Re: Errors compiling BPF programs from Linux selftests/bpf with GCC

[Ihor Solodrai <ihor.solodrai@xxxxx>]

Hi everyone.

I built and ran selftests/bpf with GCC 15-20241229, and would like to
share my findings.

Building required small adjustments in the Makefile, besides -std=gnu17

With the following change we can mitigate int64_t issue:

+progs/test_cls_redirect.c-CFLAGS := -nostdinc
+progs/test_cls_redirect_dynptr.c-CFLAGS := -nostdinc
+progs/test_cls_redirect_subprogs.c-CFLAGS := -nostdinc

Then, the compiler complains about an uninitialized variable in
progs/verifier_bpf_fastcall.c and progs/verifier_search_pruning.c
(full log at [1]):

    In file included from progs/verifier_bpf_fastcall.c:7:
    progs/verifier_bpf_fastcall.c: In function ‘may_goto_interaction’:
    progs/bpf_misc.h:153:42: error: ‘<Uc098>’ is used uninitialized [-Werror=uninitialized]
      153 | #define __imm_insn(name, expr) [name]"i"(*(long *)&(expr))
          |                                          ^~~~~~~~~~~~~~~~
    progs/verifier_bpf_fastcall.c:652:11: note: in expansion of macro ‘__imm_insn’
      652 |           __imm_insn(may_goto, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, +1 /* offset */, 0))
          |           ^~~~~~~~~~
    /ci/workspace/tools/testing/selftests/bpf/../../../include/linux/filter.h:299:28: note: ‘({anonymous})’ declared here
      299 |         ((struct bpf_insn) {                                    \
          |                            ^
    progs/bpf_misc.h:153:53: note: in definition of macro ‘__imm_insn’
      153 | #define __imm_insn(name, expr) [name]"i"(*(long *)&(expr))
          |                                                     ^~~~
    progs/verifier_bpf_fastcall.c:652:32: note: in expansion of macro ‘BPF_RAW_INSN’
      652 |           __imm_insn(may_goto, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, +1 /* offset */, 0))

BPF_RAW_INSN expands into struct init expr (include/linux/filter.h):

    #define BPF_RAW_INSN(CODE, DST, SRC, OFF, IMM)			\
    	((struct bpf_insn) {					\
    		.code  = CODE,					\
    		.dst_reg = DST,					\
    		.src_reg = SRC,					\
    		.off   = OFF,					\
    		.imm   = IMM })

This can be silenced with:

+progs/verifier_bpf_fastcall.c-CFLAGS := -Wno-error
+progs/verifier_search_pruning.c-CFLAGS := -Wno-error

Then the selftests/bpf build completes successfully, although libbpf
prints a lot of warnings like these on GEN-SKEL:

    [...]
    libbpf: elf: skipping section(3) .data (size 0)
    libbpf: elf: skipping section(4) .data (size 0)
    libbpf: elf: skipping unrecognized data section(13) .comment
    libbpf: elf: skipping unrecognized data section(9) .comment
    libbpf: elf: skipping unrecognized data section(12) .comment
    libbpf: elf: skipping unrecognized data section(7) .comment
    [...]

Test .bpf.o files are compiled regardless. Full log at [2].

Running all tests at once, as is usually done on CI, produces a too
cluttered log. I wrote a script to run each test individually in a
separate qemu instance and collect the logs.

187/581 of toplevel tests fail on current bpf-next [3]. Many tests
have subtests: toplevel test passes if all of its subtests pass.

You can find the archive with per-test logs at [4].

[1] https://gist.github.com/theihor/10b2425e6780fcfebb80aeceafba7678
[2] https://gist.github.com/theihor/9e96643ca730365cf79cea8445e40aeb
[3] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=96ea081ed52bf077cad6d00153b6fba68e510767
[4] https://github.com/kernel-patches/bpf/blob/8f2e62702ee17675464ab00d97d89d599922de20/tools/testing/selftests/bpf/gcc-bpf-selftests-logs.tgz