[Bug debug/113566] New: btf: incorrect BTF_KIND_DATASEC entries for variables which are optimized out

[Bug debug/113566] New: btf: incorrect BTF_KIND_DATASEC entries for variables which are optimized out

[david.faust at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113566

            Bug ID: 113566
           Summary: btf: incorrect BTF_KIND_DATASEC entries for variables
                    which are optimized out
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: btf-debug
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david.faust at oracle dot com
                CC: david.faust at oracle dot com, jemarch at gcc dot gnu.org
  Target Milestone: ---
            Target: all

Consider a simple program:

$ cat static.c
static int a = 5;

int foo (int x) {
        return a + x;
}

When compiled with -O2, variable 'a' is optimized away, and its use is
replaced with a literal 5 in the resulting object code.

For all targets except BPF, BTF is emitted at early_finish always.
For the BPF target, if -mco-re is in effect, then BTF is emitted at finish
rather than early_finish.

The combination of -O2 with emitting BTF at early_finish causes incorrect
BTF_KIND_DATASEC entries to be emitted for all targets except BPF CO-RE:

$ ~/toolchains/bpf/bin/bpf-unknown-none-gcc -c -gbtf -O2 -mco-re static.c -o
static.o
$ /usr/sbin/bpftool btf dump file static.o
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] FUNC_PROTO '(anon)' ret_type_id=1 vlen=1
        'x' type_id=1
[3] VAR 'a' type_id=1, linkage=static
[4] FUNC 'foo' type_id=2 linkage=global

$ ~/toolchains/bpf/bin/bpf-unknown-none-gcc -c -gbtf -O2 -mno-co-re static.c -o
static.o
$ /usr/sbin/bpftool btf dump file static.o
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] FUNC_PROTO '(anon)' ret_type_id=1 vlen=1
        'x' type_id=1
[3] VAR 'a' type_id=1, linkage=static
[4] FUNC 'foo' type_id=2 linkage=global
[5] DATASEC '.data' size=0 vlen=1
        type_id=3 offset=0 size=4 (VAR 'a')
(same for e.g. x86_64 with -gbtf)

In either case, 'a' is optimized away, and is not allocated in .data: 

$ ~/toolchains/bpf/bin/bpf-unknown-none-objdump -dh static.o

static.o:     file format elf64-bpfle

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000018  0000000000000000  0000000000000000  00000040  2**3
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .data         00000000  0000000000000000  0000000000000000  00000058  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  0000000000000000  0000000000000000  00000058  2**0
                  ALLOC
...

So, the BTF_KIND_DATASEC entry claiming 'a' is allocated in .data is incorrect.
Clang correctly does not generate such a DATASEC entry.

The only case where the entry is correctly not generated by gcc is for the BPF
target with -mco-re, since in that case the DATASEC entries will be generated
at finish rather than early finish, by which time 'a' is known to be optimized
away.

[Bug debug/113566] btf: incorrect BTF_KIND_DATASEC entries for variables which are optimized out

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113566

--- Comment #1 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by David Faust <dfaust@gcc.gnu.org>:

https://gcc.gnu.org/g:8075477f81ae8d0abf64b80dfbd179151f91b417

commit r14-9876-g8075477f81ae8d0abf64b80dfbd179151f91b417
Author: David Faust <david.faust@oracle.com>
Date:   Mon Apr 8 11:10:41 2024 -0700

    btf: emit symbol refs in DATASEC entries only for BPF [PR114608]

    The behavior introduced in
      fa60ac54964 btf: Emit labels in DATASEC bts_offset entries.

    is only fully correct when compiling for the BPF target with BPF CO-RE
    enabled.  In other cases, depending on optimizations, it can result in
    an incorrect symbol reference in the entry bts_offset field for a symbol
    which may not be emitted at all, causing link-time undefined symbol
    reference errors like in PR114608.

    The offending bts_offset field of BTF_KIND_DATASEC entries is in reality
    only currently useful to consumers of BTF information for BPF programs
    anyway.  Correct the regression by only emitting symbol references in
    these entries when compiling for the BPF target.  For other targets, the
    behavior returns to that prior to fa60ac54964.

    The underlying cause is related to PR 113566 "btf: incorrect
    BTF_KIND_DATASEC entries for variables which are optimized out." A
    complete fix for 113566 is more involved and unsuitable for stage 4,
    but will be addressed in the near future.

    gcc/
            PR debug/114608
            * btfout.cc (btf_asm_datasec_entry): Only emit a symbol reference
when
            generating BTF for BPF CO-RE target.

    gcc/testsuite/
            PR debug/114608
            * gcc.dg/debug/btf/btf-datasec-1.c: Check bts_offset symbol
references
            only for BPF target.
            * gcc.dg/debug/btf/btf-datasec-2.c: Likewise.
            * gcc.dg/debug/btf/btf-pr106773.c: Likewise.