TL;DR On aarch64 with gcc toolchain, when test_progs is linked as a PIE, reads of BTF ID array by C name return garbage because the GNU assembler on aarch64 unconditionally folds .local symbol references into section+addend form, and GOT slots cannot carry an addend per the AArch64 ELF spec. Fix by marking the test's BTF ID objects with hidden visibility, which makes gcc emit a direct access that bypasses the GOT entirely. Details below. The subsequent patches adding kfunc checks to resolve_btfids test may cause test failures on aarch64 / gcc-15: test_resolve_btfids:FAIL:kfunc_set_flags actual 13 != expected 1 test_resolve_btfids:FAIL:kfunc_set_cnt actual 0 != expected 4 The test defines its BTF ID sets with the same macros as the kernel and reads them back directly by C name (in the same way as the kernel code does). test_kfunc_set is a .local symbol emitted into .BTF_ids by inline asm and declared to the compiler as a plain default-visibility extern, that is: extern struct btf_id_set8 test_kfunc_set; Depending on the build environment, test_progs may be linked as a position-independent executable (for example, gcc defaults to -fpie [1]). In a PIE, taking the address of a default-visibility extern is routed through the GOT (Global Offset Table) [2]. The GNU assembler's adjust_reloc_syms() pass (gas/write.c [3]) replaces references to local symbols with the corresponding section symbol, folding the symbol's offset into the relocation addend. On aarch64 this conversion is unconditional: tc_fix_adjustable() is defined to 1 for all fixups (gas/config/tc-aarch64.h [4]), so even GOT-generating relocations are subject to it. The resulting object file therefore contains: R_AARCH64_ADR_GOT_PAGE .BTF_ids + 0x54 R_AARCH64_LD64_GOT_LO12_NC .BTF_ids + 0x54 However, the AArch64 ELF specification mandates that GOT-generating relocations must have a zero addend [5]. The +0x54 is therefore not honored: the linker creates a GOT slot pointing at the .BTF_ids base, and every access through that slot reads offset 0 instead of 0x54. This is purely a read-side problem, specific to the PIE test binary on aarch64 with gcc toolchain. resolve_btfids patches the set header correctly and the .BTF_ids bytes in test_progs are correct. vmlinux is unaffected because it is built with -fno-PIE [6] and reaches .BTF_ids with direct, addend-preserving relocations rather than the GOT. clang is unaffected because LLVM's assembler retains the original symbol for GOT relocations instead of converting to section+addend [7]. To mitigate this issue, mark the test's BTF ID objects hidden with a visibility pragma so that gcc treats them as non-interposable and emits a direct access instead of a GOT load. This keeps the natural by-name access, works in both PIE and non-PIE builds, and needs no change to the BTF_ID macros or resolve_btfids. [1] https://gcc.gnu.org/onlinedocs/gnat_ugn/Position-Independent-Executable-PIE-Enabled-by-Default-on-Linux.html [2] https://gcc.gnu.org/wiki/Visibility [3] https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/write.c#l922 [4] https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/config/tc-aarch64.h#l279 [5] https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst#5733relocation-operations [6] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?h=v7.1-rc6#n593 [7] https://github.com/llvm/llvm-project/blob/4b3bc46d1d794b8ed78b75ccd35a6cc30235bf31/llvm/lib/MC/ELFObjectWriter.cpp#L1213-L1224 --- --- .../selftests/bpf/prog_tests/resolve_btfids.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c index d742ecb2ca8e..d0e51979d455 100644 --- a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c +++ b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c @@ -32,6 +32,23 @@ asm ( ".balign 4, 0; \n" ".popsection; \n"); +/* + * The BTF ID arrays below are .local symbols placed in .BTF_ids by + * inline asm, and are read here directly by C name. To the compiler + * they are plain, default-visibility extern objects. + * + * When test_progs is linked as a position-independent executable (PIE), + * taking the address of such an extern is routed through the GOT. The + * GNU assembler on aarch64 unconditionally converts references to + * .local symbols into section + addend form (".BTF_ids + "), + * but a GOT slot cannot carry an addend (the AArch64 ELF spec mandates + * zero), so the linker resolves it to the .BTF_ids base. + * + * Mark these symbols hidden so the compiler treats them as + * non-interposable and emits a direct, addend-preserving PC-relative + * access instead of a GOT load, in both PIE and non-PIE builds. + */ +#pragma GCC visibility push(hidden) BTF_ID_LIST(test_list_local) BTF_ID_UNUSED BTF_ID(typedef, S) @@ -60,6 +77,7 @@ BTF_ID(union, U) BTF_ID(func, func) BTF_SET_END(test_set) +#pragma GCC visibility pop static int __resolve_symbol(struct btf *btf, int type_id) { -- 2.54.0