https://sourceware.org/bugzilla/show_bug.cgi?id=34872
Bug ID: 34872
Summary: readelf -wl: DW_LNS_const_add_pc ignores
maximum_operations_per_instruction
Product: binutils
Version: 2.48 (HEAD)
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: binutils
Assignee: unassigned at sourceware dot org
Reporter: martin.mirchev at u dot nus.edu
Target Milestone: ---
In display_debug_lines_raw() (readelf -wl / objdump --dwarf=rawline),
the DW_LNS_const_add_pc case tests
if (linfo.li_max_ops_per_insn)
where every other address-advancing opcode in both the raw and the
decoded line printers tests "== 1". li_max_ops_per_insn is never 0 at
this point (it is forced to 1 for DWARF < 4 and rejected when 0), so the
VLIW branch is dead: with maximum_operations_per_instruction > 1 the
raw dump advances the address by the whole operation advance and never
updates op_index. readelf's own -wL (display_debug_lines_decoded, which
has "== 1") and elfutils get it right.
t.s (attached) is a DWARF 4 line program with
maximum_operations_per_instruction = 2 and min_inst_length = 1:
DW_LNS_const_add_pc, DW_LNS_copy, DW_LNE_end_sequence. The operation
advance is (255 - 13) / 14 = 17, i.e. address 8, op_index 1.
$ gcc -c t.s -o t.o
$ readelf -wl t.o | grep Advance
[0x00000025] Advance PC by constant 17 to 0x11
$ readelf -wL t.o | grep t.c
t.c 1 0x8[1]
x
t.c - 0x8[1]
$ eu-readelf --debug-dump=line t.o | grep 'by constant'
[ 25] advance address by constant 8 to 0x8, op_index to 1
t-control.s is the same with maximum_operations_per_instruction = 1;
there all three agree on 0x11.
Fix (patch attached, against master 0164fa7b33f):
--- a/binutils/dwarf.c
+++ b/binutils/dwarf.c
@@ -6311,7 +6311,7 @@ display_debug_lines_raw (struct dwarf_section * section,
case DW_LNS_const_add_pc:
uladv = ((255 - linfo.li_opcode_base) /
linfo.li_line_range);
- if (linfo.li_max_ops_per_insn)
+ if (linfo.li_max_ops_per_insn == 1)
{
uladv *= linfo.li_min_insn_length;
state_machine_regs.address += uladv;
With the patch, readelf -wl t.o prints
[0x00000025] Advance PC by constant 17 to 0x8[1]
and t-control.o is unchanged. readelf -a -W --debug-dump=rawline,decodedline
output is byte-identical before and after the patch for 784 ELF files
(/usr/bin, system shared libraries, separate debug files, and the
binutils/bfd/libiberty objects of the build), all of which have
maximum_operations_per_instruction = 1. I could not run the DejaGnu
testsuite here.
This issue was found by an LLM-Enhanced Static Analysis Engine and verified
manually; the patch was manually reviewed.
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