org>]

Hi Tomasz,

On Mon, Jul 12, 2021 at 11:59:41AM +0200, Tomasz Paweł Gajc via Elfutils-devel wrote:

> convert last nested function to allow build with
> LLVM/clang. Original patch comes from
> https://github.com/OpenMandrivaAssociation/elfutils/blob/master/elfutils-0.185-clang.patch
> Tested with OpenMandriva Lx cooker and LLVM/clang-12.0.1
> [...]
> --- a/configure.ac
> +++ b/configure.ac
> @@ -127,7 +127,7 @@ void baz (int n)
>  		  ac_cv_c99=yes, ac_cv_c99=no)
>  CFLAGS="$old_CFLAGS"])
>  AS_IF([test "x$ac_cv_c99" != xyes],
> -      AC_MSG_ERROR([gcc with GNU99 support required]))
> +      AC_MSG_WARN([gcc with GNU99 support required]))

I don't think this is correct. If you believe the testcase is (now)
wrong because it tests for an unneeded feature please just adjust the
test.

>  AC_CACHE_CHECK([whether gcc supports __attribute__((visibility()))],
>  	ac_cv_visibility, [dnl
> diff --git a/src/readelf.c b/src/readelf.c
> index 161d7e65..3d6f263e 100644
> --- a/src/readelf.c
> +++ b/src/readelf.c
> @@ -8763,13 +8763,17 @@ print_debug_line_section (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr,
>        /* Apply the "operation advance" from a special opcode
>  	 or DW_LNS_advance_pc (as per DWARF4 6.2.5.1).  */
>        unsigned int op_addr_advance;
> -      inline void advance_pc (unsigned int op_advance)
> -      {
> -	op_addr_advance = minimum_instr_len * ((op_index + op_advance)
> -					       / max_ops_per_instr);
> -	address += op_addr_advance;
> -	op_index = (op_index + op_advance) % max_ops_per_instr;
> -      }
> +      bool show_op_index;
> +      #define advance_pc(op_advance_arg) \
> +      ( { \
> +        unsigned int op_advance = op_advance_arg; \
> +		op_addr_advance = minimum_instr_len * ((op_index + (op_advance)) \
> +			/ max_ops_per_instr); \
> +		address += (op_advance); \
> +		show_op_index = (op_index > 0 || \
> +			(op_index + (op_advance)) % max_ops_per_instr > 0); \
> +		op_index = (op_index + (op_advance)) % max_ops_per_instr; \
> +      } )

This doesn't compile with gcc:

readelf.c: In function ‘print_debug_line_section’:
readelf.c:8766:12: error: variable ‘show_op_index’ set but not used [-Werror=unused-but-set-variable]
 8766 |       bool show_op_index;
      |            ^~~~~~~~~~~~~
cc1: all warnings being treated as errors

Removing the unused show_op_index makes two testcases fail:

FAIL: run-readelf-line.sh
=========================

--- readelf.out	2021-07-14 19:37:00.962372827 +0200
+++ -	2021-07-14 19:37:00.971161054 +0200
@@ -42,13 +42,13 @@
  [    36] set prologue end flag
  [    37] special opcode 19: address+0 = 0x100005a4 <main>, line+1 = 6
  [    38] set column to 8
- [    3a] special opcode 47: address+8 = 0x100005a6 <main+0x2>, line+1 = 7
+ [    3a] special opcode 47: address+8 = 0x100005ac <main+0x8>, line+1 = 7
  [    3b] set 'is_stmt' to 0
  [    3c] advance line by constant -7 to 0
- [    3e] special opcode 32: address+4 = 0x100005a7 <main+0x3>, line+0 = 0
+ [    3e] special opcode 32: address+4 = 0x100005b0 <main+0xc>, line+0 = 0
  [    3f] set column to 3
  [    41] set 'is_stmt' to 1
- [    42] special opcode 108: address+24 = 0x100005ad <main+0x9>, line+6 = 6
- [    43] special opcode 76: address+16 = 0x100005b1 <main+0xd>, line+2 = 8
- [    44] advance address by 32 to 0x100005b9 <main+0x15>
+ [    42] special opcode 108: address+24 = 0x100005c8 <main+0x24>, line+6 = 6
+ [    43] special opcode 76: address+16 = 0x100005d8 <main+0x34>, line+2 = 8
+ [    44] advance address by 32 to 0x100005f8
  [    46] extended opcode 1:  end of sequence
FAIL run-readelf-line.sh (exit status: 1)

FAIL: run-readelf-multi-noline.sh
=================================

--- readelf.out	2021-07-14 19:37:10.054186557 +0200
+++ -	2021-07-14 19:37:10.062074795 +0200
@@ -112,6 +112,6 @@
  [    6e] extended opcode 2:  set address to +0x724 <main>
  [    79] copy
  [    7a] set column to 15
- [    7c] special opcode 32: address+4 = +0x725 <main+0x1>, line+0 = 1
- [    7d] advance address by 4 to +0x726 <main+0x2>
+ [    7c] special opcode 32: address+4 = +0x728 <main+0x4>, line+0 = 1
+ [    7d] advance address by 4 to +0x72c
  [    7f] extended opcode 1:  end of sequence
FAIL run-readelf-multi-noline.sh (exit status: 1)

Cheers,

Mark